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Chapter 1 SYNOPSIS: MODALITY AND EXPLANATORY REASONING

I aim to shed light on a broad family of modal notions whose most widely discussed representative is the concept of metaphysical necessity. This of topic requires little justification or explanation. Since the work of Lewis, Kripke and others ushered in the modal turn in , the concept of necessity has been at the forefront of the communal philosophical mind. Today, modality is one of the most active areas of research in , and modal notions are central to philosophical theorizing across the board—from the foundations of logic to moral theory. Given this trend, it seems to be more important than ever to gain a clear philosophical understanding of modality, not least in order to determine whether modal concepts can bear the weight that so much of recent philosophical practice has placed on them. There are different ways of elucidating a concept. For example, one can shed some light on it by clarifying its formal and logical properties, and its inferential and analytic connections to other notions. A lot of excellent work like this has been done on modal concepts. More ambitiously, one can try to give a full-blown analysis of modal notions. This task has been attempted less frequently and, in my opinion, less successfully. David Lewis’ account is the most developed and famous, but it rests on problematic ontological assumptions and it runs counter to actualist intuitions that are widely shared and, in my opinion, correct. Consequently, I think that we should look for a different analysis of necessity. We do not start this project with a guarantee of success. It may turn out that no informative analysis of modal concepts is possible, and that we should be primitivists or eliminativists about modality. But at this stage it is too early to throw in the towel. It is an important aim of this book is to advertise a new analysis of modal concepts. I will combine this objective with two other goals. Firstly, I think that a comprehensive theory of the modal domain should not stop at analyzing modal notions. It should also explain why we go in for modal thinking at all. What purpose does this practice serve? Why have creatures with our concerns and interests developed the capacity for modal thought? A comprehensive account should offer both a theory of modality and a theory of the practice of modalizing. It should, as it were, combine an internal with an external viewpoint on this practice. Before we start to philosophize about modality, we have a set of (often implicit) assumptions about modality. Philosophers provide this pre-philosophical system of beliefs with a foundation, and refine, extend and correct it from within. They act as participants of our practice of modalizing, i.e. their standpoint is internal to this practice. But philosophers should also make the practice of modalizing itself an of study. They should, so to speak, take a standpoint external to the practice, in order to describe the practice, and explain what its function is, i.e. why it exists. It is a familiar fact that these two interests can pull in opposite directions. If we concentrate exclusively on giving a metaphysical account of modality, we might end up with a theory that makes it hard to explain why we are interested in modality. It is a common charge, justified or not, against Lewis’ account that it fails in this way.1 Even if there were other possible worlds in Lewis’ sense, so the objection goes, we would have no reason to be interested in them, and Lewis’ account therefore makes it a mystery why we should bother to have modal thoughts at all. At the other end of the spectrum there are theories that do well at explaining the purpose of our modal notions, but at the expense of giving implausible metaphysical accounts of the of modality. One example is the theory that for a proposition to be necessary is for it to owe its to a convention, perhaps the convention to regard it as true come what may. If some propositions are indeed conventionally exempted from empirical testing, then a concept that singles them out is undeniably useful for our epistemic practices. It is therefore easy for the conventionalist to explain the point of our modal notions. But this advantage is purchased at the price of an implausible theory of what necessity is.

1 For discussions of the point, see Kripke 1980, p. 45, Hazen 1979, pp. 320ff., Blackburn 1984, pp. 214f., Hale 1986, p. 77, McFetridge 1990, sct. 2, and Rosen 1990, pp. 349ff. 2

The task before us, then, is to develop a theory that permits us to achieve both goals, a credible metaphysical account of modality and a plausible account of the practice of modalizing. The best way of doing this is to integrate the two goals in a single enterprise: an account of the nature of modality can be guided by a hypothesis about the ordinary- life practice in which modal concepts originated, while assumptions about the nature of modality can in turn suggest ways of developing one’s about the practice of modalizing. That is what I will try to do in this book. Secondly, it will be one of my aims to illuminate the relationship of modal notions to another family of concepts that are of central interest to metaphysics: the class of notions that contains the concept of causation and related explanatory notions. These concepts have had a career in philosophy that is similar to that of modal notions. The of causation was once subjected to a rigorous empiricist critique that convinced many philosophers that we can make sense of causal talk, if at all, only if we construe it as derived talk about regular succession, nomic entailment, or something of that kind. Consequently, for a long causal notions were made to do very little work in philosophical theorizing. That changed around the 1960’s, when causal theories began to proliferate in many areas in the philosophy of mind, epistemology, metaphysics, and related fields. Suddenly, causation was used in theories about mental states, , knowledge, explanation, reference, persistence, the direction of time, and many other topics. It would not be exaggerated to talk of a ‘causal turn.’ This historical trend also has a second, more recent chapter: the rise to prominence of another group of concepts that have an important affinity to that of causation, such as metaphysical explanation and grounding, and related notions like , real definition and fundamentality. A cursory glance at the literatures on modality and on causation and explanation leaves little doubt that there are important connections between the two areas, but the nature of these ties and their philosophical implications are controversial. While many philosophers have tried to exploit the connections to give modal accounts of causation and explanation (often counterfactual temrs), I will argue for the opposite order of explanation. Many philosophers have taken Kripke’s work to demonstrate a deep and important distinction between epistemic necessity or a prioricity on the one hand and metaphysical

3 necessity on the other. I essentially agree with this assessment, but I would describe the fundamental divide as separating, not just two notions of necessity, but two broad families of modal notions, one epistemic and metaphysical or ontic. The ontic modalities include, not merely what is commonly known as metaphysical necessity, but an entire spectrum of other notions of necessity as well. It is the ontic modal notions that form the of this book. I will argue that, just as the notions of a prioricity and epistemic necessity are constitutively tied to the epistemic domain, the notions of ontic necessity are constitutively connected to the realm of causation and explanation. As we will see, the dependence of the modal on the causal and explanatory takes several different forms. Causal and explanatory concepts figure in the analyses of the various forms of ontic necessity. The ontic modal status of individual facts derives to a large extent from their position in the network of causal and explanatory relationships. And our practice of ontic modal thinking has its origin at least in part in causal and explanatory reasoning: notions like metaphysical necessity developed as useful auxiliaries to such reasoning. What follows is a brief synopsis of the main elements of my position. My exposition in this summary will be sketchy and occasionally a little dogmatic. Detailed arguments will be given in later chapters.

