The Aristotelian Society 133rd session – 2011/2012 14 May 2012 | 16.15 – 18.00
Leibniz’s Law and the Philosophy of Mind Frank Jackson
The Woburn Suite Senate House (South Block) University of London Malet Street London WC1E 7HU United Kingdom
** non-citable draft paper **
biography Frank Jackson is a regular visiting professor at Princeton University and holds fractional research positions at The Australian National University and La Trobe University. He is a Corresponding Fellow of The British Academy. His publications include: Perception (Cambridge UP 1977), Conditionals (Blackwell1987), The Philosophy of Mind and Cognition, co-authored with David Braddon-Mitchell (Blackwell, 1996), From Metaphysics to Ethics (Oxford UP 1998), Language, Names, and Information (Wiley-Blackwell, 2010). He has held a number of visiting positions, most recently as Leverhulme Visiting Professor at Cambridge University in 2011.
Leibniz’s Law and the Philosophy of Mind Frank Jackson We draw some metaphysical conclusions about colour and belief from some epistemological commonplaces. It turns out that this requires us to challenge orthodoxy on the causal efficacy of mental properties and to rewrite the standard argument against dualism, but in a way which is good news for functionalists about the mind. Under what conditions is P evidence for Q? A comprehensive answer to that question is hard and inevitably controversial. We can however say three things that are, it seems to me, uncontroversial. Whether or not P is evidence for Q depends on i) what P is, ii) what Q is, and iii) the background evidence. The details of how one might enlarge on these three observations will inevitably be controversial but the basic thought behind each is close to a truism. The essay is about how to deploy these observations, along with some equally commonsensical observations about when we are entitled to believe that one or another property is instantiated, to reach conclusions in the philosophy of mind on subjects that have been much debated. You might describe this essay as an exercise in using the noncontroversial to adjudicate the controversial. I expect that it will, in its turn, be controversial. Much of the argumentation will employ Leibniz’s law in epistemic contexts. I know from experience that this worries people.1 We all know that epistemic contexts are opaque. This, perhaps understandably, suggests that using Leibniz’s law in epistemic contexts involves a fallacy of the famous masked man variety. This means it is sensible (essential?) to take a moment to review why it is fine to use Leibniz’s law in epistemic contexts. Our review will be conducted as a short commentary on an issue that Quine most especially put on the table many years ago, in for example (1966). I
Substitution in belief contexts and Leibniz’s law. Consider (1)
John Doe believes that George Orwell wrote Animal Farm.
John Doe believes that Eric Blair wrote Animal Farm.
It is plausible that (1) may differ in truth value from (2), despite the fact that George Orwell and Eric Blair are one and the same person. Indeed, it is plausible that there are many school children who believe that Orwell wrote Animal Farm but don’t believe that Blair did. Quine took this to be obviously true (but he used different examples to make the point). Suppose he is right and that, for some John Doe, it is the case that (1) is true and (2) is false. What should we say about the example? I want to insist (with thousands) that the one thing we must not say is that we have discovered a counter-example to Leibniz’s law; we must not say, that is, that Orwell has the property of being believed by John Doe to have written Animal Farm, whereas Blair lacks that property, despite the fact that Orwell and Blair are one and the same person. What we should say of a positive kind is not so obvious. But many say something along the following lines. Belief is a relation to a proposition. In our example, there are two propositions: the one expressed by ‘George Orwell wrote Animal Farm’, and the different proposition expressed by ‘Eric Blair wrote Animal Farm’. What makes (1) true is that the first proposition has the property of being believed by John Doe; what makes 1
Here I am greatly indebted to discussions prompted by Jackson (2009).
