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REDDISH GREEN A CHALLENGE FOR MODAL CLAIMS ABOUT PHENOMENAL STRUCTURE

Juan Suarez & Martine Nida-Rümelin Université de Fribourg, Switzerland

Abstract: We discuss two modal claims about the phenomenal structure of color experiences in the light of empirical results: the claim that violet experiences are necessarily experiences of a color that is for the subject on that occasion phenomenally composed of red and blue (the modal claim about violet) and the claim that no subject can possibly have an experience of a color that is for it then phenomenally composed of green and red (the modal claim about reddish green). The modal claim about reddish green is undermined by empirical results. We discuss whether these empirical results cast doubt on the other modal claims as well. We argue that this not the case. Our argument is based on the thesis that the best argument for the modal claim about violet is quite different from the best argument for the modal claim about reddish green. To argue for this disanalogy we propose a reconstruction of the best available justification for both claims.

1. Conceptual preliminaries and presuppositions

For a long time it has been accepted that red and green cannot combine to a new color in experience in the way red and blue can combine to form an experience of violet. This natural assumption has been challenged by the experiments of (Crane & Piantanida, 1983) and (Billock, Gleason, & Tsou, 2001). The results of these experiments if taken at face value show that it is possible for human beings to see a color that has a reddish and a greenish component. We assume that what is at issue here is the question of whether red and green can or cannot combine in experience to a new perceived color that is phenomenally composed of red and green in the way violet is composed of red and blue or orange is composed of red and yellow. To formulate the philosophical issues we are interested in will require using the concept of phenomenal composition. Before we begin we have to say a few words about how we understand the concept and we have to distinguish two related but different concepts of phenomenal composition. A few controversial assumptions will be presupposed in the present paper (and are discussed in other places)1:

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(1) We will assume that the claim

(V) Violet is composed of red and blue. is made true by a particular aspect of the subjective character of violet experiences. It is not made true, in our view, by any fact concerning the mixture of pigments or the mixture of light. (V) is true because things that look violet to a person look to that person in a particular way: they look reddish and bluish and there is no other chromatic hue component present in the color as it is perceived by the subject at issue. You may insist that (V) may be interpreted as a claim about how to mix violet with pigments on a piece of paper. This may be true but this possible interpretation of (V) is not the interpretation we will be interested in. There is a reading of (V) – or so we claim – that is made true by phenomenal facts. This is why we call the relation stated in (V) the relation of phenomenal composition.

(2) Claims like (V) about phenomenal composition are made true by phenomenal facts but they are not made true by contingent phenomenal facts. They are true because violet is necessarily experienced as being composed of red and blue. To claim that (V) is true in general is to claim that no subject in counterfactual circumstances could have an experience of something as violet without thereby having an experience of something as looking bluish and reddish at the same time. So, according to the interpretation of (V) that we will presuppose, (V) implies that it is constitutive of having an experience of violet that it is an experience of a color that is seen as containing a reddish and a bluish component. This modal claim has sometimes been invoked to argue that being phenomenally composed of red and blue is a necessary property of colors.2 Contrary to this we claim that (V) in itself (without any modal operator) is already a modal claim about violet experiences. We agree that (V) is itself a necessary truth. We agree with Hardin that violet is necessarily phenomenally composed of red and violet. But this then is to say that the color violet necessarily has the property of being necessarily experienced in a particular way. This analysis may look excessively complicated. We think the complication is unavoidable but we will not try to defend this thesis here.

(3) To avoid confusion two notions of phenomenal composition have to be distinguished. The first notion of phenomenal composition is the one used in (V). The second notion of phenomenal composition is used to describe a particular aspect of the subjective character of violet experiences (the relevant aspect we mentioned before). The fact that a particular person sees a reddish and a bluish component when having an experience of violet is expressed using the second concept. We will choose the locution "the object appears to the

2 3 person in a color that looks phenomenally composed of red and blue to that person on that occasion" to express the second concept. For our philosophical purposes it is important to be able to give phenomenological descriptions of concrete color experiences that do not imply modal claims about color experiences of the relevant type. It must be possible e.g. to claim that for Mary something that looks violet to her on a particular occasion looks in a color that has (in Mary's experience) a blue and a red component without thereby implying that what is said here about Mary on that particular occasion has to be true of her in any other moment when she has an experience of violet and has to be true for any subject that has an experience of violet. In a language that is appropriate to our philosophical purposes, phenomenological descriptions concerning a particular violet experience should not logically imply modal claims about the phenomenal character of violet experiences in general. It is quite easy to see that this constraint cannot be met without a conceptual distinction of the kind we just introduced. It can be argued that – given a number of substantial philosophical assumptions – the distinction can be neglected after all. But these reasons for abandoning the distinction cannot even be formulated without using the distinction itself.3

(4) We will presuppose a further controversial claim that might be called realism about phenomenal composition (in the sense of the second notion). According to this claim a person may actually have an experience of a color that looks composed of red and blue to him or her on that occasion without noticing that this is so. A person may even have an experience of a hue that is for her a binary hue (it contains for her two hue components) and yet be convinced that there are no two hue components in the color as she perceives it and vice versa. According to this view it is no contradiction to say that a person sees green as a unique hue and yet seriously believes that she is thereby seeing a hue that is for her phenomenally composed of blue and yellow.4 According to this realist thesis we can be wrong about facts concerning phenomenal composition (in the sense of the second notion) even with respect to our own color experiences and this even while having them and even while directing our attention towards the relevant phenomenal aspect.

2. Modal Knowledge about Phenomenal Composition. A Problem Raised by Reddish Greens

If claims like (V) are correctly interpreted as implying modal claims about the phenomenal character of violet experiences then the following epistemological question arises: How can we know that violet experiences have a certain phenomenal structure necessarily? It appears to be quite unproblematic to assume that a person can know, by carefully attending to her own violet experience while having it, that she is thereby having an experience of a color that

3 4 looks to be composed of red and blue to her on that occasion. But how can she know that this will be so again next time when she will have a violet experience? And how can she know that no subject can ever have an experience of violet without thereby having an experience of a color that looks to be phenomenally composed of red and blue for to that subject on that occasion? How can a person rationally judge about violet experiences at other times in other galaxies had by different sentient beings? Or is it maybe irrational to hold the modal claim that is implied by claims like (V)?

Our answer to these questions is that we can rationally judge that (V) is true. On our view this implies the claim that we do have epistemic access to modal truths about phenomenal composition (in the second sense). We will defend the view that a person who (a) judges on the basis of carefully attending to her own violet experiences that she has an experience of a color that is phenomenally composed of red and blue for her whenever she has an experience of this type, does indeed have reason to (b) judge that having an experience of a reddish and a bluish component is indeed constitutive of having an experience of violet. This view involves the claim that we can rationally proceed from phenomenological observations about our own experiences to modal claims about experiences of the relevant type in general. How can we do this? What is the correct reconstruction of the steps involved in a justification of the modal claims at issue? We will propose a justification of modal claims about phenomenal composition (in the second sense of the term) in a moment. But there is a potential danger for our view based on the case of reddish greens. We will now consider this danger.

In certain cases, we claim, we can rationally form judgements about the necessary structure of color experiences in general on the basis of phenomenological descriptions concerning the phenomenal structure of our own actual color experiences. This is the view we defend and the case we cited is the one about violet. However, the case of reddish greens seems to show that we are easily led astray when we dare to proceed from phenomenological descriptions of phenomenal structure to modal claims about phenomenal structure. It is a true and quite obvious phenomenological fact about the phenomenal structure of our normal human color perception that we never see a color that looks to be phenomenally composed of red and green. If we try to imagine a color that is phenomenally composed of red and green we do not succeed. In normal human color perception red and green are opponents in Hering's sense: there is no hue ever seen by a real person in real life that is phenomenally composed of red and green for that person on that occasion. We are tempted to conclude, like in the case of violet, that this particular fact about the actual structure of our color experience is a necessary truth about red and green. We are tempted to say something like this: if some arbitrary subject under counterfactual circumstances sees a color that is for that

4 5 subject phenomenally composed of two hues H1 and H2, then these hues are not green and red. It is – so we are tempted to say – constitutive of red and green that they cannot be combined in experience to a new hue in the way red and blue or red and yellow or blue and green can.5

But there are empirical results that seem to show that this is false. These results cast doubt on the claim that we have modal knowledge about phenomenal structure in the case of violet and orange as well. Our modal judgement in the case of violet is based on knowledge about the phenomenal structure of our actual experiences. Something very similar seems to go on in the case of the apparently wrong modal judgement about red and green. Maybe then we should admit that we simply do not have anything more but knowledge about actual phenomenal structure and maybe we should withdraw opinion with respect to the corresponding modal claims in general. To see if this is the conclusion to draw we will first have a look on the relevant empirical results and their interpretation.

