Conceivability and Modal Knowledge

r 2006 The Author Journal compilation r 2006 Metaphilosophy LLC and Blackwell Publishing Ltd Published by Blackwell Publishing Ltd, 9600 Garsington Road, Oxford OX4 2DQ, UK, and 350 Main Street, Malden, MA 02148, USA METAPHILOSOPHY Vol. 37, No. 2, April 2006 0026–1068

CONCEIVABILITY AND MODAL KNOWLEDGE RENE´ VAN WOUDENBERG

Abstract: This article is a discussion of Hume’s maxim Nothing we imagine is absolutely impossible. First I explain this maxim and distinguish it from the principle Whatever cannot be imagined (conceived), is impossible. Next I argue that Thomas Reid’s criticism of the maxim fails and that the arguments by Tamar Sza´bo Gendler and John Hawthorne for the claim that ‘‘it is uncontroversial that there are cases where we are misled’’ by the maxim are unconvincing. Finally I state the limited but real value of the maxim: it does help us, in certain cases, reliably to make up our minds. Along the way I show that Reid, his criticism of the maxim notwithstanding, actually employs it, and I furthermore argue that the principle What is inconceivable, is impossible is spurious. Keywords: conceivability-possibility principles, modal epistemology, David Hume, Thomas Reid.

The Treatise of Human Nature contains a section on the infinite divisibility of space and time in which Hume says: ’ Tis an establis’d maxim in metaphysics That whatever the mind clearly conceives includes the idea of possible existence, or in other words, that nothing we imagine is absolutely impossible. (1739–40, I.ii.2, 32)

This maxim was certainly ‘‘established’’ in the sense that very many of Hume’s contemporaries accepted it. Samuel Clarcke (1705, 16), for example, accepted it, as did Richard Price (1758, 34) and Christian Wolff (1731, secs. 102, 103). To be sure, Clarcke, Price and Wolff went even further. They not only endorsed, as did Hume, What is conceivable, is possible but also endorsed What is inconceivable, is impossible. With the exception of my section 3 below (and the last part of section 1), I deal here only with the first of these, Hume’s maxim. In the first section I explain what the maxim comes to and contrast it with other principles in the neighborhood. I then go on to discuss a critique of the maxim by one of Hume’s most important contemporary critics, Thomas Reid. I argue that Reid’s criticism fails. In section 3 I show that, his criticism notwithstanding, Reid avails himself of the maxim. In the penultimate section I consider some further objections that have been brought against the r 2006 The Author Journal compilation r 2006 Metaphilosophy LLC and Blackwell Publishing Ltd

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maxim and argue that they fail as well. In conclusion I state the limited but genuine value of the maxim. 1. Hume’s Maxim Explained Let us consider the various notions contained in the maxim. First, to what should we say ‘‘possibility’’ attaches? The maxim is best explained by saying that possibility attaches primarily to statements or propositions, not to objects. It is statements that can be possibleFor impossible (and hence the principle deals with possibility de dicto). For a statement to be possible means it is possibly true; for a statement to be impossible means it is impossibly true. And a statement is possibly true just in case there is at least one possible world in which the statement is true; a statement is impossibly true just in case there is no possible world in which it is true. Next, in what sense should ‘‘possibility’’ be taken, for several notions of possibility can be distinguishedFfor example, metaphysical possibility and physical possibility. Involved in the maxim is the former notion, which is broader than, and encompasses, the latter. Many statements are possible in the metaphysical sense but not in the physical sense; no statement that is physically possible, however, is metaphysically impossible. The statement that cows jump over ten-metre-wide rivers is metaphysically possibleFthat is, there is a world in which this statement is true; but this statement is not physically possible, by which is meant that it is false in all worlds in which the physical laws and constants are as in the actual world. The maxim also involves the notion of conceiving. What is it for someone to ‘‘conceive’’ a statement? I suggest that the most generous interpretation is something like the following. One conceives a statement when one can depict to oneself a consistent and coherent scenario in which that statement is true or, alternatively, when one can tell oneself a consistent and coherent story in which it is true (see Yablo 1993). In this sense of conceiving, no one can conceive of the statement that John is both taller and shorter than Jack, for although I may have an idea of what needs to be the case for that proposition to be true (viz., that John is both taller and shorter than Jack), I cannot consistently and coherently depict to myself a scenario in which that statement is true. Hume’s maxim can, therefore, be spelled out as follows: Hume’s maxim: If you can depict to yourself a consistent and coherent scenario in which a certain proposition P is true, then you may infer that P is possible in the metaphysical sense of ‘‘possible.’’ Now why has this maxim seemed so plausible to so many philosophers? The answer is: because in many (and until we have seen otherwise, in all) of its applications it seemed to yield the right result. Consider the r 2006 The Author Journal compilation r 2006 Metaphilosophy LLC and Blackwell Publishing Ltd

