Difference and Necessity

 

 

Essentialists have found in identity a shining example of their creed. Any object is necessarily identical to itself—how could that not be so? It has even been thought that the necessity of identity can be proved from Leibniz’ Law: given that a is necessarily identical to a and that a is identical to b, it follows that b is also necessarily identical to a. But far less attention has been paid to the modality of difference: if a and b are different objects, is that a necessary fact or a contingent fact? If objects are distinct in the actual world, are they distinct in every possible world? Given that I am actually distinct from Saul Kripke, are there any worlds in which we are identical? Is there any world at which the statement “Colin McGinn is identical to Saul Kripke” is true? We can clearly lack certain of our actual properties in a possible world—neither of us are philosophers in some possible world–but could we lack the property of being numerically distinct? Are there worlds in which London is identical to Paris or the moon is identical to the sun? Could twins be numerically identical in a possible world?   

            The answer is intuitively obvious: none of these things is possible. If objects are different, they are necessarily different—different in all worlds in which they exist. If a is different from b, this is part of a’s essence—it is essentially different from b. Thus objects have an enormous number of difference essences, as many as there are objects from which they are distinct. They only have one identity essence, viz. identity to themselves, but they have hugely many difference essences, according to all the objects in the universe. In fact, they have infinitely many difference essences, given the infinity of numbers or points in space: I am different from the number two, say, in every possible world, and so for any number. These essences come cheap and plentiful. Furthermore, they are a distinct species of essence, not to be assimilated to the other kinds—origin, composition, etc. Nor are they derivable from the necessity of identity; they cannot be proved from Leibniz’ Law, for one thing. The difference of a given thing from everything else is essential to what it is, as a primitive modal fact. No amount of merging or fusion can make two actually distinct things into the same thing. For instance, you might think two rivers that flow side by side could be one river in a world in which the dry land separating them is removed; but such a world is really one in which a new river exists, not a world that contains the old two rivers joined in a single river. Those actual rivers don’t exist in this possible world; we have fused two rivers into a broader river and eliminated the actual rivers. Physical merging or joining is not two things existing as one, for those things will no longer exist—any more than fission is one object existing as two, and hence not being self-identical. There are no worlds in which McGinn and Kripke both exist and yet are one person—though there is a world in which an individual has characteristics belonging to each of us.

            That is all at the level of metaphysical possibility—what could really be. I have said nothing yet about epistemic possibility, and here matters assume a different shape. For it is an epistemic possibility that McGinn and Kripke are the same individual: it might turn out that we are identical—some very elaborate trick has been performed. To take a simpler example, suppose I am dating a woman I believe to be an only child but in fact she has an identical twin. I have been seeing both twins but don’t realize it: I have mistakenly believed of different women that they are one woman. Isn’t it possible that this is true of a great many people I know? I am always assuming identity from one meeting to the next, but in fact there is a plurality—different people where I thought there was one person. We can conjure ample grounds for a skeptical argument here. So it is not an epistemic necessity that what I take to be one individual is really one individual—it might turn out that I know many more people than I realize. Just as you can mistakenly suppose that what is in fact a single object is two, as in a classic Frege case, so you can mistakenly suppose that what are in fact two objects are a single object. Judgments of identity and difference are fallible. But none of this has anything to do with questions of metaphysical necessity: just because one object can be mistaken for two, or two objects mistaken for one, has no bearing on whether identity and difference are metaphysically necessary relations. We should not confuse metaphysical and epistemic questions. How things might turn out is a quite different question from how things could be. It might turn out that there are many things where you thought there was one, or that there is one thing where you thought there were many, but these possibilities don’t show that one thing could be many things or many things one thing.

            Frege used identity statements to prove that distinct senses can correspond to the same reference, since this is what explains someone’s inability to know the identity a priori. But an exactly parallel argument leads to the conclusion that there can be many references corresponding to a single sense, namely that a person can think he is referring to one object when he is really referring to two. If I call the woman I am dating “Joan” in the morning and “Joanie” in the evening (she always seems more fun-loving in the evening), I will subscribe to the truth of “Joan is Joanie”, even regarding it as analytic. But given that I am seeing twins and don’t know it, we have to say that I really have two references, while meaning the same by the name. The twins are perceptually presented to me in the same way and I intentionally use the names “Joan” and “Joanie” as synonyms, but I refer to different individuals when I use the name in relation to the women in question. This is a case of sameness of sense combined with difference of reference. To me the statement “Joan is Joanie” is analytic, but the context of use generates distinct references.