3. Empirical studies about reddish green

In a study by Crane and Piantanida6 subjects report seeing reddish-greens and yellowish- blues. A variant of this experiment has been successfully conducted by Billock et al. 7 To our knowledge no other similar experiments have been realized. The conditions in which these studies have been made are very particular. It is highly unlikely that the perception of the so- called ‘novel colors’8 can happen outside a vision science lab. There are pertinent features of the experiments which require some introduction to non-scientists. In this section we will first introduce some of the relevant technical terms, and then describe the experiment.

3.1. A few technical terms

Stabilized images and filling in

The eye is rarely stationary for more than a few microseconds. In order to see, the eye needs contrast, which is provided by constant, very fast movements of the eye called saccades. This mechanism probably enables us to avoid seeing the blood vessels placed between the retina and the visual scene.9 If the eye totally lacks contrast for a few seconds then the image will fade out. This is what happens in the so-called ganzfeld phenomenon, in which the contrast provided by the visible parts of the subject’s face is hidden from view. C L Hardin describes an example of this phenomenon:

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"A subject’s eyes are covered with the two halves of a ping-pong ball and the room is filled with a diffuse, shadowless colored light, pink for example. Within a couple of minutes’ time, the color fades completely from view; subjects frequently claim that the light has been turned off." (Hardin, 1993) p.22

A stabilized image is an image projected on part of the visual field which follows the movements of the eye, so that the fading out of the image typical of the ganzfeld phenomenon is restricted only to the stabilized portion of the visual field. This can be done with contact lenses with a non-transparent part. In the experiments the stabilization is done with an ‘eyetracker’, a device designed to stabilize images without using contact lenses with non-transparent parts. Eyetrackers use the reflection of infrared light of the eye to track its movements and project images onto it.10 If an image is stabilized on a part of the retina for a certain time, thus producing a sort of 'informational hole' underlying the fading out phenomenon just described, then the brain tends to complete the image by filling in something using the information of the surround. The filling-in process is commonly taken to explain the fact that there is no perceived hole in our visual field corresponding to the blind spot on the retina. The blind spot is the place in the retina where the optic nerve exits the eye. In this part of the retina there are no photoreceptors, and therefore no visual information is received, and yet we do not notice this informational hole except in very exceptional circumstances. 11 So-called scotomas (lesions of the retina) can go unnoticed by the subject even when much larger than the blind spot. This again is considered to be explainable by a filling in process.

Equiluminance

There are three types of photoreceptors that participate in color vision, individuated by their sensitivity to different wavelengths. The S-cones are maximally sensible to short wavelengths, the M to middle wavelengths and the L to long wavelengths. According to a widely accepted model the ‘luminance pathway' processes the sum of the activity of the L- and M- cones.12 It is assumed that two perceived surfaces are equiluminants when the degree of activity in the luminance pathways produced by the surfaces is the same. In psychophysics equiluminance is usually defined operationally: when under certain circumstances perception of, e.g., the border between two different colored surface is minimised the two surfaces are called equiluminants.13 Because of the inter-personal differences of sensitivity to wavelengths even between normal subjects, two surfaces are equiluminants relative to a subject, and the equiluminance of two surfaces must be determined for subjects individually. Equiluminance between two chromatically different parts of the visual field tend to produce perceptions of ‘problematic borders’, fading borders and a

6 7 series of chromatic illusions. By mixing stabilized images to equiluminance, the likelihood that the borders of the contours of the image fade are increased.

3.2. The experiment

There were 7 subjects in the experiment of Billock et al, the exact number of observers participating in the experiment of Crane and Piantanida is not available.14 The subjects were presented with a red and a green stripe on a black field, such that the red and green stripes have a common border. In both experiments the red-green field was stabilized with an eyetracker. This was done in order to provoke a filling-in process in which the information from the non-stabilized parts of the image, i.e. the edges between the black field and the red and green stripe, should be used. The idea is that the subjects perceive the outer edges of the red and green stripes but not the common red-green edge. In the experiments conducted by Billock et al. the two stripes were mixed to equiluminance for each subject. Here is a list of the kinds of reports that were obtained in the two experiments:

(1) Homogenous mixture: Subjects report seeing a homogeneous color phenomenally composed of red and green whose components are as clear and as compelling as the red and blue components of a purple.15 (4 subjects out of 7)

(2) Gradient color: Subjects report seeing a gradient color that runs from red on one side to green on the other with a large region in between that appears both red and green. (5 subjects out of 7)

(3) Special arrangements of the visual field: Subjects report seeing either (a) two color stripes that seem to switch sides rapidly before fading out or regaining their places. (4 subjects out of 7) or they report seeing (b) the red and green stripes arranged horizontally rather than vertically (1 subject out of 7)

(4) Blackouts: Subjects report that the visual field turns into black (like in the ganzfeld phenomenon). (6 subjects out of 7)

(5) Transparency phenomenon: Subjects report that it looks as though the opponent colors originate in two depth planes and as though they are seen one trough the other. (4 subjects out of 7)

(6) Dots: Subjects report that the visual field appears to be composed entirely of a regular array of just resolvable red and green dots. (Number of subjects not available)

(7) Islands: Subjects report that the visual field appears as a number of islands of one color on a background of the other color. (Number of subjects not available)

In almost each trial the subject reported more than one of the kinds of experiences described in (1) to (7). In most trials where subjects reported having had experiences of the kind (1) and (2), they also reported that these experiences were preceded by experiences of

7 8 the kind described in (4) and (5). The hypothesis driving the experiments of Billock et al. is that experiences such as those reported in (1) and (2) are more likely to occur if the two stripes are equiluminant. This hypothesis was motivated by the findings of (Nerger, Piantanida, & Larimer, 1993) who had shown that differences in luminance may lessen the effects of filling in. Several trials increased the probability of reports like (1). Two subjects reported independently after the experiment that “reddish-green and yellowish-blue colors could now be imagined.” (Billock et al., 2001) p.2399

All but one of the seven subjects of Billock et al.’s study were expert psychophysicists, including some researchers in vision. Crane and Piantanida participated as subjects in their own experiment and so did two of the authors in Billock et al in their new version. Even though the experimenters made efforts to ensure that the subjects had a large color vocabulary, several subjects seem to have had difficulties in describing the phenomenon they had experienced as is documented by the following two citations:

"Although most [of the subjects] reported that regardless of where they attended in the field the color was simultaneously both red and green, some observers indicated that although they were aware that what they were viewing was a color (that is, the field was not achromatic), they were unable to name or describe the color. One of these observers was an artist with a large color vocabulary." (Crane & Piantanida, 1983) p.1079

"Our subjects (like those in (Crane & Piantanida, 1983)) were tongue-tied in their descriptions of these colors, using terms like “green with a red sheen” or “red with green highlights.” (Billock et al., 2001)p.2398 [Here the authors are referring to reports (2), (3), (5), (6), (7), above].

4. Current Interpretations of the Empirical Result

4.1. Taking the subjects reports at face value

To take the subject reports of Crane and Piantanida and Billock et al.’s studies at face value is to take these reports as literally and accurately describing the experiences the subjects were reporting. Subjects reporting percepts like (1) are thus taken to have had experiences as of binary colors with a reddish and a greenish component in the case of reddish-greens and with a bluish and a yellowish component in the case of yellowish-blues.

