Conclusion: Comparing Olfaction and Vision

Conclusion: Comparing Olfaction and Vision Th e primacy of vision over other modalities has led to a bias in our understanding of perception. Th e visuocentrism that dominates our thinking has very deep roots that are refl ected in our philosophical vocabulary. We can glimpse or see the meaning of a proposition. Using a tactile instead of visual metaphor, we can also grasp it. After we grasped enough propositions, we will form a certain view or outlook on the topic. Ideas are talked about as if they are visual objects that are placed before us. Th ey can be viewed from certain angles . Diff erent positions can be taken with respect to a certain view . Th ere are no comparable olfactory metaphors. Undoubtedly, this way of speaking has given vision a special status in philosophy of perception. Philosophy books have titles such as “Seeing and Knowing” (Dretske 1969) or “Philosophy and the Mirror of Nature” (Rorty 1981). Philosophers always have a “point of view”, whether it is a logical point of view, a scientifi c point of view, or the view from nowhere. I hope that this book will trigger a reexamination of the assumptions about perception that are built into the philosopher’s vocabulary. I do not mean to deny that in humans vision is the most important and interesting modality. Humans happen to be active during the day, when the light from the sun allows us to perceive the space around us visually. If humans © Th e Author(s) 2016 189 A. Keller, Philosophy of Olfactory Perception, DOI 10.1007/978-3-319-33645-9 190 Conclusion: Comparing Olfaction and Vision were active at night, we might have evolved echolocation like bats, to navigate in the dark. Philosophy of perception is visuocentric because at some point in our evolutionary history our ancestors found it easier to fi nd food and avoid being eaten during the day than at night. Once our ancestors became active during the day, it was important to optimize visual perception, even if it meant to reduce further the useful- ness of olfaction. Th e most important evolutionary event that shifted importance from olfactory perception to visual perception was the shift from walking on all fours to bipedalism. Our head and with it the sense organs for both vision and olfaction were moved up from the ground when we became bipedal. Th is further accelerated the evolution away from an olfactory creature to a visual creature. Close to the ground, down in the bushes, there is much less to see and much more to smell. However, at eyelevel (or noselevel) vision is much more useful than olfaction. Understanding that the diff erent ways in which humans perceive visual stimuli and odors have been shaped by the unique evolutionary history of our species leads to the realization that perception diff ers between modalities as well as between species. Human vision is diff erent from human olfaction in interesting ways. Human vision is also interestingly diff erent from fruit fl y vision. Many of the features that are associated with one modality in humans are associated with another modality in other species. In humans, visual perception has spatial structure whereas olfactory perception does not. For us, colors can be, and usually are, arranged in a spatial pattern, while it is not possible for humans to perceive a spatial arrangement of smells. Th e presence of spatial structure in vision and its absence in olfaction are due to the structure of the human sensory systems. Nothing in the nature of olfactory perceptual qualities prevents them from being spatially arranged. It is easy to imagine an olfactory system that mediates spatially structured olfactory perception. Our whole body could be covered in odor-sensitive cells. We then would experi- ence the smell stronger at the side of our body that is facing the odor source. Th is is how we perceive heat and why we can tell whether we walk toward or away from the sun based on the pattern of heat perception on our skin. Similarly, we cannot conclude that visual perception is inherently spatial from the fact that human visual perception has spatial structure. Many animal species can perceive visual stimuli, but do Conclusion: Comparing Olfaction and Vision 191 not have image- resolving eyes, and therefore do not perceive spatially arranged visual perceptual qualities. Th e simplest eye consists of only a single photoreceptor (Jékely et al. 2008). With such an eye, all that can be perceived is the level of illumination. A species of marine mollusks has hundreds of small simple eyes distributed over their body surface (Boyle 1969). Th ey perceive a light source in the same way in which humans perceive a heat source. In the light of this diversity of perceptual systems, generalizing from human vision to perception in general seems misguided. To make prog- ress, other forms of perception have to be investigated. In this book, I have considered human olfaction. Th roughout the rest of this conclusion, I will compare the central fi ndings about olfactory