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following example taken from Hume, the statement A golden mountain exists. We can, Hume holds, ‘‘conceive’’ this statement, that is, we can depict to ourselves a consistent and coherent scenario in which this statement is trueFthat is, we can depict to ourselves a consistent and coherent scenario in which a golden mountain exists. And from our being able to conceive this statement, it follows that the further statement It is possible that a golden mountain exists is true. And this is exactly the inference that the maxim means to license. Other examples indicate the same. Is it possible that John is both taller and shorter than Jack? The maxim tells us that if you want to find out, you should try to conceive the statement that John is both taller and shorter than Jack. And what this comes to is to try to depict to yourself a situation in which that statement is trueFwhich means that you can depict to yourself a scenario in which John is both taller and shorter than Jack. If you are like the philosophers who have endorsed the maxim, you will find yourself unable to do this. This inability forestalls your concluding that it is possible that John is both taller and shorter than Jack. Note that application of Hume’s maxim here has the result that one should not conclude that it is possible that John is both taller and shorter than Jack. It does not result in the stronger conclusion that it is impossible that John is both taller and shorter than Jack. That result is only gained from the application of a principle, which is distinct from Hume’s maxim (viz., principle BFsee below). A most famous application of the maxim, or something very much like it, is of course to be found in Descartes’ argument for mind-body dualism. The crucial move in this argument is that Descartes can, in a clear and distinct way, conceive the statement that he exists without his body existing. That is, in a clear and distinct way Descartes can depict to himself a scenario in which it is true that he exists, whereas his body does not. Therefore, he concludes, it is possible that he exists while his body does not. And from this he draws the further conclusion that there is a real distinction between himself and his bodyFbut that is a further step that falls outside the purview of the maxim. Next a word about the nature of the maxim is in order. Hume refers to it as ‘‘a maxim in metaphysics.’’ It is a maxim that is used in metaphysical investigations, but it isn’t a metaphysical maxim. An example of a metaphysical maxim would be Leibniz’s Law, which is a maxim that explicates what metaphysically speaking is the case. Hume’s maxim, however, doesn’t spell out what metaphysically is the case. It is an epistemic maxim that tells us what to do if one wants to find out whether a certain possibility statement is true. Hume’s maxim aims to enable its user to make up his or her mind. Accordingly, if you want to make up your mind about whether the statement possibly P is true, then try to depict to yourself a scenario in which P is true. If you are successful, that is, if you can depict such a scenario to yourself, then possibly P is true. r 2006 The Author Journal compilation r 2006 Metaphilosophy LLC and Blackwell Publishing Ltd