1.1 Ontic modality

To say that a proposition is necessary, in either an epistemic or an ontic sense, is to say that, in some sense, its truth is very secure, firm or unshakable. How this thought is to be spelled out in detail depends on the form of modality that is under consideration. To say that a proposition is epistemically necessary is to say that its truth is, in some sense, very epistemically secure. To say that a proposition is metaphysically necessary, by contrast, is to say that its truth is very firm or secure in a metaphysical, non-epistemic sense. We can approach the task of analyzing ontic necessity by trying to develop a non-metaphorical way of understanding this of metaphysical security or unshakability. I will argue in Chapter 2 that it is the same notion that we are operating with when we raise the question, concerning some fact, how easily it could have failed to obtain: the less easily P could have been otherwise, the more secure its truth. To say that P is necessary, i.e. that it could

4 not at all have been otherwise, is simply to say that it has a certain very high degree on that dimension. To see how we should understand claims about how easily a certain fact could have been otherwise, consider how we ordinarily support such a claim. When talking about a soccer game, we may say: “The game ended in a draw, but our team could easily have won. If the goalkeeper had stood two inches further to the right two minutes before the end, the other team would not have scored their goal, and our team would have won.” In less favorable circumstances, on the other hand, we may say instead, “Our team couldn’t easily have won. They would have won only if they had started their training two months earlier, had recruited Mary and Bob, and if a million other things had been different as well.” It seems that how easily a state of affairs could have failed to obtain depends on how great a departure from actuality is required in order for it not to obtain. If it fails to obtain in some scenarios that are only minimally different from the way things in fact are, then we can say that it could easily have failed to obtain. We can say the opposite if the only scenarios where the fact fails to obtain depart very significantly from actuality. It is often assumed that necessity and possibility are all-or-nothing matters. But how easily a fact could have failed to obtain is clearly a matter of degree. On my account, therefore, the feature that makes a proposition necessary (viz., its metaphysical security), and therefore necessity and possibility themselves, come in degrees. The notions of metaphysical necessity relates to one specific degrees on the possibility scale, as does the notion of nomic necessity, but there are many degrees of necessity as well. This view is also supported by linguistic evidence. ‘P could more easily have been the case than Q’ sounds on the face of it like a comparison between degrees of possibility. My account takes this impression at face value. Talk about degrees of ontic possibility is ubiquitous in ordinary life, but the idioms we use in this discourse are not always overtly modal. You are running to catch the subway train, but the doors close on you a fraction of a second before you can jump in. Frustrated, you sigh ‘I almost made it.’ That expresses the thought that you could easily have caught the train: a very minimal departure from actuality—the doors closing half a second later—is all that’s required. To take another example, suppose that your car goes into a skid on a patch of black ice, you hit the brakes and your car comes to a stop half an

5 inch from the precipice. Then you can say, ‘That was close!’ In other words, there is a scenario that is very close to actuality and where things go badly wrong. So, things could very easily have gone wrong. In other cases, we compare several different non-actual scenarios to determine which of them has a higher degree of possibility, e.g. when we say things like ‘Last season Smith got much closer to winning the championship than Jones ever did.’ Such comparisons of ontic possibility also underlie counterfactual judgments like ‘If I had pressed this button, there would have been an explosion.’ For on the standard view, which I accept, counterfactuals state claims about the relative proximity to actuality of different scenarios where the antecedent holds. The foregoing conditional, e.g., tells us that some button-pressing scenarios where an explosion takes place depart less from actuality than any button-pressing situations without explosion. The idea that notions like that of metaphysical necessity can be understood in terms of the ordering of non-actual situations by their proximity to actuality is likely to meet with immediate protest, since the very notion of a non-actual situation is often thought to be modal. Many philosophers, when they hear ‘non-actual situations’ or ‘alternatives to actuality,’ interpret it as talk about ways things could have been or possible situations (where ‘could’ and ‘possible’ express metaphysical possibility). It may therefore seem that any attempt to analyze necessity in terms of closeness of non-actual situations is bound to lead into a narrow circle. From this point of view, it may seem, not as if the concepts of possibility and necessity arise from the practice of thinking about the comparative closeness of different non-actual situations, but rather as if the practice presupposes a grasp of the concept of (a) metaphysical possibility. I think, however, that it is a mistake to assume that our attention in ontic modal thinking is restricted to scenarios that are metaphysically possible. Consider counterfactual conditionals as an example.2 A counterfactual is true just in case, roughly speaking, the consequent is true in the closest antecedent-worlds. On the assumption that all worlds are metaphysically possible, we would obtain the dubious consequence that all counterfactuals with metaphysically impossible antecedents are vacuously true (since there are no antecedent-worlds), irrespective of the specific contents of their antecedents and consequents. But that seems extremely implausible. It’s metaphysically impossible

2 Stalnaker 1968, 1984, Lewis 1973a, 1986c. 6 for Hilary Clinton to be Antonin Scalia’s daughter. But that fact does not trivialize the question of how Clinton’s views would differ if Scalia were her father. Similarly, it is metaphysically impossible for there to be no numbers. And yet, in discussing whether mathematical facts contribute to explaining the events in the natural world, we may ask - non-trivially, it seems – whether these events would unfold any differently if numbers didn’t exist.3 Since this problem arises from disallowing worlds where impossible propositions are true, the obvious remedy, suggested and developed by a number of philosophers, is to lift this restriction. Instead of using the concept of a possible world, we can formulate the account using the broader and non-modal notion of a world. Worlds are simply ways for to be, and they include both ways reality could have been – i.e. ways it is metaphysically possible for reality to be – and ways it couldn’t have been. Chapters 4 and 5 provide a more detailed analysis of worlds as classes of propositions that describe reality in a maximally detailed way. My account of worlds is non-modal. In particular, it doesn’t impose any modal constraint on the way a world describes reality: it’s not part of the account that the that a world gives of reality must be possibly true, in any sense of ‘possible.’ (I do think that there are strong theoretical reasons for imposing certain other constraints, e.g. that the description be consistent in a narrowly logical sense. As I will explain in section 1.4, I take that constraint to be non-modal.) This framework can be used to sharpen the account of possibility hinted at above. One world, called the ‘actual world,’ has the special distinction that it is a wholly correct description of reality. It gives us the truth, the whole truth, and nothing but the truth. All other worlds depart to various degrees from actuality. Worlds that misrepresent a few details about the movements of fundamental particles but otherwise contain only true propositions are pretty close to the actual world. Worlds where I am a fried egg, or where there are no mathematical entities, are much less close. Given a specific set of standards for weighing different respects of closeness against each other, we can order non-actual worlds by their overall similarity to actuality. A proposition’s degree of possibility is

