Kripke’s Critique of Materialism Debunked

In “Naming and Necessity”1placeholder and “Identity and Necessity”2placeholder Saul Kripke draws invidious distinctions between various theoretical identity statements. Specifically, he claims that both

(1) Heat = molecular motion

(2) Light = a stream of photons

are true, whereas

(3) Pain = the firing of C-cells,

an example of the mind/brain identity thesis, and hence a case of metaphysical materialism, is false. His pronouncements, held with great fanfare and received with equal acclaim, have generated a publishing industry and made for countless careers; but I will bypass all that and go back to the original source. To show that it (and hence the fanfare, the acclaim, the industry, and the careers) was based on a simple misunderstanding, and should be scrapped from the history of the subject—except, perhaps, as a warning to be more careful in the future.

The reason alleged for the truth of (1)-(2) and the falsity of (3) is always the same. (1)-(3) are identity statements, and identity statements, according to Kripke, are necessary. Therefore, if (1)-(3) are true, so are

(4) Necessarily, heat = molecular motion

(5) Necessarily, light = a stream of photons

(6) Necessarily, pain = the firing of C-cells.

But (4)-(6) do not look true, as it seems easy to imagine heat, light, and pain issuing from circumstances different from the ones that (we take it) are associated with them in the real world. And here comes Kripke’s formidable reprimand:

“We identify, for example, light with electromagnetic radiation between certain limits of wavelengths, or with a stream of photons. We identify heat with the motion of molecules; sound with a certain sort of wave disturbance in the air; and so on. Concerning such statements the following thesis is commonly held. First, that these are obviously contingent identities: we’ve found out that light is a stream of photons, but of course it might not have been a stream of photons. Heat is in fact the motion of molecules; we found that out, but heat might not have been the motion of molecules. Secondly, many philosophers feel damned lucky that these examples are around. Now, why? These philosophers, whose views are expounded in a vast literature, hold to a thesis called ‘the identity thesis’ with respect to some psychological concepts. They think, say, that pain is just a certain material state of the brain or of the body, or what have you—say the stimulation of C-fibers. (It doesn’t matter what.) Some people have then objected, ‘Well, look there’s perhaps a correlation between pain and these states of the body; but this must just be a contingent correlation between two different things, because it was an empirical discovery that this correlation ever held. Therefore, by “pain” we must mean something different from this state of the body or brain; and, therefore, they must be two different things.’ Then it’s said, ‘Ah, but you see, this is wrong! Everyone knows that there can be contingent identities.’”3placeholder

Now, however, the master is to set things right. Using the arsenal of a posteriori necessary truths, he will argue that (1) and (2) are indeed necessary whereas (3) is not—all of it, I should add, without stating or defending any theory of what the essence of heat, or light, or pain is: just voicing, with great emphasis, his own opinions. Therefore, he concludes, those naughty philosophers should not feel so damned lucky: there is no parity among these statements—on his saying so—and (3) will have to prove its shaky case on its own.

To bring some order into this mess, I begin with the remark that de dicto readings of (4)-(6), where the modality is read as relative to a sentence (Latin dictum), are obviously false: I said already that there are possible worlds in which (1)-(3) are false. As Kripke clearly means de re readings, where the modality is read as relative to an object (Latin res), that is how I will read them. But, as I pointed out in my “On Identity, Necessary and Contingent”4placeholder (from now on, INC), in order to do justice to the complexities of de re readings, we must go beyond the elementary first-order language used by Kripke, and allow for the construction of complex predicates. Whereas in his language all the structure he can bring out of (1)-(3) is

(7) □(a = b),

for example,

(8) □(heat = molecular motion),

in the language described in INC we can distinguish (by using the operator λ for the construction of complex predicates) at least the following three readings:

(9) λx□(a = x)(b)

(10) λx□(x = b)(a)

(11) λxλy□(x = y)(ab),

for example,

(12) λx□(heat = x)(molecular motion)

(13) λx□(x = molecular motion)(heat)

(14) λxλy□(x = y)(heat, molecular motion).

What, then, about these statements? Does molecular motion, say, have the property of being necessarily identical with heat?