            The same points apply to natural kinds: people can fail to realize identities and fail to realize differences. You can fail to realize that water is identical to H2O and you can fail to realize that gold is different from copper (you think they are the same metal). Nevertheless, it is part of the essence of water to be H2O and part of the essence of gold to be different from copper. There are no worlds where water is identical to some other chemical compound and no worlds where gold and copper are the same metal (though one metal might be called both “gold” and “copper” in a possible world). If pain is identical to C-fiber firing in this world, then it must be so in all worlds; and if sensations of red and sensations of green are different in this world, then they must be different in all worlds. In order to create pain all God had to do was create C-fiber firing (according to the identity theory); and in order to create gold and copper God must perform two acts of creation—he can’t skip one of these creative acts and more economically create both metals in the form of a single metal. Different things can never be other than different, and identical things can never be other than identical. Hesperus is Phosphorus in every world and Venus is not Mars in every world—despite the fact that both these propositions are not known a priori. Modality behaves in the same way for both identity and difference: the essentialist can also find in difference a shining example of his creed. We find necessity not just in the relation a thing has to itself and to no other thing but also in the relation a thing has to other things and not to itself.     

 

                                                Inner and Outer

 

 

We have the concepts of the inner and outer, but how are they to be defined? Is one more basic than the other? The following seems true: the outer is not the inner and the inner is not the outer. Alternatively, we could say the private is not the public and the public is not the private. But this doesn’t help as a definition, owing to circularity: we can’t define the inner as what is not the outer and then go on to define the outer as what is not inner. I propose that the concept of the inner is basic and not to be defined as the negation of the outer, while the concept of the outer is derivative and can be defined as the negation of the inner. We know what we mean by the inner before we get to the concept of the outer, and we can use the former concept to define the latter. The outer or public is simply that which is not inner or private; but it is not true that the inner or private is to be understood as what is not outer or public. The inner comes first conceptually.

            It is not easy to prove this, though it has strong intuitive appeal. We really do have a robust conception of the inner derived from our acquaintance with our own consciousness. Our concept of the outer is acquired by way of contrast: it is what does not lie within the realm of the inner. The outer is what lies beyond consciousness, outside of it. But the inner is not what lies beyond outer objects—say, objects of perception. We don’t understand the inner simply as what is not an outside object. We don’t conceive of a sensory experience, say, as simply what is not the outer object of that experience; we have a direct grasp of the concept. Our concept of the inner is not a negative concept. But our concept of the outer is a negative concept—the concept of what is not included in the inner. The outer is what exists on the other side of the inner—yonder, over there. The public is what is not private.

            Let us accept this conceptual primacy thesis: the inner is fundamental in the order of definition. Then the consequence to which I would like to draw attention is that the inner cannot be reduced to the outer, nor eliminated from our ontology. We need the concept of the inner in order to ground the concept of the outer—we can’t have the one concept without the other. This doesn’t yet entail that the inner has to exist, since it is possible to have the concept of Fs without there being any Fs (consider unicorn). But the best explanation of why we have the concept of the inner is surely that we are acquainted with things that are inner: we are acquainted with inner things and hence we have the concept of the inner. To deny this one would have to maintain that we have the concept of the inner, and apply it liberally, yet we have never encountered anything satisfying that concept. That would seem an unlikely state of affairs. Far more plausible is the simple thought that we have the concept of the inner because we have encountered things that are inner—namely, our states of consciousness. So (a) we need the concept of the inner to give sense to the concept of the outer, and (b) we can’t have the concept of the inner without there being instances of the inner. Therefore we cannot reduce the inner to the outer (say, behavior), nor eliminate it. It cannot be that only the outer exists.

            This refutes behaviorism. Granted that behavior is something outer, the doctrine of behaviorism maintains that there is only the outer—nothing is inherently inner. But the concept of outer behavior is defined by reference to the concept of the inner, and the concept of the inner rests upon acquaintance with inner things. So behaviorism entails the existence of the inner! There cannot be only public things because the concept of the public is defined by contrast with the concept of the private, and the concept of the private can exist only if private things exist. Put simply, I can only formulate behaviorism because I know that I have inner states that contradict it. I arrive at the idea of behaviorism by deploying the concept of the outer, in contrast to the concept of the inner, but then I need the concept of the inner, which I would not have but for the existence of the inner. So I cannot consistently assert that there is nothing but the outer: for the only way I can assert this is if it is false. Since I come to grasp the concept of the outer or public via my grasp of the concept of the inner or private, and since that latter concept is grounded in acquaintance with my own inner private consciousness, it cannot be that there is nothing in the world except what is outer and public. The simple fact is that our grasp of the distinction between inner and outer depends on our awareness of our own states of consciousness: we apprehend our consciousness as inner.