3 For reasons discussed in Chapter 3, the first example is unlikely to worry counterpart theorists like David Lewis (1968, 1971, 1986e). I think that there are strong reasons for rejecting counterpart theory (see Kment 2012). But in any case, counterpart theory doesn’t help with the second example of seemingly non-trivial counterpossibles (the counterfactual about the non- of mathematical entities). 7 measured by the degree of closeness to actuality of the closest worlds where the proposition is true. Let’s call a class of worlds a ‘sphere around actuality’ just in case it includes all worlds within a certain distance from actual world. The relation of comparative closeness to actuality generates a system of such spheres. For each sphere, there is a grade of necessity, which attaches to just those propositions that are true at all worlds in that sphere, as well as a grade of possibility attaching to all propositions that are true at some world in the sphere. The larger the sphere, the greater the associated degree of necessity. One specific sphere, described in more detail below, corresponds to the notion of metaphysical necessity: the metaphysical necessities are the propositions that hold throughout that sphere. Another, smaller sphere corresponds to nomic necessity. Other spheres give us yet further grades of necessity, some of them lower than nomic necessity, some intermediate between nomic and metaphysical necessity, and some greater than metaphysical necessity. I will argue in Chapters 2 and 3 that this account does a good job at capturing our core beliefs about what necessity is and illuminates various features of modal discourse. To complete the analysis of degrees of possibility, it will be necessary to specify the standards of closeness to actuality that underlie our repertoire of ontic modal concepts. Other worlds can resemble or differ from actuality in numerous different ways, and it needs to be explained what weight these different similarities and dissimilarities carry in determining a world’s degree of closeness to actuality. It is a commonplace observation that the standards of closeness can differ across contexts. However, following David Lewis, I believe that there is a specific set of standards that serves as our default in ontic modal thinking: we use it unless we have specific reasons to use different standards. These are the standards that figure in my analysis of ontic modal notions. The best way of determining what these standards are proceeds by reverse engineering: we collect data about the truth-values of modal judgments under various conditions, and then try to find the standards of closeness that best account for these data. I will work on that task in Chapters 7 – 9. It is in this part of the book that the constitutive tie of modal notions to the causal and explanatory realm will become apparent. For how weightily it contributes to the closeness of a world if it describes a certain fact correctly

8 depends crucially on the position of this fact in the network of explanatory relationships, as we will see next.

1.2 The explanatory source of ontic modality

1.2.1 The order of analysis

The connection of modal to causal and explanatory thought is perhaps never more obvious than when we use counterfactuals to answer causal questions. You want to know whether Fred’s tactless remark on Friday caused his fight with Susie on Sunday. Then the natural thing to do is to ask whether the fight would have taken place without the remark. And if the answer is ‘no’ – if the fight counterfactually depends on the remark – then you can infer that the tactless remark caused the fight. Counterfactuals serve as our guide to the causal facts. This observation has motivated the project of analyzing causation in terms of counterfactuals (and therefore in ontic modal terms). For a counterfactual analysis gives a very straightforward explanation of the connection between causal and counterfactual judgments: causation consists in a certain of counterfactual dependencies. To ask whether X caused Y is simply to ask whether certain counterfactuals hold. There is, however, a general limitation to any attempt to motivate an analysis of one concept D in terms of another concept C by appealing to an intuitively correct inference from a claim P(C) involving C to a claim Q(D) involving D. If the inference from P(C) to Q(D) is valid, then so is the contrapositive inference from ~Q(D) to ~P(C). We could take the former inference to motivate an analysis of D in terms of C. But we could equally well take the latter inferences to support an analysis of C in terms of D. This issue also arises for causation and counterfactuals. Just as we can infer (although defeasibly, as we will see) that A and E are causally connected if we know that E counterfactually depends on A, we can equally infer (again, defeasibly) that E is counterfactually independent of A if we know that there is no causal connection. Moreover, just as causal judgments are often guided by prior counterfactual judgments in accordance with the former inference, so counterfactual judgments are often informed by prior causal judgments in accordance with the latter inference. There is no shortage of

9 examples of the second phenomenon in the recent literature on counterfactuals.4 Here is a variant of an example due to Dorothy Edgington.5 As you are about to watch an indeterministic lottery draw on television, someone offers to sell you ticket number 17. You decline. As luck would have it, ticket number 17 wins. It seems true to say ‘If you had bought the ticket, you would have won.’ But this presupposes that

If you had bought ticket number 17, that ticket would still have won.

Contrast this with:

If a different machine had been used in the draw, 17 would still have won.

That seems false. If a different machine had been used, then 17 might still have won, or some other number might have won. It is not true that 17 would still have won. In the first case, we hold the outcome of the lottery draw fixed, in the second we do not. It’s not hard to find an explanation for the difference. Your decision whether to buy the ticket is not causally connected to the outcome. That is why you hold the outcome fixed when thinking about what would have happened if you had made a different decision. By contrast, the use of a particular lottery machine is part of the causal history of the outcome. You change that causal history when you replace the machine with another. In that case, you won’t hold the outcome of the draw fixed. In these examples, our judgments about whether certain facts can be held fixed (i.e., whether they would still have obtained if the antecedent had been true, i.e. whether they are counterfactually independent of the antecedent) are guided by prior causal judgments. Several philosophers have used this phenomenon to analyze comparative closeness to actuality (and hence counterfactuals) in causal terms.6 So far, then, there is no reason for preferring one order of analysis to the other. The only way to decide between them is to look at the two options in more detail in order to determine which of them can better account for the complex relationship between counterfactuals and causation. It is well-known that counterfactual analyses of causation face considerable problems at this point. Counterfactual dependence is not necessary for

4 As mentioned in Lewis 1986c. Examples are discussed in Adams 1975, ch. IV, sct. 8 (in particular pp. 132f.), Tichý 1976 and Slote 1978. 5 Edgington 2003. 6 See, e.g., Mårtensson 1999, Edgington 2003, Bennett 2003, ch.15, Hiddleston 2005, Kment 2006a, and Wasserman 2006. 10 causation (if A is an over-determining or preempting cause of E, then E does not counterfactually depend on A), and although it may be a near-sufficient condition, there are cases (so-called ‘backtracking cases’) that suggest that it is not quite sufficient. Additional problems arise if we try to apply the counterfactual account to probabilistic causation. Proponents of the counterfactual analysis have long been searching for solutions to these problems, but the results are not encouraging. Consequently, there is plenty of motivation for trying out a theory that rests on the opposite order of analysis, i.e. which analyzes closeness to actuality in terms of causation and explanation rather than the other way around. I will argue in Chapters 7 - 9 that such a view can account for all the data. I will give a brief summary of the basic of this account in section 1.2.3. But first, some preliminaries.