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Finally I should like to distinguish Hume’s maxim from another in its vicinity with which it could easily be conflated. But let us first note that, by modus tollens, one corollary of Hume’s maxim is A: What is impossible, is inconceivable. One cannot endorse Hume’s maxim and discard A. But one could, in principle, endorse Hume’s maxim and reject B: What is inconceivable, is impossible. After all, B doesn’t follow from Hume’s maxim. But neither do they exclude one another. I have already indicated that Clarcke, Price and Wolff endorsed B in addition to Hume’s maxim. And since they accepted it, they must have done so on grounds that are independent of the grounds underlying acceptance of Hume’s principle. The most important reason for accepting Hume’s maxim, I have suggested, is that in many (and until we have seen otherwise we may suppose in all) of its applications it yields the right result. Is there a similar reason for accepting B? We have already noted that the proposition that John is both taller and shorter than Jack is marked ‘‘impossible’’ by the application of B. For one cannot depict to oneself a scenario in which it is true. And this, says B, licenses the conclusion that it is impossible. In this case B gives the right result. Still, B is worrisome. For one may be unable to conceive a certain scenario, due to, for instance, fatigue, drugs or an uncommon moment of dullness, in all of which cases, however, B licenses the inference to impossibility. The problem with this, however, is that we would not want what we conclude to be impossible to depend on what, for reasons of fatigue, and so on, may be inconceivable. For we must suppose that many things that are possible will remain inconceivable for us as well. Many scenarios may be so subtle and so complicated that we cannot depict them to ourselves in a coherent and consistent way. But this means that B has insufficient discriminating powerFfor not only what is impossible but also many possibilities may be inconceivable to us. B, then, hasn’t much to be said for it. The same holds, of course, for the principle that, by modus tollens, follows from B, namely, that what is not impossible, is not inconceivable, or, eliminating the nots: C: What is possible, is conceivable. It should be noted that Hume’s maxim does not suffer from the problem that besets B. For the maxim as I have been explaining it does not say that if you cannot depict a coherent and consistent scenario to yourself in which P is true, then you should conclude that possibly P is false. It only says that if you cannot depict to yourself a scenario in which P is true, r 2006 The Author Journal compilation r 2006 Metaphilosophy LLC and Blackwell Publishing Ltd

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then you cannot, or should not, on the basis of the maxim alone, conclude that possibly P is false. Let us now look at some criticisms of Hume’s maxim. 2. Reid’s Official Criticism of the Maxim In Essay IV of his Essays on the Intellectual Powers of Man, Thomas Reid discusses various mistakes about conception. One of them is ‘‘That our conception of things is a test of their possibility, so that, what we can distincly conceive, we may conclude to be possible; and of what is impossible, we can have no conception’’ (1785, 327). Reid refers to all the philosophers I mentioned in the first paragraph, and to others as well. He says, ‘‘It were easy to muster up many other respectable authorities for this maxim, and I have never found one that called it in question’’ (1785, 329). Still, Reid thinks there are various reasons (by his count, four) to call it into question. Let us consider them. It should be noted that Reid’s target is both Hume’s maxim and principle B. Since, as I indicated earlier, there is already reason to be suspicious of B, I am especially interested in whether Reid’s criticism undermines Hume’s maxim. The first of Reid’s reasons is that the phrase ‘‘conceiving a statement,’’ or, as Reid says, ‘‘conceiving a proposition,’’ that occurs in the maxim may mean two different things, and that on both ensuing understandings the maxim enshrined in each is false. ‘‘To conceive of a proposition’’ may mean, first, ‘‘to understand the meaning of a proposition’’ (1785, 330). In that case what the maxim amounts to is M1: Every proposition of which one understands the meaning is possible. But this, Reid rightly says, is false. For there are propositions that I understand that are impossibleFfor example, ‘‘Any two sides of a triangle are together equal to the third,’’ ‘‘4 is prime,’’ ‘‘John is both taller and shorter than Jack.’’ Second, ‘‘to conceive of a proposition’’ may also mean ‘‘to conceive a proposition to be true’’ (1785, 331). In that case the maxim amounts to M2: Every proposition one can conceive to be true, is possible. But, asks Reid, what is the meaning of the phrase ‘‘can conceive a proposition to be true’’? Suppose it is ‘‘can give some degree of assent to a proposition,’’ ‘‘can believe a proposition to be true.’’ Then the maxim amounts to M2a: Every proposition one can believe, is possible. But this is most certainly incorrect, says Reid. For, as we know from sad experience, many people assent to impossible propositions. r 2006 The Author Journal compilation r 2006 Metaphilosophy LLC and Blackwell Publishing Ltd