(2) false is that the second proposition lacks the property of being believed by John Doe. The failure of substitutivity of co-referring terms exhibited by (1) and (2) doesn’t tell us that there is something wrong with Leibniz’s law in epistemic contexts. It tells us instead that the proposition expressed by a sentence of the form ‘A is F’ can change on substitution of co-referring names in the subject position. Nowadays many dispute Quine’s claim that (1) and (2) can differ in truth value, consistently with the identity of Orwell and Blair. They insist that if Orwell and Blair are one and the same, then (1) and (2) have the same truth value. And often their reason draws on Leibniz’s law. They insist that because Orwell is Blair, and because ‘George Orwell’ and ‘Eric Blair’ are proper names, the proposition expressed by ‘George Orwell wrote Animal Farm’ is one and the same as the proposition expressed by ‘Eric Blair wrote Animal Farm’. This implies that they agree in respect to whether or not they are believed by John Doe. It is, that is, Leibniz’s law that makes them insist that (1) and (2) are alike in truth value, given their view that there is just the one proposition in play in the example. An issue for this position is how to decide in a non-arbitrary way which truth value — true or false — the two sentences share. An issue for the alternative position is to give a plausible account of proper names that implies that the two sentences express different propositions. That completes our little review. It is far from exhaustive (obviously). All we need to take away from it is that Leibniz’s law is unscathed and, in particular, that there is no problem with using it in epistemic contexts. II
Our first example: colour. Surely we can agree that something’s looking red is a good reason for believing that it is red, other things equal. Likewise for the other colours. It is of course a defeasible reason. For example, against a background body of evidence that includes the information that the lighting conditions are especially dodgy for perceiving colours correctly, it is no reason. Some philosophers say that against a background body of evidence that includes what optical and brain science tells us, it is no reason. These philosophers hold that the deliverances of science tell us that colour is an illusion; the deliverances of science tell us that nothing is coloured just as they tell us that nothing is phlogisticated. All the same, against the kind of background evidence that Aristotle, for example, had when his vision was working well, something’s looking red is good evidence for its being red. If we call the background evidence in question ‘A’, we can say it this way: the conjunction of A with x’s looking red is good evidence for x’s being red.2 That is to say, the property of being red has the property of being such that x’s looking red conjoined with A is good evidence for holding that x has it. This may seem almost trivial but it tells us something that is far from trivial. Some say that the property of being red is having a certain surface reflectance profile. (They have in mind surface colour, and our discussion will be restricted to this case. What to say about colour as a property of regions of space and of transmitted light are issues we will leave to one side.) Exactly which profile isn’t important for what is to come. Let’s call it ‘RP’. It is clear that the conjunction of x’s looking red with A is not good evidence for x’s having RP. When x looked red to Aristotle, he did not have good evidence that x had RP. Good evidence for x having RP did not come along until quite recently. So, using Leibniz’s law, we can conclude that being red ≠ RP. It will be obvious that the same goes for the colours (hues) more generally. Thus, being blue ≠ RB, where RB is the proposed reflectance profile for blue. It will also be obvious that the same goes for candidates other than certain reflectances to be the physical properties to identify with the colours. We have, that is, a general objection to what is sometimes called the identity 2
Here and elsewhere I skate over whether we should think of evidence as what is known, what is believed, what is reasonably believed, the content of what is known, the content of what is believed etc. There are important issues here but ones that cut across our concerns.
theory of colour and sometimes physicalism about colour.3 We also have a problem to address. It is generated by the conjunction of the nature of the experience of something’s looking red to one with what we know about the causal history of the experience. I will call it the causal problem.4 III
The causal problem. The fact that the conjunction of A with x’s looking red is good evidence for x’s being red excludes being red being identical with RP (or anything of that general kind), but leaves open, for example, the possibility that being red is identical with the property of being disposed to look red, or that it is identical with some property of the experience of looking red itself, and variations on those two themes. However, the nature of the experience of something’s looking red would seem to exclude possibilities like these. The experience is, surely, an especially clear example of a state that, by its very nature, represents. As many have urged, in one formulation or another, the nature of the experience is the putative nature of what’s experienced.5 The redness of x’s looking red resides in the putative way x is, and, in the case of surface colour, it is more particularly the putative way x’s surface is. This means that looking red is evidence for how x’s surface is. It isn’t putative evidence for how we are. X’s looking red speaks to the nature of x’s surface, not to the nature of what it does to us or is disposed to do to us, and — obviously — what it ‘says’ about x’s surface is that it is red. And this would seem to mean, in turn, that x’s looking red had better be a causal response to how x’s surface is, or a causal response in normal circumstances to how x’s surface is. The problem will now be evident. We have a powerful argument for eliminativism about colour. We know, although Aristotle didn’t, that the normal cause of x’s looking red is RP (as we are supposing, and if it isn’t RP it will be some property Aristotle knew nothing about, which is all that is needed to generate our problem). We have seen that RP cannot be the property of being red. But if the normal cause of x’s looking red isn’t x’s being red, x’s looking red cannot it seems be (good) evidence for x’s being red. We have, it seems, no good reason to believe that objects are red, and the same line of argument would apply to colour in general. Some will ask, where’s the problem? Eliminativism about colour is true. You have given a sketch of one way to argue that the picture modern science gives of our world shows that colour is a pervasive illusion (a position you have in fact already alluded to). I think eliminativism about colour is an entirely respectable position (indeed, it may be the truth of the matter) but I hope you will agree with me that it cannot be that easy to demonstrate. We should look for a possible way out. I will argue later that there is a way out. But first we need to look at the case of belief, and even before that, I should say something about an objection some who like the identity theory of colour will want to press around about now. IV
Causal co-variance and representation. Here’s one way the objection might be framed. ‘You have indicated your agreement with the position that insists that x’s looking red is a state that represents that x is a certain way, and moreover that being red is the way x’s looking red represents x to be. (And I agree with you on both points.) Now, the best account of representation is one in terms of causal co-variation, one in terms, that is, of indication, as it is often put. Of course, the needed bells and whistles are matters of debate, but the basic picture is not. But we know, at least for the case of surface colour, 3
See, e. g., Armstrong (1968, ch. 12) and Byrne and Hilbert (2003) Much of what follows is prompted by objections to what I’ve said about colour in the past, an example being the objections in Menzies (2009). 5 It is diaphanous, as Moore (1922) put it. Although it is orthodoxy (orthodoxy I am taking for granted) that perceptual experiences represent, it is occasionally challenged. 4
that colour experiences co-vary with reflectances, and in particular that x’s looking red indicates that x is RP. It follows that being red is RP, just as we identity theorists hold. Your agreement with us about the representational nature of colour experience, when combined with the empirical facts, commits you to embracing the identity theory.’6 This objection would misunderstand what I mean when I say that colour experiences represent that what’s seen is a certain way (and would, I think, misunderstand what many but not all mean when they affirm their allegiance to a broadly representationalist view of colour experience and experience in general). People sometimes call out ‘I’m here’ when they want to let someone know where they are. How can this possibly help? The sentence uttered is true no matter where they are. It helps because it causes hearers to have an auditory experience which represents where the speaker is, in a sense which is available to them. As it happens, the nature of the experience co-varies with the out of phase nature of what happens at the two ears of hearers (simplifying the brain science). But that’s not something hearers know unless they have read or done the science. But they don’t need to do any science to know where their auditory experience represents the speaker as being. The utility of calling out ‘I’m here’ doesn’t await work in the laboratory. Their experience, by its very nature, issues a kind of invitation to believe that the speaker is located at a certain place, an invitation they may or may not accept; and anyone who has the experience can say where they are being invited to locate the speaker. I think the same is true of colour experiences. They represent the surface nature of objects in the same sense of representation in which auditory experiences represent where a sound is coming from, or to take a visual example, the same sense in which something’s looking to be moving represents that it is changing its location. One way to express the problem for the identity theory of colour is to note that colour experiences do not represent that an object has such and such a reflectance profile, or anything in that line of country, in this sense of representation — the sense of representation on which it is so very plausible that colour experience is essentially representational. V
The case of belief. Here is something we know, surely. Aristotle was entitled to have all sorts of beliefs about what he and those around him believed. He was not, however, entitled to believe that he and those around him were in such and such neural states. He was, for example, entitled from time to time to hold that he and others believed that it is raining. He was, that is, entitled to hold that he and others possessed the property of believing that it is raining. He was however never entitled to hold that he and others possessed the property of being in neural state N, for any N that might credibly be supposed to be the neural state that underpins the belief that it is raining. That is to say, on occasion the evidence available to him was strong evidence that something had the property of believing that it is raining, but was never evidence that something had the property of being in N. It follows from Leibniz’s law that the property of believing that it is raining ≠ being in N. The argument parallels that we offered for colour above, which is why I have given it in a short form. We saw that there is a major issue over causation in the case of colour. The same goes for belief. As the problem is especially well-known, I will put it quickly in the section that follows. VI
The causal problem for belief. Sometimes when I believe that it is raining, I tell people that it is raining. It seems that my mental state’s having the property of being a belief that it is raining causes, along with other stuff of course, my production of the words ‘It is 6
They might also add, reasonably enough, that I said something like this but with a Lockean cast, in Jackson (1998, ch. 4).