1.2.2 Ontic Explanation

I believe that causation is a special case of a more general relation that has many non- causal instances. It is the relation that we often talk about when we use phrases like the following to make non-causal claims:

x is one of the factors that are responsible for y, y is due (in part) to x, y obtains (partly) because of x, x is one of the reasons why y obtains, x is (part of) what explains why y obtains.

I will use the last of these phrases as my standard term for this general relation, mostly for the purely practical reason that ‘explain’ has an associated adjective and noun and has a passive voice, which allows us to express both the relation and its converse by forms of the same verb. But it is important to bear in mind that I am using this term in a purely ‘ontic’ or metaphysical sense, not in an epistemic sense.7 To say, in the intended sense, that x contributes to explaining y is not to say anything about the conditions under which a thinker can gain an understanding of y, or anything of that sort.8 It is to report a

7 For the distinction between ontic, epistemic and modal senses of ‘explanation,’ see Salmon 1984. 8 I will also occasionally use ‘explanation of x’ in a different but closely related sense, namely in the sense of ‘account of why x obtains’ (and I will similarly use ‘explain’ in the sense of ‘providing an acc of why x 11 relationship that holds between x and y wholly independently of human intellectual activity (though that activity may be partly aimed at gaining knowledge of the relationship). Causes explain their effects in this ontic sense, but x can also explain y without a cause of y. My first example of such non-causal explanation will stay fairly close to the causal case: effects are typically explained, not by their causes alone, but by these together with certain facts about the laws of nature. The coffee cup falls, hits the floor, and breaks into a million pieces. Why did it happen? In part, it’s because you pushed the cup off the table and because you have a planet under your kitchen floor. But another part of the reason is that there is a law of nature to the effect that any two massive bodies attract each other with a certain force. It’s partly because that is a natural law that the planet attracted the cup. This is an example of non-causal explanation: the fact that a certain law is in force contributes to explaining certain goings-on, but it doesn’t cause them (only matters of particular fact can cause things). My second illustration of non-causal ontic explanation concerns cases of what is sometimes called ‘grounding’ or ‘metaphysical explanation.’9 I will argue in Chapter 6 that there is a far-reaching structural analogy between causation and metaphysical explanation. In particular, certain general metaphysical principles, which I will call ‘laws of metaphysics,’ play essentially the same role in metaphysical explanation as the natural laws do in causation. Metaphysical laws come in different forms. They include the essential about things, where ‘essence’ is to be understood in a broadly Aristotelian sense: to a first approximation, the essential truths about X state the features of X that are part of what it is to be X. The ontological laws form another class of metaphysical laws. They include, e.g., the law of unrestricted mereological composition, or the that for any two properties there is a that is their conjunction. Metaphysically less fundamental facts are metaphysically explained by the more fundamental facts that ground them, together with certain facts about the metaphysical laws. For example, the fact that a is a gold atom is explained by the fact that a is an atom with atomic number 79, together with the fact that that’s what it is to be a gold atom. Similarly, the fact that obtains’). This is the sense in which I will be using the term when I speak of a theory’s power to explain certain data, or of inference to the best explanation. 9 See, e.g., Fine (MS), Dasgupta (MS). 12 there exists a property of being negatively charged and having a mass of n grams may be explained by the fact that there’s a property of being negative charged, the fact that there is a property of having a mass of n grams, and the fact that it’s a metaphysical law that, for any two properties, there is a property that is their conjunction. Metaphysical explanation is not causation, but it plays an important role in many causal connections. The causes of a fact F are also causes of the facts that are grounded in F. For instance, certain physical facts obtaining at one time (e.g. your biting into a piece of chalk), may cause certain physical facts (e.g. about your brain state) at a later time which, in turn, are part of what grounds a certain mental state (gustatory chalk sensations). Then the physical facts about the earlier time are also causes of this mental state. In such cases, the causes are connected to their effect by an explanatory chain whose links feature both causal and grounding relationships. The effect is explained by its causes and certain facts about the natural laws, together with the facts about the metaphysical laws that figure in the relevant instances of grounding. As the example illustrates, laws of both kinds, natural and metaphysical, can figure in causal explanation. Cases of causation differ from those of grounding inasmuch as in the latter the explanandum is connected to its explainers by metaphysical laws alone, while in the former the laws that forge the connection include some natural laws (and may or may not include metaphysical laws as well).10

1.2.3 The standards of closeness

A world’s degree of closeness to actuality is determined by its various respects of similarity and dissimilarity to the actual world. We should not assume, however, that every way in which another world resembles actuality contributes to its closeness. Some similarities may carry zero weight. One of the tasks in specifying the standards of closeness is to distinguish the relevant from the irrelevant similarities. The lottery example of section 1.2.1 provides a first set of data: as we saw, we hold fixed the outcome of the draw only if it has the same causal history in the closest antecedent-worlds as in actuality. Consider a second example with a similar structure. In

10 I don’t claim that the foregoing constitutes an exhaustive list of all kinds of non-causal ontic explanation. But the cases mentioned above are the only ones that will concern me in this book. 13 most cases of ordinary-life counterfactual reasoning, we hold fixed the fact that material objects conform to the law of gravitation. For example, we accept that

If I were to suspend my pencil in the air and then release it, it would fall to the ground.

But there are also examples that go the other way. Let ‘G’ be a name for the actual law of gravitation and consider:

If G had not been a law of nature, then events would still have conformed to G.