Here are a few at least prima facie reasons that can be advanced in favour of this interpretation:

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(i) The experiment of Crane and Piantanida has been repeated successfully by Billock et al. and it has been modified in a way that increases the likelihood of reports like (1) and (2). (ii) The only existing empirical theory which predicts the impossibility of novel colors, standard opponence process theory, is controversial in its original unmodified form and open to different interpretations that need not all imply the impossibility of experiences of reddish greens.16 According to the models proposed by the authors of the two experiments perception of novel colors is expected under some special circumstances. (iii) Some of the subjects, including Piantanida and Billock, are vision scientists and are expected to be experts in the use of color vocabulary. This is good reason to take their reports at face value. (iv) The list of different reports given by the subjects supports the idea that the subjects were trying to be precise in their description and to avoid ambiguities.

4.2. Scepticism about novel colors

This section discusses the possibility of not taking the subject reports at face value and some reasons for doing so. Crane and Piantanida’s findings were quite controversial at the time of their publication.17 This was in part due to the fact that only some of Crane and Piantanida’s subjects reported seeing novel colors. In the absence of other evidence, or a compelling explanation of why only some of the subjects in the experiment reported having seen a binary hue composed of red and green (or blue and yellow), it seems reasonable to be careful about these reports. Another source of doubt is the reluctance of the subjects to describe the colors as reddish-green. Part of this reluctance may be explained by the novelty of the experience and the fact that the subjects may have been prejudiced against the possibility of novel colors. But the reluctance may also be explained by the quality of the experiences itself, which some of the subjects described later to Larry Hardin as being dark and muddy, adding that these two qualities make colors hard to judge.18 Some of the sceptical worries are answered in Billock et al.’s study. First, there is a tentative explanation of why only some of the subjects of Crane and Piantanida’s study reported seeing reddish-greens: in order to see novel colors the stripes have to be equiluminants and some training may be needed.19 Four out of seven of Billock et al.’s subjects reported (1) and, according to Vince Billock20 “the colors we are talking about are not muddy and the subjects have rich perceptual vocabularies. The only remaining reasons

9 10 for having difficulties describing the effects are that the phenomena are novel and dynamic (multistable).”

5. A further doubt about reddish greens (a speculation)

According to the realist view about phenomenal composition that we defend a person can seriously judge that he or she has an experience of a hue that is phenomenally composed for her at that moment in a particular way and yet thereby be wrong about the phenomenal character of his or her own experience. For someone who accepts this view that explicitly endorses fallibilism about phenomenal judgements it is quite natural to consider the possibility that those subjects who described the color perceived in the laboratory as being composed of red and green for them on this occasion were misdescribing their own experience. In this section we briefly describe a way in which they might have gone wrong. The possibility we consider is a highly speculative. But the speculation might illustrate the following more general point: even in the apparently simple case of phenomenal composition of colors it can be very difficult to find an adequate and unbiased phenomenological description. The error theory we will describe is not meant to be a serious proposal of an alternative interpretation of the empirical results. To be a serious proposal it would have to be tested by further especially designed experiments.

According to the speculation we will now present subjects that described the hue they perceived as being composed (for them on that occasion) of red and green confused what we will call apparent simultaneous presence of red and green on a surface with phenomenal composition of red and green. We have to explain the phenomenal difference that we have in mind.

It will help to first remember that some subjects of the experiment described their experiences as being of two layers, one green, the other red, one behind the other, “as if looking through a green glass on a red surface”. According to this description the two surfaces appeared to be located at different distances and the hues of the two layers did not mix into one as they do in phenomenal composition. Now what would happen if the apparent difference in location where to disappear, all other qualitative aspects remaining the same? Is there some possible experience that is adequately described in this way: it is like an experience of looking through a green glass on a red surface but the two surfaces do not appear to be at different places, so that we could say equally well that the experience is like looking through a red glass on a green surface (although both descriptions are in a way inappropriate since they suggest an apparent difference in location). It is in fact possible to

10 11 produce this kind of experience by a simple experiment with goggles with one red and one green glass. When looking at a white surface with these goggles an experience of this kind may occur: (a) it is like seeing a greenish surface and a reddish surface, (b) the colors of the two surfaces do not mix into one as they do in phenomenal composition and (c) the two surfaces are one in the sense that they appear to be located at the same place.21 If we assume that there are possible color experiences that are appropriately described by (a) - (c) and that do occur when confronted with red through one eye and green through the other at the same time, then it is quite natural to speculate that those subjects who described their experience as being an experience of phenomenal composition of red and green really had an experience of this quite special kind. The speculation provides a plausible error theory explaining what then would be a phenomenological misdescription: since there is no information about difference in location phenomenally present it may well appear to be appropriate to say that the apparent surface appears to be both reddish and greenish. However, despite the fact that the two hue components are seen ‘at the same place’ they do not mix into a new hue quality (if description (b) is correct). If this speculation is correct then the experiences that have been misdescribed as experiences of reddish green require for the formation of new phenomenal concepts for their appropriate phenomenological description. They require the distinction between phenomenal composition of two hues and apparent simultaneous presence of two hues on one and the same surface. According to this distinction it is possible to see a surface as being both reddish and greenish without having an experience of a color that is (for the subject on that occasion) phenomenally composed of red and green.

6. The philosophical challenge

But let us assume now that the strongest interpretation of the experimental results is correct and that some subjects really saw a new hue that is never experienced under normal circumstances and that was phenomenally composed of red and green for them then. It follows that red and green are not incompatible in the sense of the following definition:

Definition: The hues H1 and H2 are incompatible iff (IC) It is impossible that a subject has an experience of a hue that is phenomenally composed of H1 and H2 for him or her then.

On the basis of one's own experiences of red and green it is however tempting to believe that red and green are incompatible in this sense. Phenomenological reflection on the particular character of red and green seems to reveal that these two hues cannot possibly mix into a new hue in any possible experience. Phenomenological reflection seems to support the

11 12 particular modal claim about red and green at issue. However, according to the present interpretation of the empirical results, we have to accept that the modal claim has been shown to be false by empirical investigation. We thus seem to be forced to conclude that phenomenological reflection is unreliable as a source of modal knowledge about phenomenal structure. Given this consequence we have reason to doubt other modal claims that are based on phenomenological reflection as well. We have reason to doubt, for example, that violet is composed of red and blue (if the claim is interpreted in the way we proposed).

In what follows we will argue that this doubt would be an overreaction. The possibility of experiences of reddish greens or bluish yellows should not be taken to justify general scepticism about phenomenologically based modal knowledge with respect to the structure of color experience. To justify this claim we have to show that there is a relevant disanalogy between the violet case and the reddish green case. We have to show that the best justification available for the modal claim that no one can have an experience of violet without thereby having an experience of a color that is then for that subject phenomenally composed of red and green is different in structure and stronger than the best available argument for the modal claim that no one can ever have an experience of a color that is for that subject on that occasion phenomenally composed of red and green.

7. How to justify the modal claim about violet experiences

To see that the best justification for the claim we defend (modal claim about violet) is quite different in structure from the best justification of the modal claim that is challenged by the empirical results at issue (modal claim about reddish greens) we have to propose first a reconstruction of the first justification. We have to give an answer to the following questions: Why are we justified in believing that no subject can have an experience of violet without thereby having an experience of a hue that is (for the subject on that occasion) phenomenally composed of red and blue? What are the different steps involved in the justification? What are the premises we need to accept if we take the claim to be true? What are our reasons to accept these premises?

7.1. Phenomenal criteria of concept application

In preparation to our answer to these questions we have to say a few words about the application of phenomenal concepts to one's own experiences. When learning the color language we learn not only to apply color concepts to colored objects but also to apply color

12 13 sensation words to types of sensations or experiences. These two conceptual capacities are of course closely related. For the present purposes we will focus on the second capacity. A subject who has acquired the concept of a violet experience has a number of capacities. The subject will consider an experience as falling under the concept just in case it is a violet experience (the concept of course has vague boundaries) and the subject will be able to imagine violet experiences and the subject will be able to consider the possibility that other subjects have this type of experience too. When somebody has learned to apply the concept of violet experiences correctly to his or her own experiences then he or she has learned to distinguish violet experiences from other types of experiences by their phenomenal character. He or she has learned to distinguish experiences according to a certain phenomenal criterion. In a sense the person then has learned to apply a certain criterion.

