Modal Epistemology

Philosophy Compass 3/4 (2008): 664–684, 10.1111/j.1747-9991.2008.00147.x

Modal Epistemology: Our Knowledge of Necessity and Possibility Simon Evnine* University of Miami

Abstract

I survey a number of views about how we can obtain knowledge of modal propositions, propositions about necessity and possibility. One major approach is that whether a proposition or state of affairs is conceivable tells us something about whether it is possible. I examine two quite different positions that fall under this rubric, those of Yablo and Chalmers. One problem for this approach is the existence of necessary a posteriori truths and I deal with some of the ways in which these authors respond to the problem, including the use of two-dimensional modal semantics. Conventionalism about modality offers a complementary approach to modal epistemology, prompting us to identify our knowledge of modal truths with our mastery of linguistic or conceptual conventions. Finally, I discuss an approach to modal epistemology deriving from David Lewis’s work that seeks to identify structural features of the modal space over which necessity and possibility are defined.

The principal question of modal epistemology is how we can have knowledge of modal propositions.1 Modal propositions are propositions of the form necessarily p or possibly p (alternatively: it is necessary that p, etc.). Modal idioms – which besides ‘necessary’ and ‘possibly’, include their close relatives such as ‘can’, ‘must’, etc. – have many uses. There are clearly epistemic uses in which ‘possibly p’ means something in the neighborhood of ‘p is consistent with all my evidence’ (see De Rose for more extensive treatment). There are uses connected with natural laws, as when we say that it is not possible for anything to travel faster than the speed of light. The interpretation of modal language that is at issue in discussions of modal epistemology is what is often called metaphysical modality. For example, one might argue that even though the laws of nature rule out anything’s traveling faster than light, those laws themselves could be different, and if they were, then something could travel faster than light. The ‘could’s in the preceding sentence are examples of metaphysical possibility. By contrast, however different the laws of nature might be, it is impossible for anything to be taller than itself. Hence, it is (metaphysically) necessary that nothing is taller than itself.2 © 2008 The Author Journal Compilation © 2008 Blackwell Publishing Ltd

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If something is necessarily the case, then of course it is actually the case. And if something is actually so, then it is possible. Hence, necessity and possibility are not opposites. The former entails the latter. But if we define ‘it is contingent that p’ as ‘it is possible that p and it is possible that not p’, then we have, in contingency, a modality opposed to necessity, and likewise, opposed to impossibility. So, we can see modality as exclusively and exhaustively comprising the necessary, the contingent, and the impossible. In fact, all these modalities can be defined in terms of any one of them along with negation. We have just seen how contingency is defined in terms of possibility. ‘Necessarily p’ is equivalent to ‘not possibly not p’ and ‘impossibly p’ to ‘not possibly p’ or ‘necessarily not p’. The reason that there is a distinctive question about knowledge of modal propositions is that our ordinary methods for gaining knowledge – perception, inference, memory, and introspection – seem to give us no non-trivial modal knowledge. By trivial modal knowledge I mean cases where, by knowing p, we can infer ‘possibly p’. Interesting cases of modal knowledge include all propositions of the form necessarily p and impossibly p, and instances of possibly p that cannot be inferred from knowledge of p itself. The standard methods of knowledge acquisition only inform us of what is or is not the case. They are silent about what must be and what might have been but isn’t.3 So, if (non-trivial) modal knowledge is possible, if we are not to be modal skeptics, some further account must be given of how knowledge of modal propositions is attainable by us. Why should we be interested in how, if at all, we can arrive at modal knowledge? It is often suggested that one of the chief features of human intelligence is our ability to go beyond reality as it is and concern ourselves with how things might, or might not, be different – to make modal judgments, in other words. To the extent that such thinking informs our actions, it is crucial that we be able to identify when modal knowledge is possible. In the moral arena, attributions of praise and blame are often thought to rest on judgments that someone could, or could not, have acted differently. If they do, then such attributions will be reasonable only when the underlying modal judgments are reasonable. Finally, in philosophy itself, many arguments and positions turn on the acceptability or otherwise of some modal proposition. Take, for instance, Descartes’s view that the mind and body are distinct. One of his arguments for this depends on the premise that it is possible for the mind to exist without the body. But, assuming that no actual mind does exist without a body, how are we to know whether such a thing is possible or not?4 I. Conceivability and Possibility Historically, the main view of how we can gain modal knowledge is that there is a connection between modal facts and some sort of mental activity © 2008 The Author Philosophy Compass 3/4 (2008): 664–684, 10.1111/j.1747-9991.2008.00147.x Journal Compilation © 2008 Blackwell Publishing Ltd

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(conceiving, imagining, intuiting).5 At its most programmatic the view can be expressed by the principle: CP) If it is conceivable that p, then it is possible that p.

This applies to a multitude of distinct views whose variations occur chiefly along three dimensions: (1) the nature of the relevant mental state or activity here referred to generically as conceiving; (2) the strength of the connection between conceivability and possibility, here expressed as an unqualified conditional; (3) the ground of the connection between conceivability and possibility. Variation along these dimensions occurs according to complex patterns of philosophical ideology but we can discern two broad positions defined with respect to these three parameters. The first position starts from a realist view of modality that takes modal facts to be ‘out there’ independently of our languages, concepts, conventions and responses. On this view, the connection between conceivability and possibility will be one between two independent notions. Thus, one would expect such realist views to see that connection as less strong than entailment. Rather than guaranteeing possibility, conceivability is likely to be treated as a fallible guide to it. But this, in turn, makes it harder to understand the nature of the connection. Why, exactly, should our powers of conception be any kind of guide to a realm of independent facts about what is possible (but non-actual)? Finally, such views are likely to see conceivability itself as a kind of extension of our outwardly directed cognitive capacities by means of imaginative exploration. The second position starts, conversely, from an anti-realist conception of modality that sees modal facts as somehow constitutively bound up with our languages, concepts, conventions, or responses. Such a view of the nature of modal facts will provide the grounds for some kind of analytic or constitutive connection between conceivability and possibility. And, if there are constitutive relations between conceivability and possibility, then one might expect that conceivability, at least suitably constrained and idealized, will entail possibility rather than merely gesture in its direction. Finally, the conception of conceiving associated with this line of thought will see it as a kind of inner exploration, an unfolding of something – concepts, conceptual structures, linguistic meanings – within us. No doubt, there is room for all sorts of variation on these two positions but, as stated, they fit remarkably well the views of Stephen Yablo (‘Is Conceivability a Guide to Possibility?’) and David Chalmers (‘Does Conceivability Entail Possibility?’) respectively. I shall therefore concentrate on these two authors to bring out how those who have a common allegiance to CP can differ on almost all substantive issues. Omitting the necessary relativization to persons and times, Yablo takes conceivability as follows: © 2008 The Author Philosophy Compass 3/4 (2008): 664–684, 10.1111/j.1747-9991.2008.00147.x Journal Compilation © 2008 Blackwell Publishing Ltd

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CON) p is conceivable only if one can imagine a world that one takes to verify p (i.e. of which p is true).