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The phrase ‘‘can conceive a proposition to be true,’’ however, might also mean ‘‘judge a proposition to be possible.’’ Taken this way the maxim says M2b: Every proposition one judges to be possible, is possible. But this, says Reid, is false as well. After all, what one person judges to be possible, another judges to be impossible, which implies, if the maxim is correct, that some propositions are possible and impossible at the same time, which is absurd. Does Reid’s first argument show that the maxim fails? Not necessarily. For there is yet another understanding of M2 to which Reid’s examples are not, or not obviously, counterexamples, and which at the same time seems to be the most generous reading of Hume’s maxim. Namely, M2c: Every proposition such that one can consistently and coherently depict to oneself a scenario in which it is true, is possible. It seems to me that no one can depict to herself a scenario in which ‘‘4 is prime’’ and ‘‘John is both taller and shorter than Jack’’ are true. Surely, someone may manage to believe these statements to be true; but that doesn’t mean or involve that he can depict to himself a scenario in which they are true. Again, someone may judge these statements to be true. But that too is very different from depicting to oneself a scenario in which they are true. The point is that we may believe statement P, or judge it to be possible, without having performed the act of consistently and coherently depicting (or trying to depict) to ourselves a scenario in which P is true. But what the maxim says is that if one can consistently and coherently depict a scenario in which P is true, then it is permitted, on that basis, to conclude that P is possible. In the case under consideration, however, that basis is lacking, and hence, in the absence of other reasons, it is not permitted to conclude that P is possible. Let us turn next to Reid’s second argument (1785, 331). This argument is that whoever conceives a necessarily true proposition also conceives the opposed contradictory proposition that is impossible. For example, someone who believes that 2 plus 3 necessarily makes 5 also believes that it is impossible that 2 plus 3 should not make 5. So, when we conceive a necessarily true proposition, we also conceive an impossible one, that is, one that is necessarily false. This means that we can, and in fact do, conceive impossible propositions, which shows that the maxim is false, for the maxim implies that impossible propositions cannot be conceived. Is this a convincing argument? I don’t think so. For in this argument Reid uses the expression ‘‘to conceive a proposition’’ as in M1. It means ‘‘to understand the meaning of a proposition.’’ The examples adduced are indeed counterexamples to M1. But they don’t count against M2c, which r 2006 The Author Journal compilation r 2006 Metaphilosophy LLC and Blackwell Publishing Ltd

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is the most charitable reading of the maxim. For surely one cannot depict to oneself a scenario in which 2 plus 3 is not 5. Hence the maxim, properly understood, still stands. Reid’s third argument (1785, 332) is that mathematicians have proved many theorems to be impossible and that without demonstration they would not have believed the theorems to be impossible. Now in order to prove a theorem to be impossible, it must be conceived, mustn’t it? Therefore, to conceive of a theorem doesn’t license one to conclude that the theorem is possible. Again, this doesn’t refute the maxim properly understood (that is, as M2c). For this third argument again takes ‘‘to conceive a proposition’’ to mean ‘‘to understand the meaning of a proposition.’’ But one can understand the meaning of a proposition without being able to depict to oneself a scenario in which it is true. Reid’s fourth and final argument (1785, 332) falters on the same rock. Mathematicians, so the argument goes, sometimes conceive things that are impossible in order to show that they are impossible. This is the case in reductio ad absurdum demonstrations. For example, Euclid said: Conceive a right line drawn from one point on a circle to another on the same circle. We can conceive of this, reason from it, and come to the conclusion that this is absurd. Hence, we can conceive of impossible propositions. What this argument comes down to is, again, that we are able to understand the meaning of statements that express impossible propositions. But the maxim, properly understood, doesn’t deny this. For what it says is that propositions such that one can depict to oneself a scenario in which they are true, are possible. A final note: Reid’s second, third, and fourth arguments refer to mathematics and proofs. Now I should like to suggest that we may think of the activity of trying to prove a theorem as the activity of trying to see whether one can consistently and coherently depict to oneself a scenario in which the theorem is true, that is, as the activity of applying the maxim.