raining’. It is the very fact of the belief’s being a belief that it is raining that causes me to produce the words. But we know that the property of the mental state that does the causing will be a neural property, the property we have been calling ‘N’. Our epistemic argument to the conclusion that the property of being the belief that it is raining cannot be identical with N forces us to deny the casual efficacy of belief properties. Not good, many will say. My reply to this problem will be found shocking by some. I think we should deny the causal efficacy of mental properties. I know that many of us have for decades been telling our students in philosophy of mind courses that one has to accept the causal efficacy of mental properties — usually around the second or third lecture — and then proceeding to argue that this fact, when combined with what we know about the neural origins of behaviour, provides one of the most powerful reasons for not being a dualist. I think we confused two questions: should we insist that mental properties are causally efficacious with respect to behaviour and physical events more generally? No. Should we insist that mental properties are such that behaviour and physical events more generally carry information about them? Yes. I start by explaining why I think it is open to us to reject the orthodoxy that mental properties are causally efficacious. VII
Against orthodoxy.7 Why do so many take it as obvious that mental properties cause? They have three reasons: introspection, evolution, and knowledge. None has any real force, it seems to me. The three reasons tell us that we had better believe that behaviour, and physical events more generally, carry information about mental properties, and although carrying information is a causally based property, this is consistent with mental properties not themselves doing any causing. Here are two examples where we need to distinguish the question of causation from the question of carrying information. A particle of mass m moving under the influence of two bodies of mass M1 and mass M2, respectively, is causally influenced by the gravitational fields generated by those two bodies (and, let’s suppose, nothing else). Its path through space will carry information about the resultant force acting on it, but the resultant force isn’t a cause. There aren’t three causes acting on the particle: one from the body of mass M1, one from the body of mass M2, and the resultant force. Indeed, on one plausible view, the resultant force is a convenient fiction, and so on that ground alone could not be a cause. Our second example is the way the behaviour of a refrigerator carries information about it’s having a thermostat, and let us suppose that the thermostat in question is an old-fashioned bi-metal strip. The property of being a thermostat doesn’t cause the refrigerator’s motor to turn on and off. What does that is the bending and unbending of the bi-metal strip. That is, what does the causing is a simple motion property of the strip (plus the way strip is connected into the whole system, of course), not the functional property the strip has of being a thermostat. But of course the way the refrigerator turns itself on and off carries the information that it has a thermostat. Now let’s see why those three reasons for holding that mental properties cause behaviour, and happenings in the physical world more generally, are in reality reasons for holding that behaviour and physical happenings carry information about mental properties. Let’s start with introspection. Imagine that our particle of mass m is conscious, that it is aware of the way it is moving under the influence of forces. How will things seem to the particle? It will seem to the particle that it is being acted on by a single force, the resultant force. But it isn’t. Or think of what happens when you identify someone. Sometimes you do this by consciously noticing, say, a distinctive feature of the person who has just entered the room and then saying to yourself that, as only John Doe 7
I am especially conscious of a debt to discussions with Philip Pettit and David Braddon-Mitchell in this section, but neither should be held responsible.