Yablo says nothing about what the worlds we can imagine are and fairly little about what is involved in imagining one.6 But that it is somehow a matter of looking outwards is suggested by his remark that ‘our best idea [of modal reality] is that of an external constraint on the outcome of a certain type of investigation: in the modal case, investigation by imagination’ (‘Is Conceivability a Guide to Possibility?’ 38). The degree of the strength of the connection between conceivability and possibility is clearly allowed to be one of no more than reliable indication. Often, when someone seems to have conceived of an impossibility, it will be revealed on further examination either that she has failed to conceive anything, or that she has conceived of something, but not the alleged impossibility. Nonetheless, according to Yablo, modal error is possible; one can genuinely conceive of impossibilities. If the relation between conceivability and possibility is itself not a conceptual matter, what accounts for there being a link at all? Here, as one would expect, Yablo has the least to say. Rather than offer any positive account of the connection, he starts from the view that modal appearances, just like sensory appearances, are prima facie reliable. After all, we unthinkingly treat them as such not only in ordinary thought but even when we philosophize, he maintains, no matter how skeptical we may be about such a link when we turn our sights on that particular philosophical problem itself. His strategy is to defend this common-sense view by rebutting objections to it. An interesting twist on Yablo’s position is provided by Peter van Inwagen. Van Inwagen is, as far as I know, unique in not seeking a unified account of our knowledge of possibility. He argues that we do have knowledge of various mundane possibilities, concerning, for example, the location of ordinary objects in a room, but declines to invoke any theoretical account of how this knowledge is possible. Indeed, he says he regards much of it as ‘mysterious’ (250). For more philosophically interesting cases, such as the possibility of disembodied existence, he tentatively endorses Yablo’s account but with the following twist. Although Yablo’s account is clearly developed to explain unchallenged cases of apparent modal knowledge (including the mundane kind that van Inwagen treats separately), it does not, of course, follow from his account of the link between conceivability and possibility that we can gain much knowledge of possibility this way. Conceiving, in the special sense intended by Yablo, might in fact be very hard to do properly. Van Inwagen argues that the ‘imagining a world’ part of Yablo’s definition of conceivability is generally something we cannot do in sufficient detail to be justified in taking an imagined world to verify (or falsify) a given claim. Thus, on the basis of Yablo’s account, van Inwagen supports modal skepticism about interesting possibilities such as disembodied personal existence. © 2008 The Author Philosophy Compass 3/4 (2008): 664–684, 10.1111/j.1747-9991.2008.00147.x Journal Compilation © 2008 Blackwell Publishing Ltd

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In Chalmers’s ‘Does Conceivability Entail Possibility?’, we find agreement with Yablo on CP but disagreement on almost everything else. Unlike Yablo, Chalmers wishes to show that conceivability, properly qualified and understood, is more than just a guide to possibility. It must entail possibility. For Chalmers, the trick is to provide the appropriate understanding and qualifications of conceivability. These qualifications are guided by his views on why conceivability is connected to possibility. Chalmers’s take on modality is arguably at the more anti-realist end of the spectrum. This is evident in his argument for the existence of a link between conceivability and modality. The argument is made quickly in the last few pages of ‘Does Conceivability Entail Possibility?’ and a little more fully in an earlier paper (‘Materialism’ 489–91). Chalmers calls conceivability a rational notion. This means that it is closely associated (as we shall see) with the a priori and with idealized versions of actual mental states and processes. His argument for CP is that modal notions exist as part of a theory the point of which is to explain such rational notions. The only work for modal notions to do is ‘analyzing the contents of thoughts and the semantics of language, giving an account of counterfactual thought, analyzing rational inference’ (‘Does Conceivability Entail Possibility?’ 193). Modal notions are designed to align with an interrelated set of rational notions that includes conceivability. To put it another way, Chalmers thinks that if conceivability did not entail possibility, it would be unclear what we were doing when we made judgments about possibility and necessity. To be plausible, this argument requires extensive elaboration (more, I think, than Chalmers has so far provided) in which the uses and the theoretical underpinnings of modal discourse are evaluated and an assessment is made of a variety of theories as to what the point of such discourse is.7 What conception of conceivability would one expect to go with this view of the connection between conceivability and modality? Chalmers characterizes three pairs of distinctions which qualify conceivability, the explanation of one of which I defer to section III. First, he distinguishes prima facie from ideal conceivability. Ideal conceivability is conceivability that cannot be undone by further rational reflection.8 Secondly, there is a distinction between what he calls positive and negative conceivability. Positive conceivability appears to be very close to Yablo’s notion of conceivability as stated in CON. This might be thought to complicate the contrast I am seeking to draw between Yablo and Chalmers. But in fact, Chalmers is really much more interested in negative conceivability and a closer comparison of his characterization of positive conceivability with Yablo’s would, I think, bear this out.9 Something is negatively conceivable just in case it cannot be ruled out a priori. Thus, one thesis in the vein of CP that Chalmers is sympathetic to (abstracting away from issues connected with the third pair of distinctions) is that NCP) If p is ideally negatively conceivable, then it is possible that p.10 © 2008 The Author Philosophy Compass 3/4 (2008): 664–684, 10.1111/j.1747-9991.2008.00147.x Journal Compilation © 2008 Blackwell Publishing Ltd

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In other words, if p cannot be ruled out a priori, and no further rational reflection would rule it out a priori, then it is possible. Given the argument considered above for the existence of a connection between conceivability and possibility, and given the reliance on idealization that makes it hard or impossible for an ordinary thinker to be sure she has satisfied the antecedent of an instance of NCP, it should be evident that NCP has a lot less bite to it than Yablo’s interpretation of CP. In recompense, however, Chalmers can provide some account of why there is a connection between conceivability and possibility in the first place. There are two important objections to CP that I will address. The first I shall deal with here but some of the issues raised will be further explored in section IV. The other will occupy us in sections II and III. The first objection is that CP gives us access to possibility alone. By itself, it says nothing about how we might come to know propositions of the form necessarily p. To access necessary truths along the lines laid out by CP, one would have to commit oneself to its inverse: ICP) If p is inconceivable, then it is not possible that p.

(Recall that ‘not possibly p’ is equivalent to ‘necessarily not p’.) Or to put it slightly differently, one would have to commit oneself to seeing conceivability as not only accessing possibility but as exhausting it. ICP is a lot less plausible than CP. Why should we think that conceivability gets all the way to the bottom of possibility? Why should there not be things that are possible that we cannot conceive? And if so, how can we conclude from the fact that something is inconceivable that it is necessarily not the case? How one feels about ICP is, of course, likely to be affected by the reasons for which one accepts CP in the first place. Yablo, who, as we have seen, does not really offer any explanation of why CP is true and seeks merely to defend our ordinary reliance on it, would presumably defend ICP only if our ordinary practice relied on it too. But it seems to me that both in philosophy and ordinary life, we do not unthinkingly accept ICP, or at least, we do not on anything like the scale of CP. Although Yablo does not explicitly endorse ICP he does rely on it implicitly at one point. His account of how, dialectically, we might respond to a person’s making a modal error (which I have not explained here) requires him to show how we might be justified in believing propositions of the form if q, then necessarily not p. His answer is that we suppose q and try to conceive of p on that basis. If p is inconceivable on that basis, we can conclude ‘if q then necessarily not p’ (33–6). So, Yablo should either defend ICP explicitly or amend his account of modal dialectic.11 Chalmers, however, may have better reason to adopt ICP given CP. For Chalmers, conceivability is about following out the consequences of our concepts. Although it is not to be assumed that if our concepts allow something we will be able to detect that (after all, our concepts might be © 2008 The Author Philosophy Compass 3/4 (2008): 664–684, 10.1111/j.1747-9991.2008.00147.x Journal Compilation © 2008 Blackwell Publishing Ltd