3. Reid’s Actual Acceptance of the Maxim Reid, as we saw, rejects the idea that ‘‘our conception of things is a test of their possibility.’’ In my discussion so far I have mainly focussed on Hume’s maxim that if one can conceive something, then it is possible. But what Reid means to reject is the conjunctive maxim that says: ‘‘If we can conceive something, it is posible; if not, it is impossible’’ (1785, 330). But in a famous thought experiment in the Inquiry, what he calls the experimentum crucis, Reid makes use of (the second conjunct of) this maxim himself, as I shall now proceed to show. Let me do some stage setting. r 2006 The Author Journal compilation r 2006 Metaphilosophy LLC and Blackwell Publishing Ltd

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In the chapter ‘‘Of Touch’’ in the Inquiry, Reid asks us to imagine a person who (a) is blind, (b) has lost all the experience, habits and notions he had got by touchFthat is, he has no conception of the extension of his own body or of any other, (c) has been stripped of all his ‘‘hardwiring,’’ especially of that segment of his constitution that, upon his having tactile sensations, occasions the conception of extension, and (d) has the faculty of reasoning. About this person, Reid asks the following question: Could this man, given his tactile sensations, get the notion of extension? A more perspicuous formulation of Reid’s question is: Is it possible that this person is given his tactile sensations and yet does not obtain the notion of extension?1 Reid’s answer is clearly, ‘‘Yes, that is possible. It is entirely possible that this person has had a rich diet of sensations, and still has not obtained the notion of extension.’’ The reason Reid adduces for this is that there is no internal relation between tactile sensations and the notion of extension. That is, sensations don’t ‘‘resemble’’ conceptions; the sensations that typically go with touching a smooth surface, for example, in no way ‘‘resemble’’ the surface’s smoothness. Surely those sensations usually ‘‘suggest’’ the quality of smoothness. But in Reid’s thought experiment, the subject is stripped of the mechanism that does the suggesting; to the subject his tactile sensations suggest nothing, or at least nothing about things extended in three dimensions. Reid’s thought experiment, then, tells Reid that the person he has imagined does not necessarily have the concept of extension, even though he has a variegated array of tactile sensations. What I would now like to show is that, while conducting his thought experiment, Reid is implicitly relying on the maxim. This comes out in the following quotation, taken from the passage where Reid feeds the person in the experiment with more and more new sensations: Suppose . . . that a body is drawn along his hands or face, while they are at rest: Can this give him any notion of space or motion? It no doubt gives a new feeling [sensation]; but how it should convey a notion of space or motion, to one who had none before, I cannot conceive. (1764, 66)

So Reid is reporting that he cannot conceive how certain tactile sensations could convey the notions of space and motion (and hence of extension) to the subject in the thought experiment. And what this passage suggests is that Reid concludes from this that therefore it is not possible that tactile sensations convey the notion of, among other things, extension. The passage, then, suggests that Reid reasons as follows: I cannot conceive how certain tactile sensations convey the notion of extension, therefore it 1 Van Cleve 2006 argues that Reid’s question is more perspicuously rendered as ‘‘Is it necessary that this person, given his tactile sensations, will obtain the notion of extension?’’ But given my focus on Hume’s maxim, which doesn’t talk about necessity, I prefer my own rendition of Reid’s question.

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is impossible that those sensations convey the notion of extension. In other words, this passage suggests that Reid himself is using that part of the maxim that says that if he cannot conceive P to be true, then it is not possible for P to be true.