 

Colin McGinn

 

                                                Raindrops and Persons

 

 

Consider a single raindrop: it exists for a certain period of time, eventually meeting its end through evaporation. But suppose instead that it divides into two before that happens—by wind or human intervention. Now we have two raindrops where before we had one: what is the relationship between the original raindrop and the resulting raindrops? The first thing to say is that the case is not like evaporation: the raindrop is in some sense still around after division, unlike with evaporation. It survives division, though now existing as two separate drops. It could easily be reconstituted, simply by joining the two drops together again. It has not simply gone out of existence or been totally destroyed. Yet we cannot say that the original drop is identical to the resulting drops, since they are not identical to each other; nor can we choose one of them as identical to the original and not the other. We might try saying that the original is identical to both drops together, but then we have to admit that one raindrop can exist as two.  [1]

Evidently there is a bit of a puzzle about division and identity—identity seems too simple to handle the case. So we might be inclined to say that raindrop survival does not depend on raindrop identity: a raindrop can survive even though no future raindrop (or collection of raindrops) is identical to the original. Still, we should note that a weaker identity claim is true: each of the resulting raindrops is identical to a part of the original raindrop. It had parts and they became separated, but they are the very same parts that existed earlier. And the survival of the original raindrop seems bound up with the fact that parts of it are identical with raindrops that exist in the future; the raindrop still exists because certain future raindrops are its parts. So identity is playing a role in survival, though a slightly more complex role than when the raindrop continues to exist in an undivided state.

            Now consider cell division. A cell can either divide or perish: if the latter occurs it has not survived, but if the former occurs we are inclined to say that it has survived—as two cells. Division is not the same as death. Again, we have a bit of  a puzzle about identity, since the future cells are not simply identical with the original, but one thing is clear: the future cells are identical to parts of the original cell. So identity is playing a role in survival, even if it is not the role it plays in cases of undivided survival. We can admit that in cases of cell division there is survival without identity between the original and any of its progeny, but still we have identity at the level of parts. Certainly, there is no reason to abandon identity altogether in accounting for survival in such cases, replacing it with some such notion as causal continuity or mere similarity. Future cells are literally identical with parts of earlier cells, and that is why we distinguish division from sheer destruction. The general principle seems to be that if the parts of an object are identical with any future objects then that object can be said to survive in those future objects. No doubt this principle would need to be qualified, but it roughly fits our intuitions about certain cases.  [2]If I disassemble a car engine and scatter its parts, we may suppose that the engine survives, even though it is not identical to any of these parts; still they are identical with parts of the original. Or if I saw an eight-legged table into two, thereby creating two tables, it may be said that the original table survives, despite not being identical with either table (and possibly not with the sum of both). The table certainly has more of a claim to continued existence than a table that is burnt to ashes.

            Then we come to brain division. Here again it is awkward to say that the brain survives in virtue of identity with the future brains, but we can still say that the future brains are identical with parts of the original brain—its two hemispheres. If the brain is a mouse brain and I put the two halves in separate mouse bodies, then we have two mice as a result–neither of which is identical to the original mouse. But it is still true that each mouse has a brain that is identical to a part of the original mouse brain. This is why we are inclined to say that the original mouse survives—certainly more than if it had been incinerated. An intelligent mouse would have a reason to favor brain division over brain incineration. We have survival without identity of mice—no future mouse is identical to any past mouse—but not survival without identity to any past part of a mouse. The part lives on, identically, though now separated from the other part, which also lives on, identically. It is not that identity is playing no role in survival, so that we must resort to some other notion, such as causal continuity. So far as these cases show, survival is still conceptually demanding—it doesn’t collapse into some weak notion of causal connection between successive states. It is not that these entities survive merely by causing future things to have certain states; they survive by virtue of there being future objects that are identical to original objects—but parts not wholes.

            It is the same with human persons. Fission cases show that survival doesn’t depend on identity with any future person, but they don’t show that survival is possible without identity between person parts. The cases are convincing precisely because half the brain of the original person sits in another body—and that half brain is identical to part of a past whole brain. The person survives because his two brain hemispheres are identical to the brains of two descendants—that, at any rate, is the intuition tapped into by brain fission cases. I would survive in one hemisphere because it is my hemisphere, and doubling up doesn’t undermine that; but if the future person did not possess a brain literally identical to half of my brain, I would not be so sanguine. I would certainly not be so sanguine if I was assured merely that the future person would be causally continuous with me in certain ways. Survival, intuitively, does require identity, but it can be identity at the level of parts not wholes.

