Sameness and Skepticism
The concept of identity is central to philosophy. Philosophers are characteristically concerned with whether A is identical to B. Is the mind identical to the brain, is knowledge true justified belief, is the good maximum utility, are numbers sets, is meaning reference, is the good life the intellectual life? Some philosophers favor identity, keeping things to a minimum, while others insist on difference, multiplying what there is. Naturally, then, they have been interested in identity as such: its logic, its metaphysics, and its epistemology. How is it related to indiscernibility, is it known a priori, is it absolute or relative, is it definable? Multiplicity is surely a basic fact about the world—it is presented to us as consisting of many distinct objects—and identity comes with multiplicity (this thing is not that thing but it is itself). It is natural to wonder how diverse the world really is: what is identical to what, and what is different? The very idea of conceptual analysis presupposes the concept of identity: is the concept of knowledge, say, identical to the concept of true justified belief? Synonymy is just the identity relation applied to words. Then too, there is identity through time, identity across worlds, and personal identity. Philosophers are constantly making judgments of identity and difference; you might suppose that it is their main occupation. Philosophers are identity hounds, sniffing out what is the same and what is different. That, we might say, is what philosophy is identical with.
It would be nice if such judgments were skepticism-proof. Then philosophy would enjoy an epistemic privilege. It might even elevate philosophy above science, which is not skepticism-proof. But this seems like a forlorn hope: identity judgments are often wrong. As every philosopher knows, people used to think that the Morning Star is not the same as the Evening Star, but actually they are the same planet, as we have now discovered. Now is that judgment infallible? Sadly no, since we could have made an astronomical mistake: maybe there are really two planets after all—it just seems as if there is one. Perhaps there are doppelgangers everywhere, so that we are constantly meeting different people we think are the same. It might turn out that all of our identity beliefs are false: this seems like an epistemic possibility. Nor is this possibility limited to a posteriori identity beliefs: people used to think on analytic grounds that knowledge is identical to true justified belief, but then they discovered counterexamples to that claim. And maybe that judgment will turn out to be wrong—perhaps we misunderstand the concept of justification. Might it turn out that the meaning of “bachelor” is not identical with the meaning of “unmarried man”? These are not easy questions, and it would be wrong to dismiss them as absurd. Skepticism is nothing if not resourceful.
However, it seems to me that there is one area in which skepticism about identity cannot gain a foothold—an area in which we can be certain that our identity and distinctness judgments are true. I don’t just mean with respect to the mind or numbers or meanings; I mean with respect to the external world, the skeptic’s go-to position. We may not be certain whether external objects exist or what their nature is, but we do infallibly know propositions about their identity and difference. For example, if I now judge that this cup is not identical to that computer, I cannot be wrong about that: it could not turn out that cup and computer are one and the same. Nor can I be wrong about the cup being identical to itself: this is a rock solid identity judgment. It is true that I might be wrong about whether the cup and the computer exist, or about whether I am dreaming, or about whether such objects are ideas in the mind of God; but none of that implies that I could be wrong about their identity and distinctness. Whatever the truth about the cup and computer is, I must be right in judging them non-identical. They might not have the shape and color they appear to have, but it is certain that they are not the same thing. And note that this is not a proposition about their appearance: it isn’t just that I know the appearances are non-identical. I know that whatever lies on the other side of the appearances, even if it is just non-existent intentional objects, these things are not identical. This is as certain as the proposition that each thing is identical to itself. Maybe both objects came into existence a second ago and are not identical to anything preceding them, despite the appearances, but still they are self-identical and other-distinct. Thus reality—whatever it is—is necessarily composed of distinct objects, a great many of them. Not metaphysically necessary, but epistemologically necessary: it could not turn out that our impression of multiplicity is incorrect, though there are possible worlds containing a single object. There are no illusions of multiplicity in which this cup is really identical to that computer. Maybe the cup is itself a computer, and the computer a cup, but still they are not identical. In the same way objects in dreams are not identical: it could not turn out that every object you dreamed about last night is really one and the same object. I can see that the cup and the computer are distinct, though I can’t see that they exist. It is not true to say that every question of identity is self-evident, but in some cases it is. So far as I can determine, this is the only fact about external objects that is proof against the skeptic: not existence, not materiality, not shape or color, not even being a cup or a computer. But whatever the real truth may be about these things, I at least know that they are not the same. Here appearance does entail reality. Presumably this is because identity and difference are such minimal conditions—they commit us to so little. Still, they are something: they form a roadblock to the outright victory of the skeptic. They are a counterexample to the claim that everything about our judgments concerning the external world is fallible.