has that feature, it must be John Doe. But in the more common case, you simply recognise the person as being John Doe. When this happens, it seems to you that it is the very fact that it is John Doe which is causing your judgment that it is John Doe. The causally operative property presents itself to you as the person’s being John Doe. But it isn’t. What is doing the causing will be a certain facial configuration of which you are likely unaware (finding out which facial configuration of a person M typically causes our judgement that it’s M we are seeing, is a non-trivial piece of science). What is true, though, is that a consequence of the way your judgement gets caused is that it carries information to the effect that it is John Doe who has entered the room. That which presents as causation by the property of being John Doe is in fact the carrying of information that the property of being John Doe is instantiated. What about evolution? The standard argument runs as follows. The fact that minds evolved tells us that it is very likely that mental properties are conducive to survival but that could hardly be the case unless they did some causing. But thermostats are good for survival. If my refrigerator did not have one, I would get rid of it. But it isn’t true that being a thermostat does any causing. More generally, functional properties do not do any causing (although their bases do plenty of causing), but their possession is very obviously good for survival. Finally, what about the argument from knowledge. Here is how it goes. A necessary condition for knowing the nature of some past mental state is that its nature register, causally register, on one’s current memories or on the printed record or on ... .That could only be the case if mental properties did some causing. But in fact what’s called for is that what happens downstream from a mental state of such and such a nature should carry information about that mental nature. And that, as we have seen, does not require that the mental property be a cause. To draw on the earlier example, my refrigerator’s behaviour carries information about the instantiation of the property of being a thermostat but isn’t caused by that property. Indeed, it is worth adding here the obvious point that a casual connection of a haphazard kind would be useless as a source of knowledge. What is crucial is that information be passed along. If I am right that we should deny that mental properties cause behaviour and events in the physical world more generally, we can no longer give the well-known argument against dualism that runs in outline as follows: Premise 1. Mental properties cause physical events. Premise 2. The only properties that cause physical events are physical properties. Therefore, mental properties are physical properties. We will need instead to argue as follows: Premise 1. Physical events carry information about mental properties Premise 2. The only properties that physical events carry information about are physical properties. Therefore, mental properties are physical properties. This is good news for functionalists about mental properties. We can stop being defensive about making mental properties inefficacious, and start celebrating the way our view explains how physical events, and behaviour especially, carry information about mental properties. VIII
The causal problem for colour revisited. Here was how we put the problem earlier. Isn’t it a necessary condition on looking red being good evidence for being red, that looking red be (normally) a causal response to being red? But looking red is a response to RP, and we have a powerful argument to the conclusion that being red ≠ RP. We might well have added the observation that, setting aside the epistemological issue, it is anyway intuitively 6
plausible that looking red is a causal response to being red in much the way that something’s looking to be moving is a causal response to its moving. I hope my reply will by now be obvious. The property one responds to when something looks red does indeed seem to be red itself, but it isn’t. It is like the case of face recognition, but let’s leaven the mix with a fresh example. A tennis ball can look to experienced players as if it will go out, as if it is an ‘out ball’. As it seems to them, the very fact that the ball will go out causes their judgement that it will go out. The ball presents itself to them as one that will go out. But what in fact causes the judgement are the properties of the ball just before the judgement is made. The tennis players’ judgements carry the information that the ball will land outside the court but aren’t caused by that fact. We should say the same, I suggest, about the relationship between being red and looking red: x’s looking red carries the information that x is red but it isn’t caused by x’s being red, and of course if looking red carries information about being red that allows it to be good evidence for being red. But now we have a new question. If being red isn’t the property of objects’ surfaces that makes them look red — isn’t PR — what is it? Surely we can say this much: what we have said so far gives us two constraints that need to be satisfied: first, being red is the property looking red represents things to have, and, second, it is the property that looking red carries information about. And now we have a new way to argue for eliminativism about colour: argue that no property instantiated in our world meets the two constraints. We might, for example, argue, first, that colour experiences represent that objects’ surfaces have properties that are sui generis. There is nothing more to say about how, for instance, looking red represents objects’ surfaces to be than that it represents them as being red. Red is what it is, and that’s that. The second step would be to argue that what we know about the causation of colour experiences tells us that they do not carry information about a sui generis property of surfaces. It would then follow that nothing is coloured, or at least that we have no reason to suppose that anything is coloured. How might we resist and make a case for a non-eliminativist position? The place to start, it seems to me, is by asking, what information do colour experiences carry about the surfaces of objects? That’s a question for serious colour science. But, in the broad, there will be two candidates: reflectances, which we have already argued cannot be identified with colours, and certain light accessible, similarity and difference properties of surfaces. More particularly, when objects A and B both look red to you, you are in a state that carries putative information to the effect that their surfaces are similar in a respect that is revealed to you by the light that impacts on them, a respect that is not true of objects that look green or look blue, for example. One way of tackling the task of spelling out the relevant similarity and difference relations would be to try and encapsulate everything of this kind that can be recovered from the colour solid. The second step would be to argue that colour experiences represent the similarity and difference relations among surfaces of a kind revealed by light.8 Given what we know about the causation of colour experiences, what makes it the case that there are the various similarities and differences will lie in facts about the physical nature of the surfaces and how they relate to reflectances. But what’s represented aren’t the physical natures of the reflectances as such, but the similarities and differences that supervene on them. Red looking objects are represented as being alike in a surface property that light makes accessible to us, but what that property is is not represented to us. It will be a case where similarities and differences are represented but the properties that determine those similarities and differences aren’t. Here’s a way of saying it. When two objects look red to you, you represent that there is a property that they share, that is revealed by light and is …., but you don’t represent what that property is. (And how the dots get filled in will be a matter for further discussion but not on this occasion.) It is around here that many eliminativists will object. They may well agree that colour experiences represent similarity and difference relations among surfaces of a kind 8
See Armstrong (1968, p. 278).