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opaque to us) it is certainly more plausible to suppose that we can see all the way to the bottom of our concepts than that we can see all the way to the bottom of some external modal domain to which our imagination gives us some, not very well-understood, access. In addition, Chalmers appeals, as we saw, to ideal conceivability. Again, it is more plausible to suppose that ideal conceivability exhausts what is possible than to suppose the same of ordinary conceivability. The price Chalmers pays, however, for such a defense of ICP is to distance the means by which, he argues, we have access to what is possible or necessary from ordinary human epistemic endeavor. That ideal conceivability may give knowledge of what is necessary doesn’t give us all that much reason to think that we can have any such knowledge. II. The Necessary A Posteriori I come now to the second major objection to CP, perhaps the most important objection it faces. Prior to the original publication of Saul Kripke’s Naming and Necessity (1972), it was not at all uncommon to see rational or epistemic notions, such as a priority and a posteriority, as lining up with the metaphysical modal notions of necessity and contingency. If something was a priori, it was necessary. If it could not be ruled out a priori, it was possible. If something could only be known after investigation, however, and was hence a posteriori, it was contingent – both it and its negation were possible. Kripke (Naming and Necessity) challenges this cozy alignment and argues for the existence of truths that are both necessary and a posteriori. His examples come in at least three varieties. Some concern natural kinds, for example, that water is H2O or that cats are mammals. Some concern the origins or composition of individuals, such as that my desk is made of metal. And some are identities, such as that Mark Twain is Samuel Clemens. We will not explore here Kripke’s reasons for thinking that these propositions, and others like them, are necessary but will simply assume he is right about this. (If he is wrong, so much the better for CP.) It should be clear, however, that they are indeed a posteriori. One cannot ascertain their truth merely by reflecting on them. The problem generated by such examples for CP is this. If these examples are a posteriori then, at least prior to our having conducted the requisite investigations, they could not have been known to be true. Hence, for all we knew, it might have been their negations that were true. But then surely their negations were conceivable. Yet if these examples are necessarily true, their negations are impossibilities. So their negations would be cases that were conceivable and yet not possible. Thus, if there are, as Kripke and others have thought, necessary a posteriori truths, conceivability outruns possibility and we cannot assume, just because some state of affairs is conceivable, that it might really have obtained.12 Defenders of CP have wielded an impressive array of ideas to address the problem. There are, basically, three ways in which to respond. First, © 2008 The Author Philosophy Compass 3/4 (2008): 664–684, 10.1111/j.1747-9991.2008.00147.x Journal Compilation © 2008 Blackwell Publishing Ltd

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one might simply acknowledge that such examples show that the connection between conceivability and possibility is not an infallible one and that the negations of a posteriori truths are a rich source of modal mistakes on our part. Secondly, one can deny that the negations of necessary a posteriori truths are conceivable. Thirdly, one can argue they are, at least in some sense, possible. Once again, we see a point of divergence between Yablo and Chalmers. Yablo employs a combination of the first two responses; Chalmers opts for the third. To understand Yablo’s response, let us concentrate on the example he uses, namely ‘Hesperus is distinct from Phosphorus’. Call this sentence, the negation of a necessary a posteriori truth, P.13 Assume that I know that Hesperus is identical to Phosphorus. The question before us is, is P conceivable? There are certainly some things in the neighborhood which are conceivable. For example, I can imagine having a lot of evidence for P, none against it, and believing it with excellent justification. But this does not count as conceiving that P for Yablo since that requires imagining a world of which P is true and not merely justifiably believed. I can also imagine the following: that there are two distinct heavenly bodies visible in the sky early in the evening and late in the morning; that these heavenly bodies were named ‘Hesperus’ and ‘Phosphorus’ under conditions resembling those under which the planet Venus was in fact dubbed ‘Hesperus’ and ‘Phosphorus;’ and that, in these circumstances, I believe something true which I would express with the sentence ‘Hesperus is distinct from Phosphorus’. But, according to Yablo, what I would express with this sentence under the imagined conditions is not what I actually express with it, and hence in this scenario, I am not conceiving that Hesperus is distinct from Phosphorus. And this, Yablo says, I cannot conceive. Why, exactly, does Yablo think I cannot conceive that Hesperus is distinct from Phosphorus? The reason is not that Hesperus’s being distinct from Phosphorus is a metaphysical impossibility. It is that, of course (since we have assumed Kripke is right about such cases). And one could argue at this juncture that since it is impossible for Hesperus to be distinct from Phosphorus, when I imagine a situation which I take to verify the claim that Hesperus is distinct from Phosphorus, I cannot succeed and must be guilty of misdescribing what I am imagining. The phenomenon of misdescribing the object of one’s imagination is an important one, and one that Yablo makes use of in certain contexts. But the problem with this strategy is that it simply defines conceivability in such a way as to disallow conceiving of impossibilities thereby saving CP by rendering it vacuous. In any case, this is not Yablo’s reason for insisting that, under the circumstances described, I cannot conceive P. For as we have seen, Yablo does allow that conceivability is not an infallible guide to possibility and hence that we can conceive of something even if it is impossible. Even P can be conceived by someone, if that person does not know that Hesperus is identical to Phosphorus. (Thus, as indicated above, part of Yablo’s © 2008 The Author Philosophy Compass 3/4 (2008): 664–684, 10.1111/j.1747-9991.2008.00147.x Journal Compilation © 2008 Blackwell Publishing Ltd

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response to the necessary a posteriori is to allow that conceiving is not an infallible guide to possibility.) The reason Yablo gives as to why P is inconceivable to someone who does know that it is false is that, knowing Hesperus and Phosphorus to be the same thing, it simply baffles the imagination to come up with a way in which they could be distinct. It would be like, indeed it would actually be, trying to imagine something’s being distinct from itself. I take it that Yablo’s point is that this is not a simple stipulation on the basis of P’s impossibility that we cannot imagine P. Rather, there is a genuine cognitive inability at work. One’s mind simply cannot come up with a way in which something can be distinct from itself. Try it and you’ll end up with a mental cramp. Unfortunately, it is not clear that this strategy will work for other cases of negations of necessary a posteriori truths, even in circumstances in which we know the relevant truth and know that it is necessary. Take, for example, the proposition that this (some wooden table) is made of wood. Suppose I know that it is made of wood and know (or rationally believe) that artifacts are necessarily composed of whatever it is they are actually composed of. Extending Yablo’s line of thought from the ‘Hesperus is distinct from Phosphorus’ case, one would have to say that it baffles the mind to imagine this table’s being made of, say, metal. But it is hardly obvious that such a proposition baffles the imagination in anything like the way that the proposition that Hesperus is distinct from itself does. Yablo must either insist that it does baffle the imagination in something like the same way (which is implausible) or allow that conceivability is no guide to possibility here as well, thus enlarging the gap between conceivability and possibility. It is noteworthy that although Yablo allows that conceivability is only a fallible guide to possibility, he does not, officially, simply identify the shortfall with the negations of Kripkean necessary a posteriorities. What we have been seeing in this paragraph, however, is that the class of those things which are conceivable but not possible, and the class of negations of necessary a posteriori truths, may be much closer than the official story lets on.14 III. Two-Dimensionalism Above I said that Chalmers’s reaction to the problem posed by the negations of necessary a posteriori truths was to argue that in some sense, they are after all possible and hence not counter-examples to CP. Since the negation of a necessary truth is an impossibility, it is evident that Chalmers must make some distinctions to avoid contradiction. He will thus be able to say that the negations of necessary a posteriori truths are impossible in one way (since those truths are necessary) and yet conceivable, and hence, according to CP, possible, in another way. The framework within which he distinguishes two notions of possibility and necessity is known as two-dimensional modal semantics. It is impossible here adequately to © 2008 The Author Philosophy Compass 3/4 (2008): 664–684, 10.1111/j.1747-9991.2008.00147.x Journal Compilation © 2008 Blackwell Publishing Ltd