That does not sound true. In this case, antecedent-worlds where events conform to G are no closer than worlds where they don’t. Why the difference between the two cases? In our world, events conform to G because G is a law of nature. That’s not the case in a world where the antecedent of the second counterfactual is true. While in some antecedent-worlds events conform to G, they don’t do so for the same reason as in our world, and these worlds therefore don’t come out closer than antecedent-worlds where events don’t conform to G. The situation is different in the case of the first counterfactual. Since G is still a law of nature in the closest antecedent-worlds, such worlds are closer, other things being equal, if their events conform to G than if they don’t. Such examples suggest that a similarity between world w and the actual world increases w’s closeness to actuality only if it concerns some fact that has the same explanation in both worlds. I will argue in Chapters 7 – 12 that this explanatory criterion of relevance can explain a variety of phenomena that have played a dominant role in philosophical discussions of counterfactuals. For example, in combination with the prevailing temporal asymmetry of causation, it explains the temporal asymmetry of counterfactual dependence. It also contributes to explaining the difference in counterfactual-supporting power between natural laws and accidental regularities, which has attracted so much philosophical attention since Goodman’s seminal work. While the explanatory criterion of relevance allows us to distinguish the similarities that matter to closeness from those that don’t, it doesn’t by itself tell us how different closeness-relevant similarities need to be weighed against each other. I will present my answer to that question in Chapter 7. The single most important factor is, to simplify a

14 little, match in the metaphysical laws. A world with the same metaphysical laws as actuality is automatically closer than a world where these laws are different, no matter how similar the latter world is to actuality in other respects. Worlds with the same metaphysical laws consequently form a sphere around actuality. Metaphysical necessity is the grade of necessity associated with that sphere: for a proposition to be metaphysically necessary is for it to be true in every world in that sphere. The second most important criterion is match in facts about the laws of nature. Of the worlds with the same metaphysical laws as ours, those with the same laws of nature are closer than the rest. They form a second, smaller sphere within the first sphere. Nomic necessity is the grade of necessity corresponding to that sphere. Similarities in matters of particular fact are relevant to closeness as well, although they are less important than the two aforementioned kinds of similarity. On this view, facts about the metaphysical laws have a very high degree of necessity (metaphysical necessity), facts about the natural laws have a somewhat lower degree (nomic necessity), while matters of particular fact have lower degrees still. As we will see in Chapter 10, what gives rise to these differences in modal force between the three classes of facts is their distinctive positions in the network of explanatory relationships, in combination with the explanatory criterion of relevance.

1.3 Part III: On the genealogy of modality

1.3.1 The counterfactual test and the method of difference

While Chapters 2 - 9 attempt to define the most important ontic modal notions, Chapters 10 - 12 aim to give a functional explanation of why we have developed these concepts. We can take as our starting point the observation above that counterfactuals often guide our causal judgments. The importance of counterfactual reasoning extends to judgments about non-causal explanation as well. For example, if we can show that life would never have developed if a certain physical constant had been outside a certain range, then that suggests that the existence of life is due, in part, to the value’s being within that range. I aim to explain how counterfactual dependence can serve as a useful test for causal and other explanatory relationships, despite the fact that counterfactual dependencies aren’t part of what constitutes such relationships. This will allow me to argue that the practice of modal thinking, complete with the concept of closeness, the counterfactual, and the

15 various notions of necessity and possibility, developed at least in part because of its utility in evaluating explanatory claims. To support the claim that A caused X by counterfactual reasoning, we consider a world where A doesn’t obtain but which is otherwise maximally similar to actuality in certain respects. If X doesn’t obtain in that world, then X counterfactually depends on A. Given certain background assumptions, we can defeasibly infer that A is a cause of X. This method resembles another well-known procedure for establishing causal claims: John Stuart Mill’s method of difference. Simplifying and idealizing, we can give the following description of how this method works under .

Scenario 1 Scenario 2 A B C D Ā B C D E Ē

You observe an actual scenario where A – D obtain and E obtains at the next moment. You know that all the factors that were causally relevant to the occurrence of E in Scenario 1 are included among A – D, though you don’t know which of A – D were causally relevant. If you observe another actual scenario (the ‘control scenario’) where B – D without A and where E doesn’t obtain at the next moment, then you can conclude that A must be a cause of E in Scenario 1. Mill’s method is used in a large variety of settings that range from humble instances of everyday reasoning to the controlled experiments of empirical science. Your laptop is plugged in but the battery, though nearly depleted, is not charging. This could be due to a battery defect, a malfunctioning adapter, or a dead outlet. To find out, you vary these factors one at a time, leaving everything else constant. For example, using the same battery and adapter, you plug into a different outlet. If the battery starts charging, you conclude that the issue was caused by an outlet problem. In Mill’s procedure we compare two actual scenarios, while in counterfactual reasoning we compare an actual with a non-actual situation, but otherwise the methods are closely analogous. Unsurprisingly, there is a unified explanation of how the two procedures work. My thumbnail sketch of it in the next section will focus on the deterministic case. (The indeterministic case will be considered in Chapter 12.)

16

In my view, both methods rely on an assumption that is central to our ordinary (folk) conception of causation and explanation, and which I will call the determination idea. Different versions of this assumption apply under determinism and indeterminism and to causal relata of different ontological categories, but we can give the simplest statement of the determination idea if we focus on the deterministic case and on causal between facts (rather than, e.g., events).11 Applied to this case, the determination idea tells us that the factors that contribute to explaining a fact f – i.e., f’s causes and the relevant facts about the metaphysical and natural laws – together (analytically or logically) entail f. (Talk of entailment relations between facts, in turn, can be spelled out in terms of entailment between propositions.) The determination idea is not a thesis about what causation or explanation is, nor even a necessary and sufficient condition for causation or explanation. It merely gives us a necessary condition for a certain class of facts to include all the explainers of x (under determinism, a class of explanatory factors includes all explainers of x only if it determines x). The determination idea and kindred assumptions about causation have come in for a fair amount of philosophical criticism. But for the purposes of providing a genealogy of counterfactual reasoning, it is unimportant whether the idea should be regarded as true in light of all our scientific and philosophical knowledge. What matters is that the determination idea, in either its deterministic or its indeterministic guise, plays a central relation role in our pre-scientific causal and explanatory reasoning, at least as a working assumption. That claim, which I take to be very plausible and which I will argue for in Chapter 9, receives further support from the fact that the determination idea has been one of the major paradigms in philosophical discussions of causation and explanation. The determination idea explains how the method of difference works. It implies that the causes of E in Scenario 1, together with the relevant facts about the laws, entail E. Scenario 2 shows that B – D and the facts about the laws don’t entail E. So, B – D don’t include all the causes of E in Scenario 1. Since we started from the assumption that A – D do include all the causes, we can conclude that A is one X’s causes.

