that by adopting (3), we can explain (2). The reason every instance of B is an instance of R is because R and B are the same thing. That's what explains the correlation. You can't go on to ask why (3) is true, because, as we claimed above, identity facts, unlike law facts, aren't the sorts of facts that stand in need of explanation. Things are what they are. You can ask why one should think that the identity statement is true, of course, and this is just to ask for one's evidence. In this case, the ability of (3) to explain (2) would be the answer. But you can't ask what, metaphysically speaking, makes it true. That question has no sensible answer. There can't be an explanatory gap if there's nothing (relevant) to explain.
3.6 Gappy Identities As I argued in chapter 2, I generally endorse the claim that pure identities are not suitable candidates for explanation. Yet, when we look more closely, it seems that things are not quite so straightforward. In particular, there is a sharp epistemic contrast between various standard cases of identity claims and the case of an identity claim like (3) R = B. With the standard cases, once all the relevant empirical information is supplied, any request for explanation end p.81 of the identities is quite unintelligible, as the considerations just adduced would predict. In the case of (3), however, it still seems quite intelligible to wonder how it could be true, or what explains it, even after the relevant physical and functional facts are filled in. This difference calls out for explanation. To illustrate the distinction I have in mind, consider three cases: one involving natural kinds, one involving indexicals, and one involving demonstratives. Take indexicals first. It's notorious that no purely descriptive, qualitative statement can entail one containing an indexical (see Perry 1979). No matter how full a description there is of what's happening to a certain body in a certain spatio-temporal location, it won't follow logically that it's happening to me here now. Yet, of course I can explain why I have a cut on my hand now by citing events describable in terms like “This body at time t encountered a knife while cutting a bagel.” I don't get a derivation, of course, without bridge principles, like “That body is mine” and “Now is soon after time t,” and so on. But neither the materialist nor the anti-materialist will claim that the inability to derive “My hand is cut now” from nonindexical premises shows the metaphysical irreducibility of the properties, being mine and being now. More significant than the issue of metaphysical irreducibility, however, is the question of the epistemic status of the bridge principles. After being informed that the body referred to is mine, and the time t is shortly before now, would there be any sense to the question “How could that be my body?” or “How could t be now?”? Of course I can think of circumstances that might give sense to these questions, but they involve ascribing properties to the body referred to in the premise different from those I believe my body to have. What there really doesn't seem to be is any cognitively significant content to the