Scientific IdentitiesThe problems we've seen with behaviorism prompted philosophers in 1950s and 1960s to move to new kinds of materialist theories, which get called Causal Theories of Mind.
We will look at two Causal Theories of Mind:
What these theories have in common is that, unlike behaviorism, they claim that our mental states as real inner states that cause our behavior. Behaving in certain ways is not all there is to being in pain; it just gives us evidence that someone is in pain, in the same way that smoke gives us evidence that there's a fire.
It's important to distinguish between two different kinds of claim that one thing is identical to something else:
When the identity theorists say things like:
they are not trying to tell us what the concept PAIN means. One can understand the concept of PAIN perfectly well without knowing that pain and C-fiber firing are the same thing. Rather, the identity theorists are proposing a scientific identity. They are claiming that, just as science taught us that water is H2O, and that lightning is a certain sort of electric discharge, so too has it taught us that mental states are certain kinds of brain states.
At one point in his paper, Smart does offer an account of what some sensation-concepts, like "having an orange after-image," mean. He argues that the meaning of these concepts is topic-neutral. That is, it leaves it an open question whether the sensations in question are identical to processes in the brain, or to procesess taking place in Cartesian souls, or to some other sort of process. So, in Smart's view, the meaning of sensation-concepts leaves it open whether or not sensations are identical to brain-processes. It takes scientific investigation to show that sensations are in fact brain processes.
What Reason Do We Have to Believe the Identity Theory?The natural sciences are having increasing success in explaining our behavior as the result of physio-chemical mechanisms. These scientific explanations of our behavior appeal only to the presence of certain processes in our brain (and certain neural and muscular connections between our brain and our limbs). In addition, we are getting better and better explanations of phenomena like perception, memory, and reasoning in terms of processes in our brain. So it looks like we have two choices:
Leibniz's Law and its LimitsLeibniz's Law says that if X and Y are one and the same thing, then X and Y have to have all the same properties. So if X and Y have different properties, then they cannot be one and the same thing. Here is an example. We look out the window and see Superman flying by. We wonder who Superman is. Now as Superman flies by outside the window, we see our friend Jimmy Olson standing beside us. We can reason like this:
Superman is flying outside the window.In general, this argumentative strategy is perfectly legitimate. However, we have to be careful. We can't apply it everywhere. For instance, we can't argue like this:
Lois believes that Superman is strong.For Superman is the mild-mannered reporter that Lois works with. She just doesn't know that.
Let's introduce some technical apparatus that philosophers use in discussing these cases.
The extension of a term is the set of things which it picks out in the world, as the world actually is. So, for instance, the extension of the name "Jim Pryor" is a certain person. The extension of the description "the teacher of Phil 156 at Harvard in Spring 2002" is also a certain person, in fact the same person. So the name "Jim Pryor" and the description "the teacher of Phil 156 at Harvard in Spring 2002" have the same extension. If the world had gone differently, however, and Professor Siegel were teaching Phil 156 this term, then the description "the teacher of Phil 156 at Harvard in Spring 2002" would have a different extension. In that case, its extension would have included Professor Siegel, and not Jim Pryor. So although "Jim Pryor" and "the teacher of Phil 156 at Harvard in Spring 2002" have the same extension with respect to the way things actually are, they have different extensions with respect to other, counterfactual situations.
General terms like "cordate" also have extensions. The extension of "cordate" is the set of all the things which actually are cordates. (A cordate is a creature with a heart.) So, since you are a cordate, you are included in the extension of "cordate." In other possible situations, different cordates might existed, so with respect to those possible situations, the extension of "cordate" would be different.
Next let's talk about different kinds of sentence fragments. A sentence fragment is a sentence with a hole in it. For instance:
____ is strong.is a sentence fragment with a hole in the noun phrase. If we plug that hole with the name "Superman," we get the complete sentence:
Superman is strong.A sentence fragment "___ is such-and-such" counts as extensional just in case filling in the blank with different terms having the same extension will always result in sentences with the same truth-value. (Such sentence fragments are also called "transparent.")
"___ is at least 30 years old" is an extensional sentence fragment. For example, substituting "Jim Pryor" and "the teacher of Phil 156 at Harvard in Spring 2002" into the fragment will result in sentences having the same truth-value.A sentence fragment "___ is such-and-such" counts as intensional just in case it's not extensional. That is, just in case filling in the blank with different terms having the same extension can sometimes result in sentences with different truth-values. (Notice the spelling! It's "intensional" with an "s". Such sentence fragments are also called "opaque.")
For instance, "It is necessary that ___ is a human being" is an intensional sentence fragment, because substituting "Jim Pryor" and "the teacher of Phil 156 at Harvard in Spring 2002" into the fragment will result in sentences having different truth-values. (It is necessary that Jim Pryor is a human being. It is not necessary that the teacher of Phil 156 be a human being. We might have hired a robot, or an intelligent chimp, to teach the class.)Examples like this last one show that attributions of beliefs are intensional. When you say "Lois believes that ___ is strong," putting different noun phrases in the hole can result in sentences with different truth-values, even if the words you put in the hole all have the same extension.
The same is true for attributions of any other representational or intentional state. Lois hopes that Superman will kiss her, but she does not hope that the mild-mannered reporter she works with will kiss her. She doubts whether Superman is in the office, but she does not doubt whether the reporter she works with is in the office. And so on.
What is the upshot of all of this for Leibniz's Law? It means we can't use it in connection with claims about what people believe, or disbelieve, or doubt, or hope, or know is the case. For any sentence fragment like "I believe that ____ is strong" or "I hope that ____ will kiss me" or "I doubt whether ____ is here" will be intensional. So even when we do have two expressions "A" and "B" that refer to the same man, we can't be guaranteed that plugging "A" and "B" into the holes in these fragments will result in sentences with the same truth-value. We do get different truth-values when we plug in "Superman" and "the mild-mannered reporter that Lois works with," but that does not entitle us to conclude that Superman and the mild-mannered reporter are different people.
Leibniz's Law and Scientific IdentitiesNow, what is the upshot of all of that for the identity theory?
When we're talking about proposed scientific identities like "lightning is electrical discharge" or "pain is C-fiber firing," they will only be knowable by doing empirical investigation. One can possess the relevant concepts yet fail to know that these identities are true.
As a result of this, there may be some sentence fragments that come out true when you plug "lightning" in the hole, but false when you plug in "electrical discharge." And there may be some that come out true when you plug in "pain" but false when you plug in "C-fiber firing."
For example, we cannot prove that pain and C-fiber firing are distinct by this sort of argument:
I can tell that I have pain just by attending to how I feel.That is not a legitimate use of Leibniz's Law. "I can tell that I have ____ just by attending to how I feel" is an intensional fragment. The mere fact that we get a different truth-value when we plug in "pain" than we get when we plug in "C-fiber firing" does not entitle us to conclude that pain and C-fiber firing are different states.
Smart thinks that such illegitimate appeals to Leibniz's Law lie behind several common objections to identity theory. Here are two of the ones he discusses:
Objection 1: Sensations and brain processes differ in epistemic respects. For instance, you can know lots about your sensations while being ignorant about your brain processes.
Smart replies: This does not support the conclusion that sensations and brain processes are distinct, any more than analogous reasoning supports the conclusion that lightning and electrical discharges are distinct. Whenever we're dealing with a posteriori scientific identities, the phenomena being identified will differ in epistemic respects.Objection 7: I can imagine having sensations without having brain processes.
Smart replies: This does not show that sensations and brain processes are distinct, any more than the fact that I can imagine lightning being some non-electrical, purely optical phenomenon shows that lightning has to be distinct from electrical discharge.