11 I think that the relata of the causal relation can belong to different ontological categories, including both the categories of facts and of events. I will formulate my account in terms of fact causation, partly because that makes it easiest to apply the determination idea. (See Chapter 10 for more discussion.) 17

The method of difference is limited in scope. We can use it to show that A is a cause of X in Scenario 1 only if we can either create or find an actual scenario where A doesn’t obtain but which otherwise matches Scenario 1 in all relevant ways. That’s not always possible. And the method is completely useless when our goal is to show that a certain fact about the natural or metaphysical laws contribute to explaining X, since these facts don’t vary between actual scenarios. If the aforementioned reconstruction of Mill’s method is on the right track, however, then there is a straightforward extension of it that remedies these shortcomings. To begin with, it doesn’t really matter if we can’t observe an actual scenario where B – D obtain without A. It’s enough if we can show that there are unactualized worlds (consistent, maximally detailed stories) where A doesn’t obtain but which matches actuality in B – D and the laws, and that E fails to obtain in such worlds. That’s enough to conclude that B – D and the facts about the laws don’t entail E, so that, by the determination idea, B – D don’t include all of E’s causes in Scenario 1. Given our background knowledge that A – D include all the causes, it follows that A is a cause of E in Scenario 1. This procedure won’t work in general, since there may be no worlds with the same laws where B – D obtain but A doesn’t (for B – D and the facts about the laws may entail A). Then we need to find the closest match: worlds where A fails to obtain and which match actuality as closely as possible with respect to B – D and the laws. If E doesn’t obtain in these worlds, then lends strong (though indefeasible) support to the conclusion that A is a cause of E. An exactly analogous method can also be used to demonstrate that the fact that L is a (natural or metaphysical) law contributes to explaining E. What I have described is a simplified version of what happens when we establish a claim about explanatory connections by counterfactual reasoning, and it explains one of our rationales for ordering non-actual worlds by how closely they match actuality in selected respects. This closeness ordering, in turn, is the basis for all ontic modal thinking. A systematic version of the counterfactual method for establishing causal claims requires a fixed set of standards for deciding which ~A-worlds should count as the ones that come closest to matching the actual world in all the relevant ways. In Chapters 11 and 12, I will consider in detail how we would proceed if we wanted to formulate standards of closeness that are particularly useful for the method under consideration. I

18 will argue that a very natural way of doing this results in just the principles that actually governs our practice of counterfactual reasoning (as described in Chapters 7 - 8). In particular, it results in a set of rules that generates our actual system of modal concepts, including the notions of metaphysical and nomic necessity. In other words, modal thinking proceeds just as we would expect on the assumption that it developed, at least in part, as a method for supporting causal and other explanatory claims in the way considered above. I take this to support the claim that we did in fact develop modal thinking at least partly for this purpose. As described above, the fully developed standards of closeness are formulated in terms of explanation. We therefore typically already need to have some knowledge about explanatory relationships before we can conduct counterfactual reasoning to establish claims about explanation. But there is no circularity. The explanatory knowledge needed to establish the relevant counterfactual differs from the explanatory knowledge we acquire as a result of the process. Counterfactuals mediate inferences from claims about explanation, thereby allowing us to extend our stock of explanatory knowledge. This view explains the phenomenon, mentioned in section 1.2.1, that we commonly draw inferences in both directions, from explanatory claims to counterfactuals and vice versa: both kinds of inference are involved in establishing a claim about explanation through counterfactual reasoning. Inferences of the two types are justified in quite different ways, however. The entailments from explanatory claims to counterfactuals are due to the way explanatory notions figure in the truth-conditions of counterfactuals. Counterfactuals are typically made true by, among other things, certain of explanatory relationships, and knowledge of these patterns (and of some background facts) therefore allows us to establish the conditional. The further inference from the counterfactual to the final conclusion about explanation, by contrast, is not in the same way underwritten by a constitutive connection (since counterfactual dependencies aren’t part of what constitutes explanatory relationships). Instead, it rests on the determination idea, as explained above. I mentioned above that counterfactual dependence is neither necessary nor sufficient for causation. Consequently, the counterfactual test for explanatory claims has its limitations. While my account explains how the test works in those cases where it can be used successfully, it equally predicts and explains why it doesn’t work in the problem

19 cases. The very examples that have dogged counterfactual accounts of causation and explanation provide further evidence for my alternative theory.

1.3.2 The larger picture

The significance of Mill’s method of difference in causal and explanatory thinking can hardly be overstated. It is ubiquitous in everyday life, and its application in controlled experiments is frequently hailed as the foremost procedure for the scientific study of causal relationships. If the evaluation of explanatory claims by counterfactual reasoning is an extension of this method, as I will argue, then that makes ontic modal thinking an integral component of a broad and crucially important framework for investigating the world. This framework, and in particular its application in scientific experiments, has been studied extensively by scholars from a broad range of disciplines.12 In my view, modal metaphysics is closely connected to this flourishing area of research. There is considerable potential for inter-disciplinary work that can fruitfully be explored in future work. Needless to say, I do not deny that ontic modal concepts can serve purposes other than that of mediating inferences between causal and other explanatory claims. In assessing the safety of a nuclear power plant, e.g., we may want to find out how close the plant was to suffering an accident at various points in the past, i.e. how easily such an accident could have come about. When we make practical decisions and want to know what the likely consequences of different possible courses of action are, we may ask what would happen if we were to act in such-and-such a way (i.e. what happens in the closest worlds where we perform the relevant action).13 Ontic modal beliefs – judgments about the proximity of unrealized scenarios to actuality – also has a powerful and scientifically well-documented influence on our evaluative judgments and emotional lives. Our assessment of the events that befall us, and how we feel about them, isn’t determined merely by our perception of their intrinsic pleasantness or desirability, but to an equal extent by how they compare to outcomes that ensue in other, nearby scenarios. One and the same outcome may be greeted with joy, relief or contentment by those who think that