perception with what we know about visual perception to show that many conclusions from the study of human vision do not generalize well to other modalities. At the level of perceptual qualities, olfaction is more complex than other senses. Th ere are more smells than colors or tones. Consequently, arranging the perceptual qualities of olfaction into a similarity space has not been possible yet. For colors or tones, this fi rst step of systematically describing perception has been accomplished. However, once the perceptual qualities are individuated, olfaction is much less complicated than vision. What is perceived during olfactory perception in normal cases are individual olfactory perceptual qualities. Th e olfactory perceptual qualities are not arranged spatially and the temporal structure of odor perception is much impoverished compared to the other senses. Th is is in contrast with visual perception, which normally is the perception of visual scenes that are arrangements of color qualities in space. A philosophy of perception based on smells would focus heavily on the nature of perceptual qualities rather than on how they are arranged in time and space. Consequently, entities that consist of spatially and tempo- rally arranged perceptual qualities, like perceptual events and perceptual objects, would not be part of perceptual philosophy if olfaction would be the paradigm sense. Th e lack or relative unimportance of spatial and temporal informa- tion in odor perception also has consequences for our understanding of the relation between the physical world and our percepts. In vision, relative position and distance, as well as temporal sequences, in the 192 Conclusion: Comparing Olfaction and Vision physical world are often accurately represented through perception. Th is is adaptive because when the perceiving organism responds behavior- ally to a stimulus, the responses have to be directed and timed based on the spatial and temporal structure of the physical world to have the intended eff ect on the physical world. Th e largely correct mapping of spatial and temporal similarities in the physical world onto spatial and temporal similarities in visual percepts has resulted in the idea of perception as a representation of the physical world. In olfactory perception, perceptual qualities are not arranged in space. When perception without spatial structure is considered, physical similarities between stimuli are not always mapped onto similarities in perception. In olfaction, physical similarity between odor molecules is not reliably mapped onto similarities between the perceptual qualities associated with the stimuli. 1 Because olfactory perception consists of the perception of perceptual qualities with no spatial structure, the relation between similarities in olfactory stimuli and similarities in olfactory perception is weak. If olfaction were the paradigm, the idea that perception represents the physical world would not have the same appeal it has when visual perception is considered. In olfaction, perceptual qualities are closer aligned with appropriate behavioral responses to the stimuli than with the physical features of the molecules. Olfactory perception illustrates that what perception does is not mirroring reality, but guiding behaviors. Another striking diff erence between vision and olfaction is that olfaction is more diffi cult to demarcate than vision. While diff erentiating vision or audition from other forms of perception, olfaction is easily confl ated with other chemical senses and involved in the multimodal perception of fl avors. Individuating modalities based on representations, phenomenal character, the proximal stimulus, or the sense organs will lead to dramatically diff erent results in the case of the chemical senses, while all four methods largely agree about how to individuate vision or audition. Th e diffi culty of individuating olfaction may have contributed to its relative unpopularity among philosophers of perception. Some apparent disagreements in the philosophy of olfactory perception are a consequence of olfaction being individuated diff erently by diff erent scholars. Conclusion: Comparing Olfaction and Vision 193 In addition to the more fl uid boundaries between olfaction and other sense modalities, olfaction is also diffi cult to distinguish from non- perceptual processes like emotion processing. Olfactory and emo- tional processing share the same neuronal substrate, and evaluative emo- tions such as disgust are intricately linked to odor processing. Th ere is also widespread cognitive penetration of the olfactory sensory system. Olfaction illustrates therefore the interconnectedness of the mind better than many other modalities that have a diff erent evolutionary history. Th at the diff erent evolutionary histories of the diff erent modalities result in diff erences in how they are connected to cognitive processes is also refl ected by the diff erential ineff ability of the senses.

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