But to talk of a 'phenomenal criterion' may be risky in this context. If we say that a person applies a particular phenomenal criterion in her use of the concept of violet sensations we wish to say - simply - that she recognizes a certain phenomenal commonality (something all and only violet experiences have in common) and that she applies the concept as a result of her recognition of this commonality. We presuppose here that violet experiences do have something in common and that what they have in common concerns the phenomenal character. In some cases the subject who applies a certain phenomenal criterion in her use of a phenomenal concept does not have any other concept that she could use to describe the criterion that she thereby applies. If for example, a person should say what criterion she applies when applying the concept "red experience" to her own experiences, then the person would not be able to say anything informative about the criterion without again using the concept of a red experience. The best she could probably say is that she calls an experience of her own a red experience just in case it is an experience of a color with a strongly prevailing red component. So at least the concept of a red component has to be used in the description of the phenomenal criterion applied by a person who masters the concept of a red experience. The same holds for the third person perspective. If A describes the phenomenal criterion applied by B when using her concept of a red sensation, then A will have to talk of a prevailing red component in the color experienced by B again. However, no danger of any problematic circularity arises. There is no need to describe the phenomenal criterion applied in applications of the concept of a red experience in any independent way.

In the violet case, however, we do have other phenomenal concepts that we can use to describe the phenomenal criterion that we implicitly apply when using the phenomenal concept of violet experiences. We can describe the criterion simply by saying that we call an experience a violet experience just in case it is an experience of a color with the hue violet. But we can also say the same in another way: we can say that we call an experience a violet

13 14 experience just in case it is an experience of a color that has for the subject on that occasion a red component and a blue component where none of the two prevails too much. (What would be "too much" is of course again vague.) So in the case of violet we can describe the phenomenal criterion we apply when using the concept of the type of experience at issue in terms of phenomenal composition.

It should be noted that, according to the realist and fallibilist view we defend, we may be wrong about the phenomenal criterion we actually apply. Strictly speaking we express only a hypothesis about the phenomenal criterion we actually apply when we say that we call an experience an experience of violet just in case the condition just mentioned about phenomenal composition is satisfied. The hypothesis might be wrong. A person may be convinced that the phenomenal criterion for the application of the concept of a green experience can be described as follows: the experience is an experience of a color that is for the subject at issue phenomenally composed of blue and yellow. Yet, green is a unique hue.22 So this is a false description of the phenomenal criterion we actually apply. So although we are in a sense directly aware of the phenomenal criterion that we apply we may nonetheless be wrong about the criterion (we may misconceptualize the criterion). In the present case the misconception of the phenomenal criterion is due to a false phenomenological description of one's own color experiences. A person who believes that the phenomenal criterion for green experiences is the one just mentioned will believe that she appropriately describes her own green experiences as experiences of a binary color, a color that appears (for her on these occasions) composed of blue and yellow.

How then can we rationally form a hypothesis about the phenomenal criterion that we actually apply in the case of a given phenomenal concept of types of color experiences? Our answer is that it is possible to rationally form a hypothesis of this kind on the basis of phenomenological reflex ion on one's own color experiences. By carefully attending to the subjective character of those experiences I learned to call violet, I am able to find out what they have in common and to express what they have in common in terms of phenomenal composition. Why is it allowed to base this general hypothesis on observations of my own single case? The answer is, in our view: we are entitled to assume that we did learn to use the concept of violet experiences appropriately in this sense: we learned to apply the same phenomenal criterion that is applied by other people as well when they use their phenomenal concept they express using the term "violet experience". Of course, here again, there is room for a mistake. If I am pseudonormal then what I call violet experiences is what other people call experiences of bluish green (or greenish blue). I then have not learned to use the term "violet experiences" correctly and I then associate the wrong phenomenal concept with the phenomenal term.23 Under normal circumstances however - or so we assume - we are

14 15 justified in the belief that we express the same phenomenal concept when using the term "violet experience" as other competent users of our language with normal color vision. This is to say that we apply the same phenomenal criterion when using the phenomenal concept we associate with the term as they do. But if we are entitled to believe that the phenomenal criterion is the same, then we are justified to believe that a description of the phenomenal criterion we actually apply in our own case is appropriate if and only if the description is also appropriate as a description of the criterion applied by others when using the concept they associate with the relevant term. Therefore we can arrive at justified claims about the phenomenal criterion applied in uses of the concept of violet experiences in general by carefully attending to our own single case.

7.2. Actual and counterfactual cases of color experiences

Until now we have been talking about the application of phenomenal concepts to actual color experiences only. We have not yet been talking about possible color experiences of other sentient beings in counterfactual circumstances. We have to turn to counterfactual cases however since the claim we wish to justify is a modal claim about all possible violet experiences. At this point it is important to see that phenomenal concepts are special in the following respect: the criterion for their application to counterfactual cases is the same as the criterion for their application to actual cases. If it is correct, for example, that a person who masters the concept of violet experiences applies the concept to all and only those experiences where the subject has an experience of a color that is phenomenally composed of red and blue (with none of the two components prevailing too much), then an experience in counterfactual circumstances counts as an experience of this particular type just in case it fulfils this phenomenal criterion. For a subject in counterfactual circumstances to fall under the concept of having a violet experience it is necessary and sufficient that it has an experience that fulfils precisely that phenomenal criterion actually applied by people who have the concept in the real world with respect to their own color experiences. The reason is that to fall under a particular phenomenal type is to fulfil a particular phenomenal criterion. There is nothing else that is essential for having a given phenomenal property. There is a particular aspect of the phenomenal character that is necessary and sufficient for actual experiences to fall under the type "violet experiences". If some being in counterfactual circumstances or in some other part of this universe has an experience with this particular phenomenal aspect then the experience is ipso facto an experience of violet. In other words: It is only this particular aspect of the experience that is essential for being a violet experience.24

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Note that what we just claimed for the case of phenomenal concepts is not generally true for all concepts. In many cases, as has been pointed out in the literature about natural kind terms, the criterion of application in the real world (the way we pick out what falls under the concept in the real world) may not coincide with the criterion of application in our thoughts about counterfactual circumstances. Many philosophers admit that we pick out the liquid water in the actual world by properties of water that are not essential to water. In other words: the properties that a liquid must have to fall under our concept of water in the real world are properties that a liquid in counterfactual circumstances might lack although it is appropriately considered as being water. The parallel claim is false for phenomenal concepts: if a certain phenomenal criterion has to be fulfilled by an actual experience to fall under our concept of a violet experience, then no experience in counterfactual circumstances that fails to satisfy this criterion falls under the concept of a violet experience.25 It follows that any claim about the phenomenal criterion we apply to actual experiences when using a particular phenomenal concept is also true as a claim about the criterion for counterfactual cases. Therefore the following conditional holds: If CR is the phenomenal criterion for application of the phenomenal concept C to actual experiences then nothing in counterfactual circumstances falls under C unless it fulfils CR. For the case of violet the conditional reads as follows: If it is true that the phenomenal criterion for correct applications of the concept of violet experiences is that the experience at issue is of a color that is for the person at issue on the given occasion phenomenally composed of red and blue (where none of the two components prevails too much), then no being in counterfactual circumstances has an experience of violet unless it has an experience of a color that is for it on that occasion phenomenally composed of red and blue. This is of course the result we wanted. We have justified the step from a claim about a phenomenal criterion as it is applied in actual cases to the modal claim at issue about all possible experiences.