12 See, e.g., Pearl (2000) and Spirtes, Glymour & Scheines (2000) for background about this research program. 13 See, e.g., Gibbard and Harper 1976, Lewis 1981. 20 matters nearly turned out worse, and with disappointment, regret or frustration by those who believe that a better result almost came about, even if both groups agree in their assessment of the intrinsic virtues of the actual outcome. In other contexts, the judgment that a negative event nearly happened is a source of negative emotions: you feel worse about your negligence if it nearly killed the baby than if there was never any danger. Such examples illustrate the important adaptive function that ontic modal thinking can have in regulating emotions and motivation. And there are doubtless other examples of useful applications of such thinking. Ontic modal thought was most likely molded by a variety of different functional pressures that converged on the same set of rules, or at any rate on very similar rules. That is to say, the practice emerged for more than one reason.14 Why, then, does my discussion of the genealogy of modal notions focus on their role in explanatory thinking? The reason is twofold. Firstly, it is one of my chief aims to argue that explanatory notions are more fundamental than modal ones. To support this order of explanation, it is important to show that it gives us a better account of the role of counterfactuals in explanatory reasoning than the opposite order of explanation. Secondly, by studying how ontic modal thought figures in explanatory reasoning, we are likely to shed light on a larger range of modal concepts than by investigating other uses of ontic modal thinking. For the counterfactual test for explanatory claims requires us to consider worlds that depart significantly further from actuality than those considered in other applications of ontic modal thinking. For example, when we apply counterfactuals in decision making, our attention is usually restricted to worlds that differ at most in what decisions we make, but which are otherwise pretty much like the actual world. Similarly when we emotionally respond to the thought that things could easily have turned out better than they did. In such cases, we typically only consider scenarios where the laws are the same as in actuality and where history unfolds the same way until a certain time, but where events then take a slightly different course. Such uses of ontic modal thinking don’t require us to order any more remote scenarios by their relative proximity to

14 A complete account of the various functions of ontic modal thinking would have to take into account the findings of the considerable psychological literature on counterfactual thinking, such as Au (1992), Boninger, Gleicher & Strathman (1994), Byrne (1997), (2002), Costello & McCarthy (1999), Einhorn & Hogarth (1986), Ginsberg (1986), Hilton & Slugoski (1986), Johnson (1986), Kahneman & Varey (1990), Mandel, Hilton & Catellani (2005), Reilly (1983), Roese (1994), Roese & Olson (1993), (1995a), (1995b), (1995c), Sherman & McConnell (1996), Shultz & Mendelson (1975), Wells & Gavinski (1989). 21 actuality, and they consequently don’t require us to distinguish between the different degrees of necessity that correspond to spheres large enough to include such remote scenarios. Concepts that function to differentiate such high degrees of necessity, such as the notion of metaphysical necessity, therefore cannot have their origins in these practices. By contrast, there is no limit to the remoteness of scenarios that can usefully be considered when we use counterfactual reasoning to test explanatory claims. For instance, when we investigate the explanatory role of a certain fact about the metaphysical laws, the closest worlds where the fact fails to obtain may be very far away from actuality. In such cases, we have a use for notions that distinguish between different degrees at the upper end of the necessity scale. (Needless to say, outside the philosophy and science rooms, we rarely apply counterfactual reasoning to study facts about the metaphysical laws, which is why we ordinarily have little opportunity to apply the notion of metaphysical necessity. That is perhaps the reason why this concept can appear somewhat arcane and removed from ordinary life, despite the fact that it is an outgrowth of a very familiar and extremely common cognitive practice.)

1.4 The question of reduction

My analysis of modality uses several notions – those of a proposition, essence, metaphysical laws, and logical truth – that have often been held to require modal analyses, although it is controversial in each case whether this is true. My account can count as reductive only if none of these notions are in fact modal. I tend to believe that that is indeed the case (so that Chapters 2 – 9 can be understood as offering a reduction of ontic modality), but it is beyond the scope of this book to argue for this conclusion in detail. For all I can hope to show here, my theory may be defining the various notions of ontic possibility and necessity in other modal terms. Such an account could still shed light on modality, provided that it traces what Strawson called a “wide, revealing, and illuminating circle.”15 Let me nonetheless consider the aforementioned concepts briefly and indicate in each case what understanding of them I find attractive.

15 Strawson 1992. 22

Propositions. Many philosophers think of propositions as classes of possible worlds. Given the assumption that propositions are the contents of sentences, mental states, and so on, such a view threatens to make the very notion of content modal. I prefer the opposite order of analysis. World, in my view, are classes of propositions, and propositions are sentences in a very broad sense of the term: structured entities that represent in part because of the way they are built up from certain constituents (and in part because of the semantic significance of its constituents and of the way it is composed of them). Propositions cannot be sentences of any natural language, since the expressive power of such languages is much too restricted. We need a language whose expressive resources are essentially unlimited.16 I will appeal to a variant of a language with that characteristic that was described by David Lewis, who called it ‘Lagadonian.’17 With some qualifications to be discussed later, every entity is a name for itself in that language, so that the language has a name for everything. Sentences are classes built up from such names and other expressions, such as sentence connectives, quantifiers and variables. (The Lagadonian language will be described in a little more detail in Chapters 4 and 5.)

Essence and metaphysical laws. It has long been common to define essentialist locutions in modal terms.18 On this account, a is essentially F just in case it is impossible for a to exist without being F. Building on ideas that go back to antiquity, some philosophers have recently argued that this characterization doesn’t adequately capture the concept of essence.19 As Kit Fine points out, it is a necessary feature of the number 2 to be a member of the set {2}, and a necessary property of {2} to have 2 as a member. But while having 2 as an element is part of what it is to be {2}, being a member of {2} is not part of what it is to be the number 2. The distinction between essential and accidental truths about an object cuts more finely than any modal distinction. If that is right, as I believe, then there is no apparent reason for thinking that the notion of an essential truth needs to be explained in modal terms.

16 Not quite unlimited. Some restrictions on its expressive power will be discussed in Chapter 5. 17 Lewis 1986e, Ch. 3. 18 Fine 1994, p. 3, mentions Mill (1956, bk. 1, ch. vi, § 2) and Moore (1922, pp. 293, 302) as proponents of this conception. More recent examples include Kripke (1980), and Forbes (1985, pp. 96-100), among others. 19 Fine 1994. 23

Philosophers occasionally describe the concept of essence as obscure. I am not sure what to make of this charge. If the complaint is that the concept is esoteric and somehow far removed from the ordinary thought, then I think that that is false. Essentialist idioms seem to be common in everyday life, as when we talk about what makes a piece of music punk, what it is to be courageous, or what happiness or justice consist in. (Admittedly, there are other interpretations of these utterances, but in each case it easy to image a perfectly ordinary context in which they express essentialist claims.) For reasons explained in Chapter 6, I do not believe that it is possible to define essence, in the sense of specifying what it is for something to be an essential truth about a given entity, since essence is metaphysically fundamental. Certainly, our grasp of the notion of essence does not consist in knowledge of such a definition. I think that the best strategy for elucidating essentialist terminology is to specify its conceptual or theoretical role, by describing the place of essence in our ordinary system of beliefs about explanatory connections. (Chapter 6 is intended to make a start on that.) Our grasp of the concept of essence may consist mainly or entirely in subscribing to this set of beliefs. Essence is whatever aspect of reality provides the best trade-off between fitting the essence role and other desiderata, such as naturalness. Similar remarks apply to the general concept of a metaphysical law.