Here is something Kripke says:

“What characteristically goes on in these cases of, let’s say, ‘heat is molecular motion’? There is a certain referent which we have fixed, for the real world and for all possible worlds, by a contingent property of it, namely the property that it’s able to produce such and such sensations in us. Let’s say it’s a contingent property of heat that it produces such and such sensations in people. It’s after all contingent that there should ever have been people on this planet at all. So one doesn’t know a priori what physical phenomenon, described in other terms—in basic terms of physical theory—is the phenomenon which produces these sensations. We don’t know this, and we’ve discovered eventually that this phenomenon is in fact molecular motion. When we have discovered this, we’ve discovered an identification which gives us an essential property of this phenomenon. We have discovered a phenomenon which in all possible worlds will be molecular motion—which could not have failed to be molecular motion, because that’s what the phenomenon is.”5placeholder

What Kripke is saying, modulo the verbiage, is that molecular motion cannot fail to be molecular motion. The mention of heat, insofar as it elicits the memory of sensations we have, drops out as irrelevant: we used it to focus on what we were referring to, but now we have discovered what that is (italics do a lot of work here) and we can let it go. Does that make his reasoning irreproachable?

Far from it. Since heat has dropped out, what seems to be in question is a simpler statement than (12)-(14), that is,

(15) λx□(x = molecular motion)(molecular motion)

or perhaps

(16) λxλy□(x = y)(molecular motion, molecular motion).

What about them?

(16) is trivially true, since it says that molecular motion, understood as something in this world, and molecular motion, understood as something in this world, are necessarily identical with one another. It reduces to the necessity of self-identity, and has nothing to say about the behavior of molecular motion, or anything else, in another world.

(15) is a different matter. To give the appearance that it, too, is true, Kripke proceeds as follows: He takes “molecular motion” as what theoretically identifies a phenomenon in this world; then, when he moves to another world, he looks there for something that could be identified theoretically in the same way (he looks there for “what the phenomenon is”).6placeholder If he finds it, of course it is molecular motion, and (15) is verified; if it’s not there, he will have to deal with it as with all other cases of objects (or events) missing in a given world—and in any case (15) will not be falsified. But this is too damned easy! It is like saying that the ace of spades is necessarily the ace of spades because in another world we will first look for an ace of spades and then make that what the ace of spades is there. Kripke’s position, when explained clearly, looks surprisingly close to David Lewis’ counterpart theory.7placeholder

To say something that is less ridiculous, we need to identify the “phenomenon” we are dealing with in a way that is independent from the very theoretical specification we are using. For illustration, consider the statement

(17) This storm might not have happened.

Of course, this cannot mean that in another possible world a storm is not one. What it means is that the particular weather configuration observed at time t in space s, in another world, is not a storm: an insignificant change in the initial conditions (a butterfly flapping its wings?) has caused a different course of the weather.

Same thing here. When we go looking for what molecular motion is in another world, we should not look for molecular motion. We should identify the phenomenon in this world, say, by spatiotemporal coordinates as we did with the storm—talk about it as, say, the phenomenon taking place at time t in space s—and then see how this phenomenon shapes up in that other world. And it might well be that it does not shape up as molecular motion.

Exactly the same analysis applies to the other cases, which are in fact all on a par. Heat (that is, molecular motion) is only contingently molecular motion, since in another world that phenomenon, unless you surreptitiously identify it as molecular motion, might not be molecular motion. Light (that is, a stream of photons) is only contingently a stream of photons, since in another world that phenomenon, unless you surreptitiously identify it as a stream of photons, might not be a stream of photons. And, assuming for the sake of hypothesis that the identity thesis is true, pain (that is, the firing of C-cells) is only contingently pain, since in another world that phenomenon, unless you surreptitiously identify it as the firing of C-cells, might not be the firing of C-cells. In fact, by the same kind of reasoning even pain, understood now as felt pain, is only contingently pain, since in another world that sensation, unless you surreptitiously identify it as pain, might not be pain at all, thus contradicting another Kripkean rhetorical flourish:

“Can any case of essence be more obvious than the fact that being a pain is a necessary property of each pain?”8placeholder

(In addition to italics, vain rhetorical questions are cheap ways of running an “argument” here.)

Notice that I said nothing about how to read theoretical identifications, or about whether the identity thesis is true. All of that belongs in metaphysics, or in what Kant would have called the metaphysical foundations of natural science. Though I have my own views on the matter, my point at present is only a critical one. Consistently with my conclusions in INC, it amounts to saying that you cannot hope to resolve such vast and profound metaphysical issues by a few logical tricks and a lot of chattering. A purely negative point; but one that, if taken to heart, will block a fruitless line of inquiry and have us concentrate on more promising ones.