The Identity Theory




1. Argument from violations of Leibniz’ Law (i.e. properties that mental states have but brain states do not, or vice versa)  
(a) spatial properties (e.g. having a location).  Descartes held that mental states have no location (since immaterial substance is not extended). Physical states do have a location.  So a mental state can't be identical with any physical state. There is no good reason to deny that mental states are located (unless like Descartes you're already committed to the view that mental states are modes of a nonphysical substance which is not extended, but to assume that in this context is to beg the question, since it is precisely what needs to be established). (Actually even Locke, a rough contemporary of Descartes's, and an apparent believer in immaterial substances, didn't see any reason to deny that minds and mental states had locations.)
(b) introspectability (being introspectively known as a mental event) 1. such properties are bogus (if they were genuine we could "prove" e.g. that Samuel Clemens isn’t Mark Twain, or that heat is not molecular motion)

2. Anyway, the identity theorist can argue that brain states are known by introspection:  that's what we are detecting when we notice that we are in a particular mental state! It's just that, although we are detecting a brain state, we don't realize that we are detecting a brain state.  (Compare: we are seeing an electrical discharge when we see lightning, even if we don't realize that this is what we are seeing.)

2. "species chauvinism": The identity theory seems to unjustifiably presume that beings unlike us couldn't have mental states (just as male chauvinism unjustifiably assumes that people who aren't male couldn't be intelligent, strong, worthy of the vote, or whatever).  To spell the argument out in more detail: (1) The identity theory says that every mental state is identical with some human neurophysiological state. (2) If so, then it is impossible for a creature to be in a particular mental state if the creature cannot be in the corresponding neurophysiological state. (follows from (1) by contraposition.) (3) However, it seems at least possible that there could be beings that had mental states without having a human physiology: for example, extraterrestrial intelligences, or supercomputers running AI programs, or possibly even terrestrial creatures like octopi that have a very different evolutionary history than we do. (4) Therefore, since (3) shows that the consequence of the identity theory spelled out in (2) is implausible, we should reject the identity theory. This may be a good objection to simplistic versions of the identity theory. A more sophisticated version would suggest that which neurophysiological state is identical with a given mental state is determined by which neurophysiological state occupies a certain functional role.  This could vary from one species to another. Of course, such a version of the identity theory could equally well be described as a version of functionalism. David Armstrong and David Lewis hold views of this sort.
3. Leaves out knowledge of what it’s like to be in a certain mental state (this can also be used as an argument against other versions of materialism; see Nagel, "What Is It Like to Be a Bat?" and Jackson, "What Mary Didn't Know," for defenses of this argument) perhaps this is a way of knowing rather than a content of knowledge (David Lewis has suggested this in response to Nagel's "What Is It Like to Be a Bat?" and Jackson's "What Mary Didn't Know")

Leibniz' Law (the indiscernibility of identicals):  for any objects x and y, if x = y (that is, if x and y are the very same object), then x has all the properties y has and vice versa. 

(Sometimes people use the term "Leibniz' Law" to refer to the conjunction of this principle with another, much more controversial principle, the identity of indiscernibles, which holds that if x and y have all the same properties, then they must be the same thing.  But this isn't relevant to the present issue.)

Last update: April 16, 2012.
Curtis Brown  |  Introduction to Philosophy  |  Philosophy Department  |  Trinity University