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explain two-dimensionalism.15 But I shall attempt to give the gist of it below in order to explain Chalmers’s defense of CP against the objection raised by necessary a posteriori truths. Take ‘Hesperus is Phosphorus’ again. Kripke, in introducing such examples of necessary a posteriori truths, also wanted to do justice to an intuition we might have that, in a certain sense, before we found out that Hesperus was Phosphorus, the question of their identity might have turned out either way. It is possible that there might have been two, distinct heavenly bodies which nonetheless appeared exactly as do Hesperus and Phosphorus. Before we found out that Hesperus was Phosphorus, we would not have been able to tell, a priori, whether our reality was one in which the body we called ‘Hesperus’ was identical to the one we called ‘Phosphorus’, or whether it was a reality in which those bodies were distinct. As Kripke puts it: being put in a situation where we have exactly the same evidence [as we in fact did have], qualitatively speaking, it could have turned out that Hesperus was not Phosphorus; that is, in a counterfactual world in which ‘Hesperus’ and ‘Phosphorus’ were not used in the way that we use them, as names of this planet [Venus], but as names of some other objects, one could have had qualitatively identical evidence [to the evidence we actually had] and concluded [correctly] that ‘Hesperus’ and ‘Phosphorus’ named two different objects. (104)16

Let us put this by saying that, while it is metaphysically impossible that Hesperus is distinct from Phosphorus, it is (or was) epistemically possible for them to be distinct.17 Now, what are we to say about epistemic possibility, so defined, and its relation to metaphysical possibility (which, remember, is the kind of modality the epistemology of which we are investigating)? There are, very broadly speaking, two approaches to this question. The first is to say that epistemic possibility does not co-incide with metaphysical possibility. This view itself has two important sub-varieties. Some hold that, despite the name ‘epistemic possibility’, something’s being epistemically possible is not a way of being possible at all. Epistemic possibility is simply not a variety of possibility. (This is the position taken by van Inwagen; Seddon.) The second sub-variety, advocated by Soames, is to treat epistemic possibility as a genuine kind of possibility but one that is broader than, and hence includes, metaphysical possibility. That Hesperus is distinct from Phosphorus would be an example of something that was metaphysically impossible but epistemically possible. Kripke, I think, takes one of these positions though it is unclear which one. (For some purposes, the distinction between them may not be too important.) There is close link between epistemic possibility and conceivability.18 Just as we said above that it is (was) epistemically possible that Hesperus and Phosphorus are distinct, so we might say that it is (was) conceivable © 2008 The Author Philosophy Compass 3/4 (2008): 664–684, 10.1111/j.1747-9991.2008.00147.x Journal Compilation © 2008 Blackwell Publishing Ltd

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that they were. Thus, to take the first approach to epistemic possibility and its relation to metaphysical possibility, in either of its sub-varieties, would be tantamount to giving up any interpretation of CP on which conceivability entailed possibility. So, as we would expect, Chalmers actively resists the first approach.19 The second approach to the question of the relation between epistemic and metaphysical possibility, developed by Chalmers, is provided by two-dimensionalism. Given the background of our knowledge that in fact, Hesperus is Phosphorus, we cannot conceive them to be distinct. To suppose they were would be, as Yablo put it, like supposing that something was distinct from itself. So here is a sense in which ‘Hesperus is distinct from Phosphorus’ is impossible. It is impossible, given that Hesperus is identical to Phosphorus. But if we consider matters in abstraction from our knowledge of the actual world, it is, as we have seen, epistemically possible that Hesperus is distinct from Phosphorus. If ‘Hesperus is distinct from Phosphorus’ is possible, then there must be some possible states of affairs that would make it true. What are these states of affairs? According to Chalmers, they are states of affairs in which there are two distinct heavenly bodies, one of which looks just like Hesperus actually does, the other of which looks just like Phosphorus actually does, etc. These states of affairs, of course, are themselves metaphysically possible by anyone’s reckoning. So, what makes it true that it is (epistemically) possible that Hesperus is distinct from Phosphorus is just plain old metaphysical possibility after all. And in this sense, it is possible that Hesperus is distinct from Phosphorus. Chalmers calls the two notions of possibility at work here primary and secondary rather than epistemic and metaphysical. (Yablo, ‘Coulda, Woulda, Shoulda’ 449 lights on the happy epithets of ‘counteractual’ and ‘counterfactual’ possibility.) A sentence p is primarily (or counteractually) possible if there are ways the world might actually be such that, if the world is like that, then p is true. Thus, at least before the discovery that Hesperus and Phosphorus were identical, there were metaphysically possible ways the world could have been such that the body called ‘Hesperus’ would be distinct from the body called ‘Phosphorus’. By contrast, p is secondarily (or counterfactually) possible just in case, given how the world actually is, i.e. given that Hesperus is actually identical to Phosphorus, there are ways it could have been such that p would have been true. It is not secondarily possible that Hesperus is distinct from Phosphorus. Now it is likely to be objected at this point that the metaphysical possibilities that make it true that it is primarily possible that Hesperus is distinct from Phosphorus are simply not possibilities in which Hesperus is distinct from Phosphorus but rather possibilities in which some other heavenly bodies that look like Hesperus (i.e., Venus) does in the evening and Phosphorus (i.e., also Venus) does in the morning are distinct from each other (or in which Hesperus/Phosphorus is distinct from some other heavenly body). (That is why, in describing primary possibility in the © 2008 The Author Philosophy Compass 3/4 (2008): 664–684, 10.1111/j.1747-9991.2008.00147.x Journal Compilation © 2008 Blackwell Publishing Ltd