7.3. Summary of the proposed reconstruction

In our thoughts we use phenomenal concepts to attribute phenomenal properties to other people or to other sentient beings. We use the phenomenal concept of having a violet experience to attribute the phenomenal property of having a violet experience. In many cases we can use language to express what we think. We use the predicate "has a violet experience" in order to attribute the property of having a violet experience under its phenomenal conceptualization. This does not mean that a person who does not have the phenomenal concept of a violet experience cannot use the corresponding phenomenal term to attribute a certain phenomenal property. A blind person may lack the phenomenal concept of a violet experience and yet use the term "violet experience" with the intention to ascribe a

16 17 particular phenomenal property to another person. Yet, there is a close relationship between the phenomenal concept of having violet experiences and the corresponding phenomenal predicate "has a violet experience". We can describe the relation as follows: If a person thinks that some X has a violet experience and thereby uses the phenomenal concept of violet experiences, then that person would use the sentence "X has a violet experience" correctly if she used it to express that thought. To have a name for this relation between phenomenal concepts and phenomenal predicates we will say that the concept is associated with the predicate and we will thereby presuppose something like the following definition:

Definition: A phenomenal concept C is the concept associated with the phenomenal predicate PP just in case the following condition is fulfilled: If a person A has the thought that X has the property P and conceptualizes the property P via the phenomenal concept C and if A expresses this thought saying "X has P", then A uses PP correctly.

According to this definition what phenomenal concept is associated to a particular phenomenal term depends on what concept is involved in a thought that can be correctly expressed using the term. With this terminological preliminary in place we can reformulate and summarize the argument developed in the two preceding sections.

The justification of the modal claim that no sentient being can have an experience of violet without thereby having an experience of a color that is for it then phenomenally composed of red and blue can now be formulated in the form of an argument. (Since phenomenological descriptions of one's own experiences play a role we will formulate the argument in the first person.)

Premise 1 (hypothesis about my own phenomenal criterion): The phenomenal criterion for my application of my phenomenal concept to my own experiences can be formulated as follows: I accept something as being a violet experience just in case it is an experience of a color that is for me then phenomenally composed of red and violet (where none of the two prevails too much).

Premise 1 expresses a phenomenological description. A person who accepts that premise accepts a certain phenomenological description of all those experiences he or she subsumes under the phenomenal concept of violet experiences.

Premise 2 (assumption of correct language use): My phenomenal concept of having violet experiences is the concept associated with the predicate "has violet experiences".

This premise is meant to imply:

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Consequence 1: My phenomenal criterion for the application of my phenomenal concept of violet experiences coincides with the phenomenal criterion for the concept associated with the term "has a violet experience".

Premise 1 and consequence 1 imply:

Consequence 2 (correct application of the corresponding term): An actual experience falls under the phenomenal concept associated with the predicate "has a violet experience" iff the subject at issue has an experience of a color that is phenomenally composed of red and blue for it then (where none of the two components prevails too much).

Premise 3 (counterfactual extension of phenomenal concepts): In the case of phenomenal concepts the phenomenal criterion of application to actual experiences coincides with the phenomenal criterion of application to counterfactual cases.

Consequence 2 and premise 3 imply:

Consequence 3 (the modal claim formulated in terms of concept application): No experience of a being in counterfactual circumstances falls under the phenomenal concept of having violet experiences unless it has an experience of a color that is for it then phenomenally composed of red and blue.

We arrived, almost, at the modal claim we wanted to justify. We still have to get rid of talk of concept application since we wish to talk of the type of experience itself. To do this we simply have to assume that what falls under the phenomenal concept of violet experiences in counterfactual circumstances is a violet experience.

Premise 4 (link between concept application and the exemplification of the corresponding property in counterfactual circumstances): A being (in counterfactual circumstances) falls under the concept of having a violet experience iff it has a violet experience.

We now can conclude (using premise 4 and consequence 3):

Consequence 5 (the modal claim about violet experiences): Necessarily, if a being has an experience of violet it has an experience of a color that is for it then phenomenally composed of red and blue.26

We normally are in an epistemic position where is justified to assume the first two premises. Premise 3 is a general conceptual truth concerning phenomenal concepts and premise 4 is trivial. So the modal claim at issue has been shown to be justified (for every person in a normal epistemic situation). Of course this does not exclude the possibility of error. We cannot be certain that our phenomenological description of our own experiences is correct (we might be wrong about the phenomenal criterion we apply, premise 1 may be false). Premise 2 might be false too. No singular person can exclude that he or she is a deviant color perceiver and that her or she systematically misunderstands phenomenal terms for that

18 19 reason. However, there normally is no good reason for this doubt. In the normal case it is possible to form rationally justified belief about modal claims concerning phenomenal composition in color experiences of a given type. A person can (a) form a hypothesis about the relevant phenomenal criterion for her own application of the phenomenal concept of the type of color experience at issue. He or she can then (b) assume that this criterion coincides with the phenomenal criterion of application for the phenomenal concept associated with the corresponding predicate and (c) conclude that this criterion coincides with the criterion of application for that concept in counterfactual cases. Step (a) involves a phenomenological analysis of one's own experiences of the relevant type, step (b) presupposes conformity of one's own understanding of a phenomenal term with the understanding of competent speakers of the language and (c) uses a conceptual fact about phenomenal concepts.27

8. How to justify the modal claim about reddish greens

Does the case of reddish greens undermine the modal claim about violet? The possibility of experiencing reddish greens would undermine the modal claim we just defended if the best justification for the intuitively plausible modal claim that it is impossible to experience a reddish green were relevantly similar to the justification we gave for the modal claim about violet. If that justification looks very much like the one we gave for the modal claim about violet then (given the empirical results reported above) we have reason to doubt the argument that led to the modal claim about violet as well. To see if this is the case we have to find a justification for the modal thesis about reddish green that is plausibly the best candidate for a rational reconstruction of the reasons we have for believing in that claim.

It may be appear as though we are able to judge that red and green cannot possibly combine to a new hue just by attending to the particular quality of red and green. Given their intrinsic quality (somebody might say) it is evident that they cannot be present in the sense of phenomenal composition in the experience of one hue. If this is the way to justify our belief in the modal claim at issue about reddish greens, then the way we arrive at the modal thesis is quite direct. According to the present view about how the claim could be justified a person who has the phenomenal concept of having red experiences and the phenomenal concept of having green experiences and who understand the notion of phenomenal composition can see (intuit) that there is no possible hue such that a person could have an experience of that hue and thereby have an experience of a color that is phenomenally composed of red and green. According to this proposal, the modal claim about reddish greens is justified by what has been called rational intuition: understanding the notions involved suffices for the capacity

19 20 to have the modal insight.28 Let us call this view about the best justification of the modal claim about reddish green the rational intuition view (about the claim).

According to the rational intuition view the best justification of the modal claim about reddish green is quite different in structure from the best justification of the modal claim about violet. According to that view understanding the notions involved makes the modal claim compelling or intuitively attractive. A person who understands the concepts involved cannot but intuit that the modal claim is true. According to our reconstruction of the way the modal claim about violet can be justified, the situation is more complicated and involves a number of different steps. There are no analogous steps present in the justification of the modal claim about reddish greens according to the rational intuition view. No premise about the phenomenal criterion of application of phenomenal concepts plays any significant role. There is thus no premise needed that would be analogous to the first premise of the above argument. The first premise in that argument is the result of phenomenal reflection on particular color experiences belonging to the kind in question. It is clear that the case for the modal claim about violet cannot start with a phenomenological description of experiences of the type we call green experiences or the type we call red experiences. That green cannot combine in the way at issue with red is nothing that can be part of a phenomenological description of a green experience. By carefully attending to violet experiences we are able to discern a reddish component. If we discern the reddish component, then we have been able to isolate (in reflection) a particular aspect of the phenomenal character of experiences of the relevant kind. By contrast, the impossibility of green to combine with red is not an aspect of the subjective quality of the experience of green in itself. Therefore, phenomenological reflection in the strict sense of finding out what phenomenal aspect experiences of a particular kind have in common is not the starting point for any possible justification of the modal claim about reddish greens.

Let us turn to the second premise. A person who asserts the modal claim at issue wishes to say something about red and green experiences. Red and green experiences are those experiences that are attributed in thought using the phenomenal concepts associated with the term "red experiences" and "green experiences". If a person has reason to doubt that he or she understands the terms "red experiences" and "green experiences" correctly then she cannot use her conceptually based intuition to judge the truth of the modal claim (that is of course formulated using the linguistic terms "red experience" and "green experience"). Therefore, on the rational intuition view, a justification of the modal claim has to include an assumption analogous to premise 2 in the above argument. So here we find a parallel. The parallel, however, is a trivial one.