I think we can generalize this point beyond perceptually presented objects. We know with certainty that red is distinct from blue and triangles are distinct from rectangles. What these properties ultimately are, and whether they are ever instantiated, is as may be; but that they are distinct is indisputable. Nor is there any doubt that red is identical to itself (etc.). Again, appearance and reality cannot diverge. Likewise, I can be sure that 2 is not 3 and that pain is not belief and that democracy is not monarchy. These identity facts are infallibly known to me—and they are objective facts not facts about the mind. When they are known they are incorrigibly known. They are as certain as the Cogito. Indeed, the Cogito presupposes such facts, since we must accept that thinking is not identical to the self that thinks (or else it is a mere tautology). All our judgments presuppose conceptual distinctness, ruling out the possibility that our concepts are all identical. The skeptic may convince us that meanings don’t exist, or that we don’t know what they are, but he can’t convince us that the meaning of “plus” is the same as the meaning of “minus”. If he could, we could turn round and ask him whether “ignorance” means the same as “knowledge”. Both skeptic and anti-skeptic presuppose that they know that words and concepts are distinct from other words and concepts. Did Quine ever argue that “rabbit” might mean the same as “table”, or Kripke’s Wittgenstein contend that “plus” might mean “pain”? As with external objects, such judgments are invulnerable to skeptical challenge, which puts them in a very special class. And don’t I also know with complete certainty that I am not you? Descartes could have proposed the “distinct-self Cogito”: I think, therefore I am not you. That is, my knowledge of myself rules out the possibility that I am (so to speak) someone else—I cannot suppose that this self might actually not be distinct from another self. It could not turn out that I am you. That is not a real epistemic possibility. Maybe you don’t exist at all, but if you do I know with certainty that I am not identical to you. There may not be any other minds, but I know that my mind is not the same as yours if there are other minds. I know that I am identical to myself and that I am distinct from you: no skeptical scenario could convince me otherwise. I also know with certainty that I am not identical to this cup, so my distinctness from other things is guaranteed. No evil demon can persuade me that I am merely dreaming that I am different from other selves and from the objects around me. This is why I never dream such things: the very idea is preposterous verging on nonsensical.
Given this, the philosopher’s interest in identities and differences is not vulnerable to the skeptic’s corrosive doubts. Such knowledge is uniquely positioned to avoid these doubts because beliefs about identity and difference are so epistemologically undemanding. It is not like inductive knowledge or inference to the best explanation, which are exercises in epistemological adventure and full of risk. This doesn’t mean it is easy to acquire, but once acquired it is hard to dislodge: we really do know that knowledge is not identical to true justified belief, that use and mention are distinct, that goodness is not the same as pleasure. There are no skeptical scenarios like the brain in a vat that can persuade us that these are spurious distinctions; we couldn’t be just dreaming that use is not mention. The scientist must face the possibility that nature is not uniform, but the philosopher needn’t worry that everything may be one. We know perfectly well that the world (whatever it is) contains a multitude and that each of its elements is identical to itself. This may not be much, but it is something. It curbs the skeptic’s enthusiasm.
 Non-existent objects can be the same or different: Hamlet is not identical to Macbeth, though he is identical to himself (and to the Prince of Denmark).
 In Berkeley’s system distinct objects are distinct ideas in the mind of God; there is no suggestion that object distinctness might be an illusion—that there might really be just one Big Idea. Nor have I ever heard of a skeptic who contends that multiplicity might be a perceptual illusion, as external existence might be an illusion. Even the brain in a vat is confronted by multiplicity—all those fake tables and chairs. Illusions of existence don’t entail illusions of identity.
 Let this point not be underestimated: with respect to any object or property X I know with certainty that X is identical to itself. The skeptic can never take this away from me: it is a piece of substantive knowledge concerning any object that comes to my attention. Such identity knowledge is invulnerable to skepticism (and it is not trivially tautological). When Frege said, “Identity is that relation a thing has to itself and to no other thing” he was stating a profound truth—and one that places a limit on the scope of skepticism.
 I am not here talking about philosophical monism in any of its varieties; I am talking about commonsense judgments of distinctness, as that my two cats are distinct or that Tuesday isn’t Wednesday. The world might be composed of one type of thing or stuff, but we can rule out the possibility that all our ordinary objects are really identical (e.g. Mount Everest is identical to London, Queen Elizabeth is identical to Mars, everything is identical to Brad Pitt). Even if the world is one giant particular, it has numerically distinct parts. We can be certain that the world is Many (though not necessarily existent).