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previous paragraph, I put things in terms of the names ‘Hesperus’ and ‘Phosphorus’ rather than in terms of Hesperus/Phosphorus itself.) Granted that these are metaphysical possibilities, why should they have anything to do with whether it is possible (in any sense) that Hesperus and Phosphorus are distinct? The answer that Chalmers, and others, give is two-dimensionalism. The basic idea is to associate with a given sentence, such as ‘Hesperus is identical to Phosphorus’, two distinct aspects to its meaning. (These are often called intensions or propositions.) Very roughly speaking, one of these, what Chalmers calls the primary intension, is that something that looks like Hesperus (or something that is called ‘Hesperus’ under qualitative evidential conditions similar to those in which, in fact, Hesperus is called ‘Hesperus’) is identical to something that looks like Phosphorus (or mutatis mutandis). The other, secondary, intension is that (the actual, honest-to-goodness) Hesperus is identical to (the actual, honestto-goodness) Phosphorus. The first of these intensions is both contingent and a posteriori. Hence its negation – that something that looks like Hesperus actually does is distinct from something that looks like Phosphorus actually does – is also contingent and a posteriori. It is, in fact, false, but it could have been true. It is metaphysically possible that it is true. The secondary intension of ‘Hesperus is identical to Phosphorus’ is necessary and a priori; its negation – that Hesperus/Phosphorus is distinct from itself – is (metaphysically and epistemically) impossible. Neither one of these intensions or propositions is both necessary and a posteriori; rather the sentence ‘Hesperus is identical to Phosphorus’ is necessary a posteriori because its primary intension is a posteriori and its secondary intension is necessary.20 If this sketch of a theory can be worked out and defended, it provides the answer to the question at the end of the previous paragraph: why is the metaphysical possibility of a situation in which, not Hesperus and Phosphorus, but some other heavenly bodies that look like Venus in the evening and Venus in the morning are distinct, relevant to the possibility that Hesperus is distinct from Phosphorus? The answer is that that possibility is, in some complex way, part of the very meaning of the sentence ‘Hesperus is distinct from Phosphorus’. Not only does Hesperus/ Phosphorus itself feature in an account of the semantics of the sentences ‘Hesperus is identical to Phosphorus’ and ‘Hesperus is distinct from Phosphorus’. So do other possible things that would have appeared as Hesperus does in the evening or Phosphorus does in the morning.21 We are now in a position to complete Chalmers’s analysis of conceivability, a task we left unfinished in section I. Besides being prima facie or ideal, and negative or positive, conceivability may also be primary or secondary. Something is primarily conceivable if it is conceivable considered as how things might actually be – that is, in abstraction from our knowledge of how they in fact are; it is secondarily conceivable if it is conceivable considered as counterfactual (i.e., given that the world is actually as it is). © 2008 The Author Philosophy Compass 3/4 (2008): 664–684, 10.1111/j.1747-9991.2008.00147.x Journal Compilation © 2008 Blackwell Publishing Ltd

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So, ‘Hesperus is distinct from Phosphorus’ is primarily conceivable and primarily possible, but not secondarily conceivable or secondarily possible. And (one of ) Chalmers’s preferred precisification(s) of CP is that PCP) ideal primary conceivability entails primary possibility.

(I gloss over difficulties arising from the difference between positive and negative conceivability.) IV. Conventionalism about Necessity In the previous two sections we have been dealing with the problems generated by necessary a posteriori truths for CP. The form of the problem was this: according to CP, conceivability is a guide to, or entails, possibility. Take some truth that we know is necessary and a posteriori, such as that Hesperus is identical to Phosphorus. Its negation seems to be conceivable but impossible. Hence it is potentially a counter-example to CP. There is, however, a further problem to address. The objection just outlined starts with the assumption that we know that it is necessary that Hesperus is identical to Phosphorus and hence that it is impossible that they are distinct. Yet, we may ask, how can we come by that knowledge? It should be stressed here that our problem is not how we know such things as that Hesperus is identical to Phosphorus or that water is H2O. Such things can be known through ordinary empirical methods.22 What is not obvious is how we can know that such things are not just true, but true necessarily. One solution to this problem is provided by conventionalism about modality.23 Conventionalism, in the sense in which I am interested in it here, is a theory about the nature of modality; but as we have seen, one’s views about modal epistemology are likely to be seriously affected by one’s theory of modality. And conventionalism, as argued for in the work of Alan Sidelle, is largely supported on the grounds that it, and it only, can show how we can have knowledge of necessities. The basic idea is that necessity is to be explained in terms of analyticity (truth in virtue of meaning) which, in turn, is accounted for by the existence of conventions or rules about how to use language. Necessity is thus, according to conventionalism, a purely linguistic phenomenon and our knowledge of it derives from our knowledge of the conventions that govern our use of language. Consider the classic example, ‘all bachelors are unmarried’. The conventionalist argues that this sentence is analytic because there exists a rule, or convention, that the word ‘bachelor’ should not be applied to anyone who is married. If we know of the existence of this convention, then we know that not only is ‘all bachelors are unmarried’ true; so is ‘necessarily, all bachelors are unmarried’. There cannot be a married bachelor because the relevant linguistic convention precludes anyone who is married from being correctly described as a bachelor. It is, of course, a © 2008 The Author Philosophy Compass 3/4 (2008): 664–684, 10.1111/j.1747-9991.2008.00147.x Journal Compilation © 2008 Blackwell Publishing Ltd

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contingent fact that the word ‘bachelor’ in English is governed by this particular convention. But it would be a mistake to infer from this that ‘necessarily, all bachelors are unmarried’ cannot be true. ‘Necessarily, all bachelors are unmarried’ is quite distinct from ‘necessarily, the sentence “all bachelors are unmarried” expresses a truth’ which is ruled out by the contingency of our conventions. Conventionalism, of course, needs extending if it is to be made to work for necessary a posteriori truths since these are clearly not themselves analytic. Someone seems quite well able to master the names ‘Hesperus’ and ‘Phosphorus’ and yet still be ignorant as to whether Hesperus is identical to Phosphorus. Again, one seems able to master the term ‘water’ without knowing whether water is H2O or not. (To deny this would commit one to holding that the word changed its meaning once it was discovered that water is H2O.) The extension offered by Sidelle is ingenious. A number of people have argued that necessary a posteriori truths are the result of plugging some empirical, a posteriori information in to a higher-order principle that says that such types of claims are, when true, necessarily true. For example, we learn a posteriori that water is H2O and plug that in to a schema according to which chemical kinds (i.e., kinds like water) have their chemical structures necessarily. From that, we can infer that necessarily, water is H2O. Or, we learn empirically that Elizabeth II’s father was George VI, plug that in to a schema according to which persons have the parents they do necessarily, and infer that necessarily, Elizabeth II’s father was George VI. Sidelle’s treatment of necessary a posteriori truths is a version of this approach. For anyone who accepts something like this approach and wants to give an account of modal knowledge, the obvious next question is, how do we gain knowledge of the schemata that chemical kinds like water have their chemical structures necessarily or that people have their parents necessarily? The conventionalist simply applies to these higher-order principles the move that was made to explain the necessity of analytic truths like ‘all bachelors are unmarried’. Chemical kinds have their chemical structures necessarily because it is analytic that they have their actual chemical structures. And this analyticity, in turn, is explained by conventions governing how to apply expressions like ‘chemical kind’.24 If we have knowledge of the relevant conventions, we can infer the consequence that chemical kinds have their actual chemical structures (whatever those are) necessarily just as, if we know the conventions governing ‘bachelor’, we can infer that it is necessary that all bachelors are unmarried. Now, armed with knowledge of the necessity of the higher-order schema, we can plug in a piece of ordinary, a posteriori information, such as that the actual chemical structure of water is H2O, and infer that this too, is necessary. So, the higher-order schemata are analytic, and the knowledge of them a priori, even though the results of applying them to particular empirical claims (such as that the actual chemical © 2008 The Author Philosophy Compass 3/4 (2008): 664–684, 10.1111/j.1747-9991.2008.00147.x Journal Compilation © 2008 Blackwell Publishing Ltd