revealed by light, but will insist that they do more than this. They represent the intrinsic properties of surfaces in virtue of which they stand in these similarity and difference relations. This, they may well argue, is how we are able to make sense of the inverted spectrum cases that dominate so much of the philosophical literature on colour and colour experience. Who is right? I don’t know. I will content myself by reminding you of the Charlie Brown position. We can think of our eliminativists as setting very high standards to be as colour experiences represent things to be, and their opponents as setting somewhat lower standards.9 Charlie Brown says that there is really nothing to fight over: by one set of standards there are no colours, by another there are. Let’s leave it at that. There is a final matter we need to attend to: the distinction between properties and states. IX
Properties and states. We have argued that the property of believing that it is raining cannot be identified with N. Does this mean that we cannot identify the state of believing that it is raining with N? No. That possibility remains open for all we have said. Suppose, as many hold, that some kind of functionalism is true for belief. To believe that it is raining is to be in a state that plays a certain functional role R. What is the property of believing that it is raining in that case? It is the property of having in one the state, the kind of state, that plays role R at the time in question. The state may vary from time to time and need not be the same state in different believers that it is raining. What has to remain constant across the believers and times is the role that needs to be occupied. The situation is akin to that which obtains for the property of being grammatical in a language. For a sentence to be grammatical is for it to be of a kind that plays a certain role in a language — the ‘it is OK’ role, we might dub it. However, the kind may vary from time to time — what’s grammatical in a language can and does change over time, and of course varies greatly from one language to another. But although the kind may vary, the role played in order to be grammatical remains the same. The upshot is that if functionalism is true, there is an important distinction between the property of believing that it is raining and the state kind that plays the role. Nothing I say in this essay rules out the position that holds that the state that plays the role for me now is N and that, moreover, that state is my belief that it is raining now; and that, at other times and for other people, each and every belief is some neural state or other.10 What is ruled out is the property of believing that it is raining being N, or indeed any neural state. Something similar needs to said about lightning (and indeed for many similar examples drawn from science that were appealed to in expositions of the mind-brain identity theory). Certain of Aristotle’s experiences gave him reason to believe in lightning. They did not give him reason to believe in electrostatic discharges in the atmosphere. It follows that the property of being lightning is not that of being an atmospheric electrostatic discharge. But surely scientists are right when they say that lightning is an atmospheric electrostatic discharge. What we have to say is that being lightning is being the (or a) kind that plays a certain role. Aristotle knew about the role and was entitled to believe that the role was played on various occasions. What he did not know was what kind played the role, and it is, of course, the kind that science has discovered to be an electrostatic discharge.
Not too low. There must be light accessible similarity and difference relations of a not too gerrymandered kind, and that is a matter open to debate. 10 A view I in fact hold.
References Armstrong, D. M. 1968: A Materialist Theory of the Mind. London: Routledge & Kegan Paul. Byrne, Alex and David R. Hilbert 2003: ‘Color Realism and Color Science’. Behavioral and Brain Sciences, 26 (1), pp. 3–21. Jackson, Frank 1998: From Metaphysics to Ethics, Oxford: Clarendon Press. Jackson, Frank 2009: ‘On the Metaphysical Implications of Some Epistemological Commonplaces’. In Heather Dyke (ed.), From Truth to Reality: New Essays in Logic and Metaphysics. London: Routledge, pp. 99–111. Menzies, Peter 2009: ‘The Folk Theory of Colour and the Causes of Colour Experience’. In Ian Ravenscroft (ed.), Minds, Ethics, and Conditionals, Oxford: Clarendon Press, pp. 141–160 Moore, G. E. 1922: ‘The Refutation of Idealism’. In Philosophical Studies. London: Routledge & Kegan Paul, pp. 1–30. Quine, W. V. 1966: ‘Quantifiers and Propositional Attitudes’. In The Ways of Paradox. New York: Random House, pp. 183–194.
27 April 2012 The Australian National University (School of Philosophy, ACT 0200), La Trobe University, Princeton University