Readers of Your Brain and You will know that I deny that sensations are identical with neural events. Neural events cause sensations, but the effects are different from their causes. The qualities that distinguish sensations – colors, tastes, smells, itchiness, hunger, and so on are found in the sensations, not in the coordinated groups of neuron firings that cause the sensations.
But I do agree that water is identical with H2O and that the heat in, say, a frying pan, is nothing but the average energy in the motion (or, mean kinetic energy) of the molecules that compose the pan.
So, what is the difference between these two kinds of case? In a nutshell, the identities I accept are backed by explanations, but the alleged identity of sensations and neural events is a bald piece of ideology that is not backed by explanations.
Of course, physicalists, who propose that pain is nothing but neural firing of a certain kind (let’s call it Neural Kind NK1), and having an orange afterimage is nothing but having a neural firing of a different kind (let’s call it NK2) will not take this claim of difference lying down. They’ll say “What do you mean “is backed by an explanation”? And they’ll ask whether there is really any difference in the explanatory value of “water = H2O” and “pain = NK1”.
Those are fair questions, so let’s try to answer them. I’ll address the first one by focusing on just one case, which is representative of many others. Namely, ‘water = H2O” is supported by its explaining why water dissolves salt.
What the chemists tell us is that H2O molecules, in virtue of their chemical properties, can surround individual molecules of salt (i.e., NaCl molecules). So, when we put salt into water, its molecules don’t stay together, they get separated so that H2O molecules lie between the salt molecules. And that’s just what we mean by “dissolves salt”. So, the hypothesis that water = H2O explains why salt dissolves in water. And its giving us access to such an explanation is a reason for accepting the identity of water with H2O. Other cases go similarly, and give us further support.
Well, not so fast! While there’s a good idea expressed here, it won’t do as it stands, because it’s simply not true that “dissolves salt” just means “has its molecules surrounded by molecules of the solvent”. After all, people knew that water dissolves salt long before they had any ideas about molecules. And they knew that it also dissolves sugar, for example, but not gold.
So, how can we rescue the good idea without having to say something that is plainly false? Well, how did people know that water dissolves salt when they didn’t know anything about molecules? They put salt into water and, after a bit of stirring, they couldn’t see it any more. But if they put their wedding ring in water and stirred for a long time, there would be the ring, as plainly visible as when they took it off their finger.
What this example suggests is that a better description of what “water = H2O” explains, in our particular case, is why things look the way they do. When we put salt into water and stir, we can’t see it any more. Given that we can’t see individual molecules, the chemists’ story about what happens when we put salt into water explains why we can’t see it after stirring.
The generalization is that identity claims like “water = H2O”, together with other claims we already accept (such as that we can’t see individual molecules) explains why our evidence is what it is.
Here are a few other examples to illustrate the point. (1) Water evaporates, desks do not. E.g., if there is water on a kitchen counter and we don’t do anything, the counter will be dry a few hours later; but our desks don’t disappear. That’s something we see. Chemists’ stories about strength of bonding and the transfer of kinetic energy explain why we see the dry counter, but don’t lose our desks.
(2) We can put a cool thing up against a warm thing, and feel that the first has gotten warmer after a while. If heat is just the motion energy of molecules, and motion energy can be transferred by contact, we have an explanation of why the cooler body warms up.
(3) It would be a thankless task to try to define “life”. But we can see that things we agree are living typically grow. A strictly biological story (ingestion, digestion, metabolism) explains why organisms get bigger. This supports the idea that living things are nothing but biological systems; and similar explanations of other typical functions converge on the same result.
What about the second question? If “pain = NK1” were like “water = H2O”, then there being NK1 should explain why some piece of evidence is what it is. No such explanation seems to be in the offing. If “having an orange afterimage = having NK2” were like “heat = mean kinetic energy” there should be some piece of evidence that is explained by NK2. Again, no such explanation seems available. It’s not even clear why any neural event should ever be correlated with any sensation at all.
Physicalists may suggest, however, that such explanations are available. Namely, we see that pains cause pain behavior (e.g., withdrawing, favoring injured parts, avoiding repeated contact with the source of the pain), and we see that having orange afterimages cause reports of their occurrence. “Pain = NK1” and “having an orange afterimage = having NK2” explain these pieces of evidence.
But these causal connections are not pieces of evidence: they are inferred conclusions. Moreover, the inferences depend on assuming the identities. Actions, including utterances of reports, are – as physicalists agree – caused by neural events. The neural events are themselves adequate to bring about the behavior. The causal relevance of pain or having an orange afterimage comes in only by using the identity claims. The pattern is: NK1 causes behavior B, pain = NK1, therefore pain causes behavior B.
It follows that these alleged pieces of evidence cannot be used to support identity claims; for to make such a use would be circular reasoning. The pattern would be this:
 pain = NK1
 NK1 causes behavior B
 Therefore, pain causes behavior B.
 We can explain why (or how) pain causes behavior B by assuming that pain = NK1.
 We are entitled to identity claims if they are backed by explanations.
 Therefore, we are entitled to the identity claim that pain = NK1.
But you can’t get to  without assuming ; and the legitimacy of that assumption is the conclusion of this reasoning. So this kind of “support” for “pain = NK1” depends on assuming what was to be supported.
Even those who think that “pain = NK1” or “having an orange afterimage = NK2” are true ought to be able to see that such claims cannot be supported in a way that parallels our reasons for accepting “water is H2O”, “heat is mean kinetic energy” and similar identity claims.