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structure of water is H2O) are necessities that are not themselves analytic or knowable a priori. Knowledge of necessities, on the conventionalist view, thus turns on knowledge of linguistic conventions. How do we obtain knowledge of these? It might be objected that knowledge of conventions governing language can only be gained by empirical observation of people’s linguistic behavior and hence that, not only is the existence of the conventions contingent, but knowledge of them must be a posteriori. I do not think this is a very plausible claim but in any case, strictly speaking, it is not fatal to conventionalism. For it is the existence of the conventions that makes certain claims analytic and hence necessarily true. Conventionalism would, if the conjecture being considered were true, have to shed the view that analytic truths are knowable a priori. All necessities, depending for their truth on linguistic conventions, would be knowable only a posteriori. But since, as we have just seen, the conventionalist can account for a posteriori knowledge of necessities, this is not a serious problem.25 In any case, Sidelle does not accept that linguistic conventions can be known about only by empirical observation of linguistic behavior. Rather, Sidelle appeals to CP. We can gain explicit knowledge of the conventions governing our language by trying to conceive of things describable in our language and succeeding or failing. We try to conceive of a person whom we would correctly describe as a married bachelor. In failing, we gain explicit knowledge of the convention governing the word ‘bachelor’. Conceiving is thus treated as a way of probing the conventions governing the use of our language to find out what they allow and what they do not. A few comments on this are in order. First, as we saw at the end of section I, it is not CP that a conventionalist needs at this point but ICP, the principle that says that if something is inconceivable then it is impossible. However, if our conceivings are just ways of probing for the presence of linguistic conventions tied to words of which we have mastery, ICP is not so implausible (as it would be if it were supposed to yield access to necessities wholly independent of our own thoughts and conventions). Secondly, if conventions are the origins of necessity, then CP or ICP are not the only ways of gaining modal knowledge. For example, if I explicitly introduce a term t with the stipulation that it apply to all and only things that are F, I do not need conceivability or inconceivability to help me know that necessarily all ts are F. But presumably, to the extent that conventionalism does see modal epistemology as tied to principles like CP and ICP, it is because it holds (plausibly enough) that the relevant conventions are not like my imagined stipulation. Not being made so decisively they cannot be known so directly. Thirdly, various of the relevant conventions have obvious analogues in the material mode. For example, a convention to the effect that if a name N applies to a person, and that person’s parents are x and y, then N cannot correctly be applied to anyone whose parents are not x and y has as a (generalized) counterpart the © 2008 The Author Philosophy Compass 3/4 (2008): 664–684, 10.1111/j.1747-9991.2008.00147.x Journal Compilation © 2008 Blackwell Publishing Ltd

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metaphysical view known as the necessity of origin. This is the view, propounded by Kripke, that things have their origins necessarily. These metaphysical principles stand halfway between the often implicit conventions governing our language and concepts and the consequences of those conventions in particular claims of the form necessarily, a is F. If conventionalism is right, the obvious and most direct way of arguing for such principles would be by bringing to light the underlying linguistic conventions. If, however, one looks at how metaphysicians do in fact argue for things like the necessity of origin, it does not always seem as if this is what they are doing. Colin McGinn, for example, argues for the necessity of origin on the basis of its relation to facts about identity and continuity, which he takes to be necessary. But the necessity pertaining to identity and continuity is taken as given. There is no attempt to link it to the existence of conventions governing our language or concepts. The motivation for conventionalism is to be able to explain modal truth and modal knowledge without mystery, mystery that is thought to beset any realist account of these phenomena. When earlier incarnations of the view fell under suspicion generated by Quine’s attacks on analyticity and truth by convention (see especially ‘Two Dogmas’), the impulse that had fueled those conventionalist views found other outlets.26 In particular, work on ethics was thought to provide some illumination. Moral claims, such as that a given act is good, apparently face a number of the same problems faced by modal claims. Metaphysical issues about what in the world could make true such claims and epistemological issues about how we could ever know the truth of such claims arise in both cases. Expressivist theories of morality took a radical approach. Although utterances of sentences like ‘helping others is good’ seem as if they are assertions, capable of being true (or false) and potential objects of knowledge, according to expressivists they are none of these. Such sentences are used, rather, to express the speaker’s attitudes or feelings to the action in question. If these sentences are not used to make statements, then issues of truth and knowledge simply fall away. Moral utterances are no more true or false than are ejaculations like ‘Hooray for helping others’. Hence, there is no knowledge that needs explaining. This kind of approach in ethics was sometimes extended to modality as well, particularly in the work of Simon Blackburn. Modal claims, too, it was held, do not make statements. Hence they cannot be true or false and are not appropriate objects of knowledge at all.27 There are, in the end, no difficult questions about modal knowledge that need answering.28 V. Structural Possibilities A final approach to the epistemology of modality that I shall deal with here is of a completely different kind from the various views we have examined so far. Up to now, we have considered methods for making © 2008 The Author Philosophy Compass 3/4 (2008): 664–684, 10.1111/j.1747-9991.2008.00147.x Journal Compilation © 2008 Blackwell Publishing Ltd

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isolated forays into the realm of the possible. Through intuition, through imagination, by probing the limits of our concepts or our linguistic conventions, we can, according to the various theories at which we have looked, come to know whether this or that state of affairs is possible or necessary. The work of David Lewis suggests another question: can we discern anything about the structure of possibility as a whole? Are there ways in which we can, as it were, map features of the whole of modal space? Lewis answers in the affirmative. He argues for a principle of modal plenitude according to which there are ‘no gaps in logical space’ (86). Plenitude comes in several different forms. One form it takes is as a principle of recombination. The basis for this principle is Lewis’s commitment to a Humean view that nothing logically depends for its existence on the existence of any distinct thing. This leads to a mix-and-match extravaganza in which for any two possible things, it is possible for them to co-exist. Of course, this tells us nothing about which things are possible in the first place; it simply allows us to recombine things the possibility of which we already know into further possibilities. Thus, if it is possible that a dragon exists and possible that a talking donkey exists, then it is possible that there co-exist a dragon and a talking donkey. It also allows us to subtract possibilities. If my cat is a distinct existence from everything else that actually exists (except her parts), then recombination assures me of the possibility that my cat might not exist while everything else does. There are further ways in which commitment to the plenitude of logical space can help us know about certain possibilities. It also applies to structural features of the world. A fuller treatment of this issue, in the spirit of Lewis, is given by Phillip Bricker, concentrating specifically on spatio-temporal structures. Bricker argues that, given that some structures are possible, so must certain other ones be. Suppose we know that it is possible that space has three dimensions. According to Bricker (and Lewis), it would be an arbitrary limitation in logical space if spaces with all other numbers of dimensions were not possible too. The main work to be done in defense of a view like this is to characterize, as we did in the case of recombination, just which structures are guaranteed to be possible relative to which other ones. There must be no arbitrary limitations in logical space, but just which limitations are arbitrary? Bricker’s final formulation of plenitude as it applies to structures such as space-time structures is that a structure is possible if it belongs to a class of such structures which is a natural generalization of a class of possible structures. In other words, if some structures are possible, then any structure which belongs to a natural generalization of the given structures must itself be possible. Without getting into more detail about how this view is reached and what exactly it means, suffice it to say that it ends up relying on an undefined notion of naturalness as it occurs in the notion of a natural generalization. Since arbitrariness is, more or less, equivalent in this context to unnaturalness, the answer is not going to persuade anyone who is not © 2008 The Author Philosophy Compass 3/4 (2008): 664–684, 10.1111/j.1747-9991.2008.00147.x Journal Compilation © 2008 Blackwell Publishing Ltd