            Does any of this conflict with our commonsense view of persons? No, it is all part of common sense. It no more conflicts with common sense than the analogous view of raindrops, cells, and mice: we quickly see that in cases of division we need to tinker with identity to explain our sense that there is survival in such cases. But we are not metaphysically committed to a mistaken view of the nature of persons—any more than with raindrops and cells. Nothing radical is demonstrated by fission cases in any of these areas. If personal fission were as common as other kinds, we would have no hesitation in describing fission cases as survival without identity (of whole persons); but we quickly see the point once an imaginary case is constructed. Our concept of a person prepares us for the possibility of fission cases, and our knowledge of the brain makes this concrete because of the facts of brain anatomy (two separate hemispheres). There is no revisionary metaphysics here, no deep error about the ontology of persons. We don’t need to replace our ordinary notion of the self with a new one that weakens it to something like mere continuity of psychological state. Brain division cases by themselves have no such dramatic consequences.

            It is sometimes claimed that we have an unduly simple view of the self and its survival because of the word “I”: the word looks simple, so we suppose that its bearer must be. This is a preposterous suggestion: who would commit such a gross non sequitur? Simple things can have complex names and complex things can have simple names. Do we have overly simple views of time and space because of the simplicity of “now” and “here”? And people often have long proper names, as well as lengthy descriptions, not just short personal pronouns. It would be absurd to infer a simple view of the self merely from the syntactic simplicity of some terms for selves.

Nor is it true that we naturally have an all-or-nothing view of personal survival, failing to recognize the possibility of degrees of survival. We don’t have an all-or-nothing view of survival in general (sand dunes, cities), and we recognize that the person of childhood may not fully survive into adulthood, as well as that dementia can weaken personal survival. We are well aware that personal survival requires psychological overlap and that survival can be a matter of degree. Just consider pushing fission cases further so that we halve each hemisphere and retain only certain aspects of the original person—we rapidly conclude that full survival is not guaranteed in such cases. There may be partial survival, which is something, but not survival of everything that matters. We don’t have an all-or-nothing view of the survival of persons (or animals). We know that in certain cases there is no survival at all, but we don’t have the naïve idea that survival is always either complete or completely absent. Maybe there are some people who do, but it is not part of general common sense. So common sense has no need of revision in this regard.

            If there is anything especially puzzling about personal fission, it is that we don’t have a clear idea of a person having parts in the way a physical object has parts. A person’s brain has literal parts and that is what guides our intuitions in fission cases, but it doesn’t follow that persons have parts: I don’t divide into two person-like hemispheres. Thus we cannot easily say that the resulting two people are identical to parts of the original person, because we don’t normally think that persons have persons as parts. But split brain cases suggest that we really do have parts that are persons—that our sense of the unity of the self is mistaken. In any case, puzzles about the divisibility of the self should not be taken to show that fission cases undermine the basic model supplied by raindrops and cells, namely that survival arises from identity as to parts. Raindrops and cells, like brains, are complex entities that have parts; these parts can be detached from each other and the results are also raindrops, cells, and brains; these results count as the survival of the original entity, in contrast to more drastic changes. We have no more reason to revise our common sense view of persons than we do of raindrops and cells in the light of fission cases. No doubt the self is philosophically puzzling, and skepticism about its existence can be pressed, but brain fission cases show nothing remarkable about personal survival. Survival without identity is commonplace, intelligible, and non-revisionary; and it involves identity anyway (as to parts). By judicious deployment of parts you can make two persons from one: in such a case the original person can be said to survive—in virtue of those parts. Similarly, you can make two raindrops from one, and you also get survival from the persistence parts. Logically, the two cases are on a par.  [3]

 

  [1] I won’t explore this possibility further in this essay, but I don’t think it is absurd. For purposes of argument I will suppose that it is ruled out; we can still maintain an identity view of survival, as I shall argue.

  [2] No just any parts: reducing an object to its atoms looks a lot more like destruction than other sorts of division into parts. I won’t here consider what notion of part is needed to make the right distinctions.

  [3] As will be obvious to many readers, I have been discussing views associated with Derek Parfit without going into questions of precise attribution. If Parfit’s views differ from those I discuss, my criticisms don’t apply to him. However, I think the views I discuss correspond closely with what he has maintained. I have not considered whether it is possible to motivate a continuity view of the persistence of the self on the basis of considerations independent of fission cases; my point is that such cases fail to take us in that direction.