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In the argument for the modal claim about violet the conceptual claim about phenomenal concepts, premise 3, plays a prominent role. There is no such step involved in the justification of the modal claim about reddish greens if the rational intuition view is the one to adopt. We do not, in that case, first form a hypothesis about phenomenal criteria of concept application to actual cases and then generalize to counterfactual cases (using premise 3), since there is no hypothesis about phenomenal application to actual cases involved in the first place. Rather, according to the rational intuition view, we go on directly to a claim about counterfactual cases by modal intuition based on concept understanding.

We conclude that - according to the rational intuition view and presupposing the above justification concerning the violet case as the best justification available - there is no structural similarity between the two justifications. The two justifications are fundamentally different. Therefore, the empirical results that undermine the impossibility claim about reddish greens cannot undermine the necessity claim about violet.

To arrive at this result we assumed, however, that the rational intuition view is the one to accept. But there might be a better justification of the modal claim about reddish green that is also more similar in structure to the argument given for the modal claim about violet. If this were true, then the empirical results about reddish greens would undermine the necessity claim about violet after all. How could a justification of this kind look like?

A justification of this kind might start with the observation that (a) by carefully attending to our own color experiences we can find out that we never have an experience of a hue that is phenomenally composed for us then of green and red and (b) that whenever we try to imagine an experience of a hue that is phenomenally composed of red and green we do not succeed in fulfilling that imaginative task. The first observation, however, is a bad starting point. There simply is no rational way from the observation that we do not ever have an experience of a color that is for us then phenomenally composed in a particular way to the belief that necessarily no subject can ever have an experience of a color that is for him or her then phenomenally composed in the way at issue. More to the point, no premise analogous to premise 3 can help to get from the observation about actual cases to the claim about counterfactual cases. Premise 3 can help to make that step only if we have a phenomenological description of concrete actual cases as a starting point. But the observation mentioned in (a) does not provide a phenomenological description of actual cases. So maybe our incapacity mentioned in (b) is a better starting point. But how can we rationally conclude from the mere fact that we are unable to imagine a reddish green to the modal claim that it is impossible to experience a reddish green? Maybe somebody can come up with a convincing justification of this step. If so, however, it certainly will not have the

21 22 structure of the argument for the modal claim about violet. Again, no premise similar to premise 3 can possibly be used in an argument defending the step from our incapacity of imagination to the relevant impossibility claim. The reason is the same as before. Premise 3 can help if we can start with a phenomenological description of actual experiences that belong to a particular kind. But the observed incapacity mentioned in (b) does not provide a phenomenological description of actual cases.

Our main point here is not to attack the modal claim about reddish green or to show that it is irrational to hold that claim. We rather would like to be neutral on this point for the moment. What the discussion should show is this: there is a fundamental disanalogy between the best way to justify the modal claim about violet and the best way to justify the modal claim about reddish green. Therefore, it would be a mistake to think that the empirical results about reddish greens can challenge the modal claim about violet (and other similar claims about phenomenal composition).

9. Objections

First objection Simplifying a bit we may say that the above argument implies the following claim: A person who forms (on the basis of carefully attending to her own color experiences) the belief that all her violet experiences are experiences of a color that is for him or her then phenomenally composed of red and blue has reason to believe that nothing can count as a violet experience unless it has that particular property. But now imagine the case of a person, Rosa, who for some contingent reason never sees any yellowish red but only pure red or bluish red. When Rosa carefully attends to her own color experiences she will observe that all her red experiences have the following property: they are either an experience of a unique hue or they contain a bluish component. Can she rationally conclude that all experiences of red necessarily have that particular property? Intuitively it seems that she cannot and obviously the conclusion would be false. But the case seems to be perfectly parallel to the violet case. So something must have gone wrong.

Answering the first objection The objection rests on a misunderstanding of the first step in the argument. To form a hypothesis about a phenomenal criterion is not the same as the observation of something that all experiences of a particular type have in common. Suppose that a person, we will call her Violetta, never sees any violet except for a few rare occasions where she is presented with violet stars of different shades painted on a white surface. Suppose somebody tries to

22 23 teach the term "violet experience" to Violetta. There are two possibilities: either Violetta understands the explanation and forms the phenomenal concept associated with the term violet or she does not. If she does understand the explanation then she would apply that concept to every violet experience. In this case, if somebody were to present an object to Violetta that looks violet and circular, then Violetta would consider it to be one further case of a violet experience. If this first possibility is realized, Violetta would be aware of the fact that she herself actually never had (and might not ever have in the future) any violet experience that is not at the same time an experience of something with the shape of a star. However, Violetta would also be aware of the fact that the shape is irrelevant for the application of the phenomenal concept that she has been taught. The example illustrates that we have to distinguish - as said before - between the observation of a commonality between all experiences of a particular type that somebody has ever had or has in his or her entire life- time and the forming of a hypothesis about the adequate phenomenal criterion. If however, Violetta did not understand the explanation, then she may have formed a different concept. The phenomenal concept of the experience of something that looks both violet and star- formed. In this case she would not be willing to apply her newly acquired concept to the experience of a violet elephant.

Let us now turn to the case of Rosa. In Rosa's case we have to distinguish the same two possibilities: (a) Rosa has the concept of a violet experience and believes correctly that this concept is the one associated with the term "violet experience". (b) Rosa has the concept of an experience of red or bluish red and she believes incorrectly that this is the concept associated with the term "violet experience". If the possibility (a) is realized, then Rosa will actually classify violet experiences according to the phenomenal criterion mentioned before. She would subsume an experience of her own under her phenomenal concept of a violet experience if and only if it is an experience of a color that is phenomenally composed of red and blue (where none of the two components prevails too much). Rosa has the phenomenal concept of a violet experience only if she would apply that concept in this way. This possibility can be subdivided in two further possibilities: (aa) When reflecting upon her own violet experiences Rosa is able to find out that the criterion she applies when classifying them as violet experiences is the one mentioned before. She then will accept premise 1 despite her special and slightly impoverished experiential background. If this is the case under consideration, then Rosa poses no problem for the argument. Her special situation will entail no epistemic mistake and she will rationally arrive at the correct modal conclusion. (ab) When reflecting upon her own violet experiences Rosa will come up with a false hypothesis about the phenomenal criterion that she actually applies. Although she would classify slightly yellowish reds as experiences of red if she ever had this kind of color

23 24 experience she is not aware of this fact. In this case Rosa will introduce a false premise. The premise corresponding to premise 1 in the above argument in her corresponding argument would be false. This possibility however provides no argument against the proposed reconstruction. False premises may lead to false conclusions. Of course a person may construct an argument of the above structure and yet arrive at a false conclusion if she introduces at one step a false premise. Let me no return to the possibility mentioned above under (b). Rosa then has not acquired the concept of red experiences on the basis of her red experiences but another phenomenal concept that is narrower (it only includes experiences of particular shades of red). In this case Rosa may arrive at the wrong conclusion mentioned in the objection. But this will be due to another mistake: she will believe, incorrectly, that her phenomenal concept at issue is the concept associated with the term "red experiences". Again, Rosa's case poses no problem. She arrives at a false conclusion although she uses a good type of argument for the simple reason that she introduces at one point a false premise. (The premise corresponding to premise 2 in her argument would be false).