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already sympathetic to the Lewisian idea. But it is useful to see Lewis’s idea articulated in such detail. Two further reflections on this Lewisian approach to the epistemology of modality are in order. First, as already indicated, and as emphasized very clearly by Bricker, principles of plenitude (including recombination and plenitude of structures) do not tell us anything about what is possible absolutely. What they do is extend our knowledge of what is possible, given the possibility of certain other things. So, as a general epistemology of modality, they need to be supplemented by some account of how we can know of any such possibilities in the first place. Of course, one such source of knowledge is our knowledge of what is actually so. Physics, an empirical science, tells us about actual space-time. From this, we can infer something about other possible space-time structures. Ordinary perception tells us about cats’ heads and dogs’ bodies. From this, by recombination, we can infer the possibility of a creature with a dog’s body and a cat’s head. Bricker is actually more permissive than this in how he allows us to know of possibilities to which to apply such principles as plenitude of structures and recombination. He suggests that we are warranted in believing in the possibility of anything that has played an explanatory role in our theorizing about the actual world. (I generalize his formulation to extend beyond structures.) So, for example, even though space is not actually three-dimensional and Euclidean, we are warranted in believing in the possibility of such a space because it did genuinely, at one time, play an explanatory role in our physics. Secondly, although Lewis rejects CP – he calls imaginability ‘a poor criterion of possibility’ (90) – he offers an explanation of why it seems to work when it does. At least one capacity of imagination is to recombine elements of our experience in novel ways. We can imagine a gorgon by recombining our visual impressions of snakes with our visual impressions of a person’s head. Since recombination is itself a key to what is possible, it is evident that imagination should often be able to inform us of what is possible. It has the same mix-and-match feature that characterizes logical space itself. Acknowledgment Many thanks to Susanna Siegel and Amie Thomasson for advice and suggestions. Short Biography Simon Evnine works on epistemology, metaphysics, and the philosophy of mind. His new book Epistemic Dimensions of Personhood is published by Oxford University Press (2008). He teaches at the University of Miami and before that was at California Polytechnic State University. He has a © 2008 The Author Philosophy Compass 3/4 (2008): 664–684, 10.1111/j.1747-9991.2008.00147.x Journal Compilation © 2008 Blackwell Publishing Ltd

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Bachelor of Music degree from King’s College London, an M.Phil in philosophy from University College London and a Ph.D. in philosophy from UCLA. Notes * Correspondence address: Simon Evnine, University of Miami, PO Box 248054, Coral Gables, FL 33124-4670, USA. Email: [email protected]. 1 Knowledge or justified belief. In this article I will not be concerned with what distinguishes them. 2 The profusion of terminology regarding varieties of modality and the fact that there is considerable disagreement over whether there are substantive rather than merely terminological differences in play make the whole issue of varieties of modality a minefield. I will have something to say on this in section III below. The reader might consult Divers 3–9; Fine; van Inwagen for some discussion of these issues. 3 Susanna Siegel argues that perception can give us some knowledge of mere possibility. Perception of a rolling soccer ball, she suggests, tells us in what direction it would have continued rolling had it not been kicked. I take no stand on such examples. Even if they are good, they do not, as Siegel agrees, give us very much knowledge of non-trivial possibility. 4 In fact, because Descartes’s line of reasoning is very much alive in contemporary treatments of the mind-body relation, much of the discussion of modal epistemology occurs in that context. 5 See Yablo, ‘Is Conceivability a Guide to Possibility?’ 1–4 for a brief but good run through of some of its historical proponents and critics. An excellent collection of articles on this subject is Gendler and Hawthorne. 6 One important fact he does tell us is that to imagine a world, it is not necessary to imagine all of it. 7 Something in the spirit of this enterprise is undertaken in Blackburn and some of the literature cited therein. 8 Though I think this is in the spirit of Chalmers’s views, it is a non-trivial alteration of how he actually characterizes ideal and prima facie conceivability. Literally, Chalmers treats these as varieties of judgments about conceivability. Something is ideally conceivable if someone could judge that it was conceivable (that the ‘relevant tests’ for conceivability were passed) and have a justification for that judgment that could not be overturned by better reasoning. On this way of understanding the matter, something’s being ideally conceivable does not entail that it is conceivable (at least, not without further philosophical argument). 9 Unfortunately, I don’t have the space to make this comparison here. 10 Chalmers’s final view is that this principle is either equivalent to its counterpart involving positive conceivability or falls slightly short of it and hence is not actually true without qualification. I will ignore these issues here for want of space. 11 George Bealer argues that we should understand CP in terms of intuitions rather than conceivability. Modal intuitions are a priori, intellectual (i.e., non-sensory) seemings. (Bealer then justifies a link between modal intuitions and modal facts by appeal to his general theory of the reliability of intuitions.) On this basis, an intuition of possibility is close to what Yablo describes as conceiving (as Bealer notes): a seeming that p is possible. On Bealer’s account, however, there can be, alongside intuitions of possibility, intuitions of necessity as well. Bealer, therefore, is an advocate of a version of CP who is immune to the objection I am pursuing here. 12 Kripke also argues for the existence of contingent a priori truths, though these have proved more controversial than necessary a posteriorities. It would take too much space to explain Kripke’s examples of contingent a priori truths. Suffice it to say that if there are such, the challenge they pose in modal epistemology is to ICP, the inverse of CP, the thesis that holds that conceivability exhausts possibility. The negations of contingent a priori truths would be cases where possibility outran conceivability; where things that might be true could nevertheless be ruled out a priori and hence could not be conceived.