Logical truth. My conception of propositions as entities with syntactic structure makes it natural to think of (narrowly) logical truth, as it applies to propositions, as truth purely in virtue of logical structure and the meanings of the logical constants. (The notions of logical consistency, logical consequence, etc. can be understood in the corresponding ways.) Logical truth, thus understood, is not the same thing as logical necessity, a special modal status believed by some philosophers to be distinctive of the logical truths.20 (It is controversial whether the logical truths do indeed have such a special modal status – I will eventually argue that they don’t – but let us assume for the sake of the argument that that is the case.) Logical truth is a proposition’s property of being true for a certain reason (namely that it has a certain logical structure and that certain meanings attach to the logical constants). Logical necessity, by contrast, doesn’t relate to the reason why a

20 Kit Fine makes a similar point in his 2002, pp. 264f., when he distinguishes between logical truth and logical necessity. 24 proposition is true, but to the fact that its truth is especially metaphysically secure (a fact that may, of course, be explained by the fact that the proposition is a logical truth). These are different properties, despite being coextensive. Logical necessity, if such a form of modality exists, is evidently a modal property, but it is far from obvious that the same is true of logical truth. The matter cannot be adjudicated without spelling out the foregoing characterization of logical truth in more detail, e.g. by saying more about the relevant notions of logical structure and logical constants, and clarifying how facts about logical structure and the meanings of the logical constants can explain the truth of a proposition. I am somewhat undecided between the different existing accounts of this and have no new ideas to offer. I have no argument that ontic modal notions won’t be needed to develop the theory of logical truth, but I also don’t see a compelling reason for thinking that it will. In any case, even if a general account of logical truth turns out to require ontic modal notions, this need not frustrate the reductive aspirations of my view. For we could define a non-modal syntactical notion that is coextensive with logical truth over the domain of Lagadonian sentences and then substitute this concept for the notion of logical truth throughout my analysis of ontic modality.

1.5 Modality in metaphysics

I mentioned two important developments in metaphysics during the last half century: the ‘modal turn,’ which conferred renewed prominence on modal notions, and the causal turn, which brought various causal and explanatory concepts back into the limelight. It’s one of the goals of this book to shed light on the complex connections between the two families of notions, and I hope that that will ultimately give us a better understanding and evaluation of the two historical developments. It will be largely left to future work to develop the details of this assessment, but some of the later chapters of this book will indicate the direction that this work may take. In the course of the modal turn, the notion of metaphysical necessity began to play an increasingly central role in metaphysical theories about a multitude of different topics. To mention just a few examples, it has been suggested, as mentioned before, that it is the defining feature of a thing’s essential properties that they attach to the object as a matter

25 of necessity, that the concept of truth-making can be explained in terms of a modal relationship between worldly entities and true claims, and that the idea that the B-facts are ‘nothing over and above’ the A-facts, or that the A-facts are more fundamental than the B-facts, can be captured by a supervenience thesis. And counterfactual conditionals, too, have been made to do heavy metaphysical lifting in a range of areas. These developments made it natural to think of the exploration of modal facts as one of the metaphysician’s chief occupations. More recently, this idea has come in for criticism. Not that there aren’t important connections between many metaphysical theses on the one hand and modal claims on the other. The claim that the B-facts are nothing over and above the A-facts, appropriately understood, may entail a (suitably formulated) supervenience thesis. The view that x is essentially P seems to entail that x is necessarily P (if x exists). And perhaps the thesis that x is a truth-maker of claim P entails that x’s existence necessitates P’s truth. The problem is that in all of these cases the entailment goes only in one direction—from the metaphysical claim to the modal one. The supervenience of B-facts on A-facts does not entail that the former are in any interesting sense nothing over and above the latter, a necessary property of a thing need not be essential to it, and P’s truth-maker may not be the only entity whose existence necessitates P’s truth.21 There is therefore no obvious way of formulating modal claims that are equivalent to the relevant metaphysical theses, let alone modal claims that capture the intended contents of these theses. Examples like this have recently motivated a trend among some philosophers to doubt that modal concepts deserve the central role in metaphysical theories that they were given in the wake of the modal turn. In the context of what I called the ‘causal turn,’ in particular its second stage, the recent rise in interest in such notions as metaphysical explanation or grounding, it has become common in some circles to suspect that these concepts should do the work that had previously been assigned to modal notions. Maybe the best way of spelling out the idea that the B-facts are nothing over and above the A- facts is in terms of metaphysical explanation, and perhaps the distinctive feature of the

21 In many metaphysical applications of counterfactuals, the inference arguably fails in both directions. For example, contrary to classic formulations of the counterfactual analysis of causation, standing in the ancestral relation of counterfactual dependence is neither necessary nor sufficient for causation (see Chapter 10 for more discussion). 26 essential truths about a thing is their special explanatory role. And the truth-maker of a true proposition P may be thought of as some entity whose existence is responsible for P’s truth. I am sympathetic to this shift of focus from the modal to the explanatory domain and my account promises to provide a theoretical foundation for it. On my view, facts about explanatory relationships are more fundamental than modal facts and therefore better candidates to be the subject matter of fundamental metaphysics. At the same time, my theory makes it unsurprising that modal considerations have figured so prominently in philosophical debates whose ultimate concern is with explanation. For it entails that some modal facts (e.g. facts about counterfactual dependencies, supervenience, or about what’s metaphysically necessary) reflect facts about explanatory connections or facts that are of metaphysical interest because of their central and fundamental explanatory role (e.g. facts about the of things and other facts about what the metaphysical laws are). Modal facts therefore constitute an important set of data in metaphysics. As I will try to illustrate with some examples in Chapter 7, we can evaluate hypotheses about essence, the metaphysical laws, relations of metaphysical explanation, relative fundamentality, and so forth, by their consistency with the modal data and their ability to explain them.22 (Of course there will often be different and conflicting hypotheses that can explain the same data, in which case the tie can only be broken by appealing to other criterion of theory evaluation.) In all of these cases, modal facts are not themselves the ultimate objects of investigation. They are of interest solely in their role as evidence. That, I think, is the main way in which modal facts are important in metaphysics.

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