Second objection Despite the differences between the two justifications there is a common element that makes them sufficiently similar for the empirical results about reddish green to undermine the modal claim about violet as well. Let us assume that the rational intuition view is rejected and that we are left with the imagination argument as the best argument for the modal claim about reddish green. Then the common element is the role of imagination in both arguments. Obviously in the second argument (for the impossibility claim concerning reddish green) imagination plays a crucial role. From the fact that we cannot imagine an experience of a color that is phenomenally composed of red and green we conclude that no experience of this kind is possible. In the first argument, as it has been presented, no imaginative act seems to be important at any point. A closer look upon the argument, however, reveals that imagination does play an important role in the justification at issue. We see this if we have a closer look at the way we arrive at premise 1. How can I find out that my phenomenal criterion for applying my concept of a violet experience is appropriately described in the way that has been proposed? It is not sufficient to consider actual cases of experiences that I had in the past. I have to form an opinion about how I would react to imagined cases as well. (This is what we learned from the answer to the first objection. Rosa can form an appropriate hypothesis about the criterion she applies only by considering new cases that she never has been confronted with.) So we do have to rely on our capacity to imagine color experiences. To form a belief that we can state as premise 1, we have to be sure that (a) if we were confronted with a color that is not phenomenally composed in the relevant way, than we would not consider it as falling under the concept and that (b) if we were confronted with a

24 25 color that is composed in the right way, then we would subsume it under the concept. To test (a) and (b) by imagination we cannot do anything else but this: we try to imagine an experience of a color that is phenomenally composed in the right way and that we would not count as falling under the concept and we try to imagine an experience of a color that is composed in the right way but that we would not consider as falling under the concept. If we fail in both imaginative acts, then we accept premise 1. This is - it seems - the best and only way to justify premise 1. But if so, then imagination and in particular the incapacity to imagine an experience with certain properties, is crucial for the first argument just like it is crucial for the second and so the analogy is re-established.

Answer to the second objection We must admit that imagination plays a role in the justification of premise 1 in the way described in the objection. However, this role of imagination is quite different from the role of imagination in the justification of the modal claim about reddish green. In the case of justifying premise 1 of the argument for the modal claim about violet imagination is necessary to test one's own phenomenal criteria of concept application. If we do not succeed in imagining a hue experience that fulfils a certain criterion but that we would not count as falling under a particular concept and vice versa, then we have reason to believe that we found the criterion that we actually apply. The test is not infallible of course. We might fail to imagine a particular experience that would falsify our hypothesis about our own application criterion associated with a given phenomenal concept, but still the imagination test is a legitimate test of a given hypothesis about our own phenomenal application criterion. Contrary to this, imagination is not used to test phenomenal application criteria in the argument for the modal claim about reddish green. In this case we try to imagine an experience that fulfils a particular phenomenological description (the description of being the experience of a hue that is phenomenally composed of red and green). If we find ourselves incapable to fulfil this imaginative act, then we take this to be an argument for the impossibility of an experience that fulfils the description. So the role of imagination in the two cases is radically different in two ways: (a) In the first case we try to imagine a color that fulfills a certain phenomenological description concerning phenomenal composition and if we succeed we go on to test whether this would count as falling under the phenomenal concept at issue. We can test our hypothesis about the phenomenal criterion of application only if we succeed in imagining an experience that fulfils the relevant phenomenological description. Contrary to this, in the second case, we try to imagine an experience that fulfils a particular phenomenal description concerning its phenomenal composition and take our failure to do so as support for the modal claim.

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(b) In the first case, the imagined experience is used to test our phenomenal application criteria of a given phenomenal concept. This is clearly a legitimate test that is not in need of any special additional argument. In the second case, the incapacity to imagine an experience that fulfils a particular phenomenal description is taken to be an argument for the impossibility of an experience of this kind. This is clearly a step that does need an additional argument. While it is unproblematic to use imagined experiences to test one's own application criteria it is highly problematic to take one's incapacity to imagine something as an argument for its impossibility. No such step from our incapacity to imagine to a claim about metaphysical impossibility is in the first argument concerning violet. We conclude that imagination does play a role in both arguments but that the role is so different that the possibility of experiences of reddish greens would not undermine the modal claim about violet.

Third objection The argument for the modal claim about violet is correct but the consequence depends on a terminological (or conceptual) decision. We decide to call an experience a violet experience just in case it is an experience of a color that is phenomenally composed of red and blue (where none of the two components prevails too much). An experience has to fulfil this condition in order to fall under the concept by stipulation. We could however just as well stipulate that nothing falls under the concept of being an experience of green unless it is either an experience of pure green or an experience of green with a slightly yellowish or slightly bluish component. Given this stipulation, an experience with a reddish component does not count as an experience of green. In this way we exclude the possibility of experiencing reddish greens by stipulation. We could do the same for the concept of a red experience. An experience with a greenish component would not count as a red experience and so again - by stipulation - experiences of greenish red are excluded. We thus arrive at the modal claim that it is impossible to have experiences of reddish green or greenish red but we arrive at this conclusion by mere stipulation. It is clear that we cannot decide any interesting substantial issue about what is essential for certain kinds of experiences merely by stipulation. So the modal claim at issue really turns out to be quite uninteresting and it is not open to any empirical rejection. If someone claims - like the subjects in the experiences at issue - to have been able to see a reddish green or a greenish red, then we simply can reply that he or she is misusing language and we can reply in this way simply because we decided to use the concepts involved in that particular way. This reasoning about the reddish green case has consequences for the modal claim about violet as well. What seems to be a substantial claim about metaphysical necessity really is nothing but a trivial consequence of our decision to use the concept violet experience in a particular way.

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Answer to the third objection According to this objection both modal claims are the trivial result of our decision to use the concepts involved in a particular way. Let us call this view the stipulative view (about the modal claim under consideration). The stipulative view is however false for both cases. We will first discuss the stipulative view about the impossibility claim concerning reddish greens.

According to the third objection we could take the following decision: If a person had an experience of a color that is phenomenally composed for her then of red and green, then we will neither count this experience as being an experience of a reddish green nor count this experience as being an experience of greenish red. We simply decide that color experiences do not count as experiences of red if they contain a greenish component and do not count as green if they contain a reddish component. We thus arrive at the modal claim at issue in a trivial way. There are two points to be made about this proposal (A) The question was whether it is possible to have an experience that has both a red and a green component. We cannot exclude this possibility by deciding that we would not call an experience of this particular kind an experience of reddish green of greenish red. In formulating his stipulation the proponent of the present objection uses the term “red” and the term “green” to refer to a specific potential phenomenal component of a color as it is perceived by a subject. But once the reference to these components is assumed as fixed we can ask whether or not it is possible to have an experience of a color that contains for the person at the relevant moment both a reddish and a greenish component. According to the modal claim the answer to this question is “no”. The proposed stipulation however is silent about the answer to this question. The proposed stipulation excludes that we call an experience of a color an experience of reddish green or greenish red but it does not exclude that a perceived color might be phenomenally composed of red and green. So, contrary to the above objection the modal claim we are interested in no trivial consequence of the proposed stipulation. (b) A second point to stress is that the proposed stipulation is unacceptable given the use of the rest of our phenomenal terms. We call an experience an experience of a reddish blue just in case it is an experience of a color that is phenomenally composed of red and blue (where the blue component prevails). To decide that an experience that is in fact an experience of a color that is for the subject on that occasion phenomenally composed of red and green (with e.g. a prevailing green component) should not be called an experience of reddish green would be a highly misleading deviation. – From a more abstract point of view we may say that a person who finds the proposal plausible might not have seen that there are two different uses of the term “red” in descriptions of experiences. We use the term “red” in “has a red experience” to express the property of having an experience of a color that comprises many different shades (from almost orange to almost violet). To have a red experience is to have an experience of a color that has a sufficiently prevailing red component where a blue

27 28 or a yellow component can be present as well. The second use of “red” in the context of the description of experiences is its use to refer to a particular phenomenal component in a color as it is perceived by a subject. Once the meaning of our color component talk is fixed (once we know what specific potential components we refer to using the words “red” and “green”) the modal question at issue can be formulated and its answer cannot be found by any decision about how to use the broad concept of having an experience of red in the sense that comprises a great range of color shade experiences. – We conclude that the stipulative view about the impossibility claim concerning reddish greens is false.