4. Cases Where We Are Led Astray by the Maxim? The fact that Reid’s criticisms against Hume’s maxim don’t cut ice, and that Reid himself employs it, doesn’t, of course, imply that the maxim is sound. Many think it is not. Tamar Szabo´ Gendler and John Hawthorne, for example, suggest that ‘‘it is uncontroversial that there are cases where we are misled’’ by the conceivability-possibility arguments, that is, by the application of the maxim (2002, 9–10). They refer to Greek philosophers who found it conceivable that stars are holes in the sky. And to Berkeley, who found it conceivable that the sun, the moon, the mountains and all the rest of the furniture of the earth are mere bundles of ideas. And to various mathematicians who have found it conceivable that Goldbach’s conjecture is wrong. And to various metaphysicians who have found it conceivable that the morning star is not the evening star, or that water is not H2O. So should we, for reasons other than Reid’s, reject the maxim? Now I myself am not at all sure that in these cases we are indeed misled by the conceivability-possibility move. For instance, I don’t really see what is wrong with concluding that it is possible that stars are holes in the sky on the basis of one’s ability to conceive that they are. Nor do I see anything wrongheaded in concluding that, since one can conceive that the furniture of the earth is all a bundle of ideas, it is possible that the furniture of the world is all a bundle of ideas. We can depict to ourselves a scenario in which stars are holes in the sky, and we can describe to ourselves a scenario in which the furniture of the earth is all a bundle of ideas. The maxim then licenses us to conclude that these scenarios are possible. As I said, I don’t see what is wrong with that. Certainly I don’t see that it is ‘‘uncontroversial’’ that the maxim gives the wrong results in these cases. In the Goldbach case the maxim doesn’t uncontroversially give a wrong result either. This case differs, however, from the two just mentioned. The question here is not whether the possibility conclusion can be drawn from from the conceivability premise. Rather, the question is whether we can really conceive of Goldbach’s conjecture being wrong. That is, can we really envisage a situation in which each even natural number is not the sum of two primes? I am simply not sure about this, that is, I don’t really see that we can envisage such a situation, or that we can’t. Accordingly, I am inclined to think that the maxim does not give us guidance here. For it only licenses a possibility inference from what can be conceived. And it is not clear that this is the case. r 2006 The Author Journal compilation r 2006 Metaphilosophy LLC and Blackwell Publishing Ltd

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With respect to the evening star/morning star case: what can be conceived, I should think, is that the last star shining in the morning sky is not the same object as the first star shining in the evening sky. The maxim licenses the conclusion that it is therefore possible that the last star shining in the morning sky is not the same object as the first star shining in the evening sky. And I can’t really see what is wrong with that. Again, the maxim doesn’t uncontroversially mislead us. The evening star/morning star case can, however, also be interpreted differently. On this alternative interpretation the maxim asks us to see whether we can conceive that the object we refer to as the morning star is not the same object as the object we refer to as the evening star. But I don’t think we can conceive this. For conceiving this would require that we depict to ourselves a possible world in which a certain object, the planet Venus, is not identical with itself. And this we cannot do, for the simple reason that a thing is the thing it is and not something elseFwe cannot conceive a thing to be something else. Hence, on this interpretation, the maxim does not license us to conclude that Possibly, the morning star is not identical with the evening star is true. And that seems to be the right result. Finally, is it conceivable that water is not H2O, and if so, should we conclude from this, as the maxim urges us to do, that it is possible that water is not H2O? And isn’t this an excellent reason to reject the maxim? I don’t think so. For I don’t think it is even conceivable that the stuff we refer to by the word water could be something other than the stuff we refer to by the designation H2O. Of course, the word water could have been used to refer to something other than water; and of course H2O could have been used to refer to something other than water. But this doesn’t mean that we can conceive the stuff we refer to as ‘‘water’’ to be something else than the stuff we refer to as ‘‘H2O.’’ For an item is the item it is, and not some other item. My conclusion so far, then, is that the maxim still stands. We haven’t been given uncontroversial cases in which it gives wrong results.

5. The Limited but Real Value of the Maxim As I said at the end of the first section, the maxim enables its user to make up his mind about whether a certain possibility statement is true or not. ‘‘To make up one’s mind’’ can mean different things, however. For the maxim will lead us into three different sorts of situation. First it will lead us into situations where it is clear that we can conceive a scenario in which a certain proposition p is true. This, I contend, is the case with the following proposition: William the Silent died of natural causes. r 2006 The Author Journal compilation r 2006 Metaphilosophy LLC and Blackwell Publishing Ltd

