Mind, identity theory of

2. Early objections

Here are some of the many objections that greeted the identity theory when it became well known through, especially, Smart (1959).

It was objected that the ancients knew about mental states while knowing next to nothing about the brain. How could this be if mental states are identical with brain states? However, as identity theorists observed, science has established many identities that were unknown to the ancients. They did not know that lightning is identical with an electrical discharge, temperature in gases is mean molecular kinetic energy, and water is H₂O. The identity of mental states with brain states is, identity theorists urged, of a piece with scientific identities in general. We learned, for example, that temperature is mean molecular kinetic energy by discovering that it is mean molecular kinetic energy that is responsible for the phenomena we associate with temperature. In the same general way, identity theorists claim, we have discovered that mental states are brain states by discovering that brain states are responsible for the behavioural phenomena that we associate with mental states, and we will discover which particular brain states are which particular mental states by discovering which particular brain states are responsible for the behavioural phenomena associated with those mental states.

Identity theorists also noted that water, lightning, and temperature do not present themselves to us as H₂O, electrical discharge, and mean molecular kinetic energy, respectively; so it cannot be an objection to their theory that mental states do not present themselves as brain states.

Many objected that the identity theory violates Leibniz’s Law that if x = y, then x and y share all properties (see Identity), on the ground that mental states and brain states differ in their properties. An after-image is, say, yellow and in front of my face, but my brain states are not yellow, and are inside my head; again, an itch is, say, in the middle of my back, but my brain is not in the middle of my back. They also pointed out that brain states are at a certain temperature but that it is surely absurd to hold that my belief that the Earth is not flat is at a certain temperature.

Identity theorists replied by arguing that our talk about mental states is, from a logical point of view, misleading. We talk as if mental states involved relations to mental objects. We say, for example, ‘I have a headache’, and this apparently has the same logical structure as ‘I have a car’: in both cases we seem to be asserting a relation between a person, me, and an object – a headache in one case, a car in the other. However, as identity theorists noted, cars are very different from headaches: cars can exist without their owners (if I die suddenly, my car will go on existing), but headaches cannot exist independently of being experienced (if I die, any headache of mine necessarily dies with me). Headaches, and mental states in general, are necessarily someone’s. In consequence, they argued, we should think of headaches in the way we think of limps: limps are not things we have when we limp. When I say that I have a limp, I am simply saying that I limp; similarly, to say that I have a headache is to say that my head aches. Attributing properties to aches and limps is really attributing properties to achings and limpings. To have a bad limp is to limp badly, and to have a winning smile is to smile winningly; likewise, to have a bad headache is to have a head that aches badly (see Mental states, adverbial theory of). Strictly speaking, there are no mental objects, and so, in particular, no after-images to be yellow and in front of faces, and no itches to be in backs; there are experiences of having after-images and of having one’s back itching. These experiences are not yellow, not in front of us or in our backs, and so the fact that brain states are not yellow, not in front of us and not in our backs is no objection to the identity theorist’s claim that these experiences are identical with brain states.

The discussion of belief followed a different course. There is an important distinction between my believing something, my state of belief, and what I believe. My believing that there is a tiger before me is, most likely, caused by seeing a tiger and is a state of mine, whereas what I believe – that there is a tiger before me, the proposition believed, as it is often put (see Propositional attitudes) – is not caused by my seeing the tiger and is not a state of mine. Again, there is my belief that the Earth is not flat, thought of as a state of mine that causes me, say, to reassure a traveller that they do not have to worry about falling off the edge, and there is the proposition that the Earth is not flat, which is what I accept. Now, the identity theory is not a view about the objects of belief, the propositions; it restricts itself to claiming that the state of believing is a state of the brain. And, identity theorists argued, although it would be absurd to hold that the objects of belief are at a certain temperature, it is not absurd to hold that the believings are.

Finally, some philosophers objected that the behaviour associated with having a mind, and in particular, that associated with intelligence, rationality and free action, displays a flexibility and sophistication incompatible with a purely material etiology. There are complex issues here, but we can note two serious problems for this objection. First, computers have enlarged our conception of the behavioural flexibility and sophistication compatible with a purely material etiology; and, second, it is hard to see how having an immaterial etiology would make any difference to the conceptual issues at stake (and, of course, quantum mechanics has broken the tie between having a material etiology and being determined).