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‘Hesperus’ was the name given by the ancient Greeks to the planet Venus when visible in the early evening sky; ‘Phosphorus’ was the name given to that same planet when visible in the late morning. At some point it was discovered that Hesperus was identical to Phosphorus. 14 Though it should be said that Yablo nowhere relies on the alleged distinctness of those two classes. 15 Readers can consult Chalmers, ‘Two-Dimensional Semantics’ for an overview of different types of two-dimensionalism and ‘Epistemic Two-Dimensional Semantics’ for an exposition of his own favored variety. Jackson is close to Chalmers’s view. Soames is a hard-hitting and detailed critique of two-dimensionalism in general and Chalmers and Jackson in particular. 16 This passage and a similar one (Naming and Necessity 141–4) are illuminatingly discussed in Soames 71–80. 17 A big caveat is required here concerning terminology. First, in the context of his discussion of this issue, Kripke talks of an epistemic ‘might’ (in ‘it might have turned out that Hesperus and Phosphorus were distinct’) that means no more than that the question of their identity was as yet unsettled. This is not the same as what I am here calling epistemic possibility. (Yablo, ‘Is Conceivability a Guide to Possibility?’ 22–5 is, I think, somewhat misleading on the terminology in his discussion of this passage in Kripke.) Secondly, there is a confusing range of terms, such as ‘epistemic’, ‘conceptual’, and ‘logical’, that are used to qualify possibility by different authors to mean different things in different contexts. Some authors speak of conceptual possibility or logical possibility to mean something like what I am here calling ‘epistemic possibility’. ‘Logical possibility’, though, is used by Chalmers as synonymous with ‘metaphysical possibility’. For some views, these distinctions are important, for some they are not. Chalmers, ‘Materialism’ 477–80 diagnoses sustained misunderstanding over this terminology even among philosophers of the highest caliber. The moral is: caveat lector! 18 I leave the relation at an intuitive level here. 19 See also Jackson 68ff; Soames passim on the relation of epistemic to metaphysical possibility. 20 It may be wondered why, if the secondary intension associated with the sentence is a priori, we should not be able to know the truth of the sentence that has that as its secondary intension a priori after all. The answer is that it is not a priori what the secondary intension is that is associated with the sentence. 21 There is thus a sense in which one could see two-dimensional semantics as an attempt to yoke together traditional Kripkean modal semantics (for the second dimension) and a version of David Lewis’s counterpart-theoretic semantics (for the first dimension). 22 There is a problem, proper to modal epistemology but which I cannot explore here, as to why we should need empirical investigation to learn that Hesperus is identical to Phosphorus, given that it is necessarily true. See Stalnaker for one answer to this question. 23 Conventionalism has been pursued independently of two-dimensionalism and the two theories are logically independent of each other. They are, however, in some sense complementary. 24 Getting the exact formulation of these and related claims right turns out to be a bit tricky but I shall skip over the details here. 25 Furthermore, the conventionalist could still make a principled distinction between what were formerly thought of as the necessary a priori and the necessary a posteriori. The empirical information needed to know the former would be restricted to knowledge of linguistic conventions. That required to know the latter would also include information about a kind’s chemical composition and so on. 26 Although I have not discussed this at all here, Sidelle (133ff ) and other latter-day conventionalists (such as Thomasson, Ordinary Objects 28–53; Forbes 224–37) have a lot to say in response to Quine’s arguments against analyticity. 27 In fact, Blackburn combines his expressivist views of both morality and modality with what he calls ‘quasi-realism’, which is an attempt to explain how we get a semblance of truth, knowledge and assertion in the relevant domains. I omit this crucial element of his position here. 28 Amie Thomasson (Ordinary Objects 54–72; ‘Modal Expressivism’) develops an interesting combination of conventionalist and expressivist ideas about modality. According to her, modal claims are explicit demonstrations of the rules that govern the terms used in those claims. Thus, ‘necessarily, all bachelors are unmarried’ is a demonstration of the linguistic rule that tells us not to apply the word ‘bachelor’ to someone who is married. (So is the analytic claim ‘all © 2008 The Author Philosophy Compass 3/4 (2008): 664–684, 10.1111/j.1747-9991.2008.00147.x Journal Compilation © 2008 Blackwell Publishing Ltd

684 Modal Epistemology bachelors are unmarried’. The function of ‘necessarily’ is to indicate that we are dealing with a demonstration of a rule rather than an empirical generalization.) Modal knowledge is acquired when we know how to move from uses of the term ‘bachelor’ (for example, to describe particular people) to the demonstrations of the rule governing the word. Explicit knowledge of the convention itself is not required. 29 Lewis makes certain qualifications to all this that I pass over here. In particular, I completely ignore the issue of transworld identity versus counterpart theory that affects everything Lewis says on this topic.

Works Cited Bealer, George. ‘Modal Epistemology and the Rationalist Renaissance’. Conceivability and Possibility. Eds. Tamar Szabo Gendler and John Hawthorne. Oxford: Clarendon Press, 2002. 71–125. Blackburn, Simon. ‘Morals and Modals’. Essays in Quasi-Realism. Oxford: Oxford UP, 1993. 52–74. Bricker, Phillip. ‘Plenitude of Possible Structures’. Journal of Philosophy 88.11 (1991): 607–19. Chalmers, David. ‘Does Conceivability Entail Possibility?’. Conceivability and Possibility. Eds. Tamar Szabo Gendler and John Hawthorne. Oxford: Clarendon Press, 2002. 145–200. ——. ‘Epistemic Two-Dimensional Semantics’. Philosophical Studies 118.1–2 (2004): 153–226. ——. ‘Materialism and the Metaphysics of Modality’. Philosophy and Phenomenological Research 59.2 (1999): 473–96. ——. ‘Two-Dimensional Semantics’. The Oxford Handbook of Philosophy of Language. Eds. Ernest Lepore and Barry Smith. Oxford: Oxford UP, 2006. 574–606. De Rose, Keith. ‘Epistemic Possibilities’. Philosophical Review 100 (1991): 581–605. Divers, John. Possible Worlds. London: Routledge, 2002. Fine, Kit. ‘The Varieties of Necessity’. Conceivability and Possibility. Eds. Tamar Szabo Gendler and John Hawthorne. Oxford: Clarendon Press, 2002. 253–81. Forbes, Graeme. The Metaphysics of Modality. Oxford: Clarendon Press, 1985 Gendler, Tamar Szabo and John Hawthorne. Conceivability and Possibility. Oxford: Clarendon Press, 2002. Jackson, Frank. From Metaphysics to Ethics. A Defense of Conceptual Analysis. Oxford: Clarendon Press, 1998. Kripke, Saul. Naming and Necessity. Cambridge, MA: Harvard UP, 1980. Lewis, David. On the Plurality of Worlds. Oxford: Blackwell, 1986. McGinn, Colin. ‘On the Necessity of Origin’. Journal of Philosophy 73 (1976): 127–35. Quine, W.V. ‘Two Dogmas of Empiricism’. From a Logical Point of View. Cambridge, MA: Harvard UP, 1953. 20–46. Seddon, George. ‘Logical Possibility’. Mind 81 (1972): 481–94. Sidelle, Alan. Necessity, Essence, and Individuation. A Defense of Conventionalism. Ithaca, NY: Cornell UP, 1989. Siegel, Susanna. ‘The Visual Experience of Causation’. Philosophical Quarterly, forthcoming. Soames, Scott. Reference and Description. The Case Against Two-Dimensionalism. Princeton, NJ: Princeton UP, 2005. Stalnaker, Robert. ‘Assertion’. Context and Content. Essays on Intentionality in Speech and Thought. Oxford: Oxford UP, 1999. 78–95. Thomasson, Amie. ‘Modal Expressivism and the Methods of Metaphysics’. Philosophical Topics, forthcoming. ——. Ordinary Objects. New York, NY: Oxford UP, 2007. Van Inwagen, Peter. ‘Modal Epistemology’. Ontology, Identity, and Modality. Essays in Metaphysics. Cambridge: Cambridge UP, 2001. 243–58. Yablo, Stephen. ‘Coulda, Woulda, Shoulda’. Conceivability and Possibility. Eds. Tamar Szabo Gendler and John Hawthorne. Oxford: Clarendon Press, 2002. 441–92. ——. ‘Is Conceivability a Guide to Possibility?’ Philosophy and Phenomenological Research 53.1 (1993): 1–42.

© 2008 The Author Philosophy Compass 3/4 (2008): 664–684, 10.1111/j.1747-9991.2008.00147.x Journal Compilation © 2008 Blackwell Publishing Ltd