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Now this proposition is in fact false. Still, we can conceive of a scenario in which it is true. And hence application of the maxim gives the unambiguous result that the statement It is possible that William the Silent died of natural causes is true. But, second, the maxim will also bring us into a situation where it is clear that we cannot conceive of the truth of a certain statementFa situation, that is, where it is clear that we cannot depict to ourselves a scenario in which it is true. This situation obtains, I assert, for most of us with respect to the following statements: John is both taller and shorter than Jack. 3 is not prime. The maxim here doesn’t license us to conclude that these statements are possible; nor does it license us to conclude that they are impossible. So, in these situations, the maxim won’t help to make up one’s mind. And it is explicitly implied that it won’t. (This is, of course, a disappointing feature of the maxim, since we do know that these statements are impossible. But in order to get to that result, other maxims will be required.) Third, the maxim will bring us into situations where it is not clear whether we can conceive of a certain statement, unclear that we can depict to ourselves situations in which a certain statement is true. This is the case, I claim, with the following statements (taken from Van Inwagen 2001): Transparent iron exists. There is a time machine. Since it is really unclear that we can depict to ourselves scenarios in which these are true, the maxim does not license us to conclude that these statements are possible. The maxim does not help to make up one’s mind here. The maxim, therefore, is only a limited guide to the modally perplexed. But in its limitations it can be worthwhile nonetheless. The guide is limited in that it doesn’t always tell us what to think about possibility statements (it only does so in those situations where it is clear that we can conceive a certain statement, not in those situations where it is clear that we cannot, or in those cases where it is not clear that we can). But it is still a helpful guide, worthwhile to listen to. For it will help us to make up our minds about certain statements. And where it does not help, it will at least say ‘‘Here I cannot be of help, go and look for other help.’’ And with such r 2006 The Author Journal compilation r 2006 Metaphilosophy LLC and Blackwell Publishing Ltd

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a guide one is really much better off than with a guide that always confidently leads the way, but often in the wrong direction.2 Department of Philosophy Vrije Universiteit De Boelelaan 1105 1081 HV Amsterdam The Netherlands [email protected] References Clarcke, Samuel. 1705. A Demonstration of the Being and Attributes of God. Page references are to the edition edited by Ezio Vailati. Cambridge: Cambridge University Press, 1998. Gendler, Tamar Szabo´, and John Hawthorne. 2002. ‘‘Introduction: Conceivability and Possibility.’’ In Conceivability and Possibility, edited by Tamar Szabo´ Gendler and John Hawthorne, 1–70. Oxford: Oxford University Press, 2002. Hume, David. 1739–40. Treatise of Human Nature. Page references are to the edition edited by L. A. Selby-Bigge and P. H. Nidditch. Oxford: Clarendon Press, 1990. Price, Richard. 1758. A Review of the Principal Questions in Morals. Page references are to the edition edited by D. D. Raphael. Oxford: Clarendon Press, 1948. Reid, Thomas. 1764. Inquiry into the Human Mind on the Principles of Common Sense. Page references are to the edition edited by Derek R. Brookes. Edinburgh: Edinburgh University Press, 1997. FFF. 1785. Essays on the Intellectual Powers of Man. Page references are to the edition edited by Derek R. Brookes. Edinburgh: Edinburgh University Press, 2002. Van Cleve, James. 2006. ‘‘Touch, Sound, and Things without the Mind.’’ In this issue of Metaphilosophy 37, no. 2 (April):162–182. Van Inwagen, Peter. 2001. ‘‘Modal Epistemology.’’ In Ontology, Identity, and Modality: Essays in Metaphysics, 243–58. Cambridge: Cambridge University Press, 2001. Wolff, Christian. 1731. Philosophia prima, sive Ontologia, methodo scientifica pertractata, qua omnis cognitionis humanae principia continentur. Francofurti et Lipsiae. Yablo, Steven. 1993. ‘‘Is Conceivability a Guide to Possibility?’’ Philosophy and Phenomenological Research 53:1–42. 2 Many thanks to Arianna Betti, Todd Buras, James Van Cleve, Rebecca Copenhaver, Terence Cuneo, Lieven Decock, Cornelis van Putten, Wim de Jong, Dale Tuggy and Marie¨tte Willemsen for comments on an earlier draft of this article.

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