The reconstruction we have proposed for the best justification of the necessity claim concerning violet might invite the stipulative view about this modal thesis. After all the argument uses assumptions about the use of the phenomenal concept of violet in two places: in premises 1, where a claim about the phenomenal criterion of application is at issue and in premise 3 where it is assumed that in the case of phenomenal concepts there is no difference between the criterion of application to actual or to counterfactual cases. However, the stipulative view misinterprets the status of premise 1. Premise 1 is not the result of an arbitrary decision. It is not open to us to decide about the phenomenal criterion associated with the use of the phenomenal concept of violet experiences. Rather, we already have the phenomenal concept of violet experiences and then try to find a description of the phenomenal criterion we actually apply by trying to become aware of and to conceptualize the phenomenal aspect shared by those and only those experiences that we are (or would be) ready to subsume under the concept. Also, the stipulative view misinterprets the way we arrive at the phenomenal criterion for the application of the concept to counterfactual cases. Here again we do not simply decide that we would not count an experience as being an experience of violet unless it is an experience of a color that is phenomenally composed for the subject at issue on the relevant occasion of red and blue. Rather, we are forced to accept this constraint on the application of the concept to counterfactual cases if our phenomenological hypothesis in premise 1 is correct. But premises 1 is a phenomenological hypothesis that might be wrong. So we are not free to decide about how we apply the concept to counterfactual cases. How we have to apply it to counterfactual cases depends on how we do apply it to actual cases and the latter is already fixed. This dependence is of course based on the conceptual insight expressed in premise 3. Here again, somebody might see room for an arbitrary decision. Couldn’t we decide to use our phenomenal concepts in a way that makes – contrary to premise 3 – other aspects of the experience essential for having an experience of the relevant kind? The answer is again no. Our phenomenal concept of having violet experiences is a phenomenal concept and this implies that nothing but the particular phenomenal ‘feel’ is relevant to for the categorization we make using the concept. It is not up to us to decide what we consider as essential. In introducing a

28 29 concept of “having a violet experience” where it is not the phenomenal criterion that is relevant for the application to counterfactual cases but something else (say the way the experience is caused or the way it is realized in the brain) would be to introduce a different concept and thereby to change the topic.

10. Concluding Remarks

We have argued that the modal claim about reddish greens has been challenged by the empirical results at issue but that the modal claim about violet has not been challenged. Could there be other empirical results that could give us reason to reject the claim that no subject can have an experience of violet without thereby having an experience of red and blue? Could it be that somebody constructs an experiment tomorrow where under unusual circumstances people seriously and correctly claim to have seen a violet that did look neither reddish nor bluish to them then but e.g. looked like a unique hue or looked like a color phenomenally composed of red and yellow? According to the view we have proposed an empirical rejection of the modal claim about violet is not a priori excluded. We might be wrong in our assumption about the phenomenal criterion we actually apply when using the phenomenal concept of violet experiences. Premise 1 is open to empirical counterarguments. However, if we all associate the same phenomenal concept with the term “has a violet experience” and if the phenomenal criterion for applications of that concept is the one we mentioned then the possibility of empirical rejection of the modal claim is excluded. An empirical argument against the modal claim would have to be an argument against one of the preconditions just mentioned.

These reflections may raise the issue about the epistemic status of modal claims about phenomenal structure of color experiences. In the case of reddish greens, can we arrive at the modal claim a priori? If the rational intuition view is correct, then the answer seems to be “yes”. According to that view to have the relevant concepts is sufficient to rationally intuit the modal claim. But note that the modal claim is nonetheless open to an empirical rejection. Also, acquiring the concepts involved requires phenomenological reflection on one’s own color experiences. Something similar applies to the modal claim about violet. Who has the relevant concepts is in principle able to see the truth of premises 1. Premise 1 therefore seems to be a claim that we can know in a sense a priori. However, the claim involves an empirical thesis about the actual criteria of application of one’s own phenomenal concept that is open to empirical falsification. On a closer look it might well turn out that phenomenologically justified modal claims about phenomenal composition (like the one

29 30 about violet) do not fit well into the traditional distinction between a priori and a posteriori knowledge. But we have to leave these related epistemological issues to another occasion.

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31 1 See Nida-Rümelin, 2005a and Schnetzer & Nida-Rümelin, 2005. 2 See (Hardin, 1984) p.492-493. 3 For more about this point see M. Nida-Rümelin, 2005a 4 The status of green in this respect has actually been an issue among philosophers and among scientists as well. See e.g. 5 Note that the fact that green and red when mixed on a piece of paper may appear grey or brownish is no objection. These cases are not cases of phenomenal compositions in the sense at issue. (Nida-Rümelin & Schnetzer, 2004). 6 (Crane & Piantanida, 1983) 7(Billock et al., 2001). The perception of so-called ‘novel colors’ has also been reported by synaesthetes. Color-blind subjects reported seeing synaesthetic ‘Martian colors’, normal subjects have reported the perception of new colors. Fiona Macpherson (personal communication) has drawn my attention to these reports. Cf. (Ramachandran & Hubbard, 2001); (Ramachandran, 2001) 8 Cf.(Thompson, 1992) and (Macpherson, 2003). 9 Cf. (Hardin, 1993)p.22. 10 For technical information cf. (Billock et al., 2001) p.2402. For the specific sort of eyetracker used in the experiments cf.(Crane & Steele, 1985). 11 For interesting ‘do it yourself’ experiments involving the blind spot cf. http://faculty.washington.edu/chudler/chvision.html 12 The complete story about the luminance pathway is of course more complicated. Cf. (Cavanagh, 1991) for a review on empirical work on the luminance pathway. Cf. (Billock & Tsou, 2004) for a general account of the role of luminance in visual perception. 13 Cf. (Billock & Tsou, 2004) p.86. 14 In Crane and Piantanida’s study the only information available is that more than 12 subjects were submitted to the stimulus. 15 For the subject reports see (Billock et al., 2001) pp. 2398-2399. We have put between parentheses the number of subjects per percept. This information was available only for the study of Billock et al. 16 Cf. (Dacey & Packer, 2003) and (Lennie, 2000) for reviews on this subject. 17 As reported in (Billock et al., 2001) p.2398 and (Hardin, 1993) p.xxix. 18 In the second edition of Color for Philosophers p.xxix. Cf. also (McLaughlin, 2003) pp.113-114. 19 Apparently Crane and Piantanida knew also that the experiment worked better at equiluminance: “By the way, when I called Hewitt Crane to tell him about our results, he told me that he and Piantanida knew that the forbidden color effect worked better with equiluminant colors, but they never wrote a follow-up paper on the effect, so they had no venue to publish the fact in.” Vince Billock (Private communication). 20 Private Communication. 21 This is a description of the experience made by one of the authors when using goggles of this simple kind (MNR). 22 This claim is controversial. See (Sternheim & Boynton, 1966) ; (Boynton & Olson, 1990) for linguistic studies showing that green is a primary hue and (Gordon, Abramov, & Chan, 1994) for a defence of the methodology. For Brentano about this issue see (Nida-Rümelin & Schnetzer, 2004) (Schnetzer, 2005). For a non-realist position about phenomenal composition in general and about the issue of the uniqueness of green see (Mizrahi, 2005). 23 I then misapply the term but do I also misapply the corresponding concept? The answer is a bit tricky: I associate the wrong concept with the term but I do not misapply the phenomenal concept. For the case of pseudonormality see (Nida-Rümelin, 1999), for the impossibility of misapplying phenomenal concepts see (Nida-Rümelin, 2006). 24 These claims are closely related to the view about phenomenal concepts developed in (Nida-Rümelin, 2006). 25 The same intuitive idea is formulated by Kripke in his famous claim that the concept of pain fixes its reference by way of a non-contingent reference fixer. According to one of the authors the idea is however better captured in a framework that differs from Kripke's, see (Nida-Rümelin, 2006), last section. 26 In this step we presuppose the following principle: Necessarily a being has property P1 if it has property P2 iff in all counterfactual circumstances a being that has P2, also has P1. 27 The conceptual fact used here has been expressed in the two dimensional framework saying that these concepts have identical primary and secondary intension see (Chalmers, 1999) for that they are actuality- independent, see (Nida-Rümelin, 2006). Note that this is true only for the phenomenal concepts and arguably false for the corresponding phenomenal terms see (Nida-Rümelin, 2006) section *. This difference, however, does no harm to the above argument which is - for that reason - formulated with respect to concept extension (and not with respect to extension of terms). The argument would of course go through in a version that uses the extension of linguistic terms but it would have to be formulated in a slightly different and a bit more complicated manner. 28 For a theory about rational intuition see e.g. (Bealer, 2000)