3.4.3 - Mind-brain type identity theory

The Identity Question

The mind-body problem asks how mental states fit into a physical world. When you feel pain, believe that it is raining, remember a name, or want a drink, something mental is going on. The physicalist challenge is to explain what that "something" is without adding a non-physical mind or non-physical properties to reality.

Mind-brain type identity theory gives a bold answer:

All mental states are identical to brain states.

The word identical is doing important work. The theory does not say mental states are merely caused by brain states, correlated with brain states, or accompanied by brain states. It says a mental state just is a brain state, in the way that lightning is not merely correlated with an electrical discharge but is an electrical discharge.

The theory must cover every kind of mental state (pain, belief, desire, memory, fear, visual experience), and it identifies each with a brain state: a physical state or process in the brain or central nervous system.

So type identity theory is stronger than saying every particular mental event happens to be physical. It says each mental kind lines up with a physical brain kind:

Pain = brain-state type B1
Believing that p = brain-state type B2
Wanting X = brain-state type B3

Those labels are schematic. The identity theorist does not need to know the exact neuroscience before stating the theory, but the theory does need there to be such identities. If there are mental types that cannot be matched with brain-state types, the theory is in trouble.

Ontological Not Analytic Reduction

The central distinction is between ontological reduction and analytic reduction.

If mental states are brain states, then there are not two things, a mental state plus a brain state. There is one physical state described in two ways. On this view, the ontology, the inventory of what exists, contains brain states rather than extra non-physical mental items.

An analytic reduction of the mental would say that a mental term means the same as a brain-state term, or that mental language can be translated into brain language without loss of meaning.

Type identity theory accepts the first and rejects the second:

J J C Smart's defence of identity theory relies on this distinction. His point is not that ordinary people secretly mean "brain process" when they say "pain" or "after-image". The point is that a mental report may refer to a state whose scientific nature is a brain process, even if the speaker does not know that scientific description.

The Smart-style move can be reconstructed like this:

Claim at issue: Sensations and other mental states can be identical with brain processes.

1. P1: Some true identities are empirical discoveries rather than truths of meaning.
2. P2: In such cases, two terms can refer to the same thing without being synonymous.
3. P3: "Mental state" and "brain state" are not synonymous, but this does not show that they refer to different things.
4. P4: Scientific investigation may reveal that what we report as a mental state is in fact a brain process.
5. Therefore, C: It is coherent to claim an ontological reduction of mental states to brain states without an analytic reduction of mental terms to brain terms.

Hidden assumption: mental-state terms can refer to states whose underlying nature is discovered by empirical science.
Inference type: conceptual clarification supported by empirical analogy.
Main pressure point: whether sensations are enough like scientific natural-kind cases for the analogy to work.

This matters because a common objection would otherwise be too quick. Someone might object that they can know they are in pain without knowing anything about brain states, so pain cannot be a brain state. Smart's reply is that this only shows that the concepts differ. It does not prove that the state picked out by one concept is ontologically different from the state picked out by the other.

Why The Theory Attracts

Type identity theory is attractive because it offers a simple physicalist answer to mental causation. Mental states seem to do things. Pain causes wincing and avoidance. Beliefs and desires help explain action. Fear affects heart rate and attention. If all these mental states are brain states, then mental causation becomes part of ordinary physical causation.

This avoids a problem for dualism. If mental states were non-physical, we would need to explain how non-physical states interact with the physical body. Type identity theory removes that gap by saying there is no extra interaction to explain: the mental cause is the brain cause.

The positive case can be put like this:

Claim at issue: Type identity is a strong physicalist explanation of mental states.

1. P1: Mental states appear to have physical causes and physical effects.
2. P2: Brain states are well-placed to explain those causes and effects within a physical scientific picture.
3. P3: If mental states are identical with brain states, then mental causation does not require a separate non-physical substance or property.
4. P4: A theory that explains mental causation without adding extra kinds of entity is ontologically economical.
5. Therefore, C: It is reasonable to identify mental-state types with brain-state types.

Hidden assumption: explanatory economy is a good reason to prefer one metaphysical theory over another when it preserves the relevant phenomena.
Inference type: abductive reasoning, an inference to the best explanation.
Main pressure point: whether type identity really preserves all mental phenomena, especially qualia and intentionality.

The theory also makes physicalism more definite. General physicalism says everything is physical or supervenes on the physical. Type identity theory says exactly how mental states are physical: they are brain states. That gives the theory its power, but also creates its vulnerability. Because it makes a strong one-to-one claim, objections can attack the claim that mental kinds and brain kinds line up neatly.

Dualist Arguments Applied

Dualist arguments can be turned directly against mind-brain type identity theory. The point is not to re-teach dualism from scratch, but to ask whether arguments for distinctness between mind and body undermine the identity claim.

Most dualist pressure uses this principle:

If X and Y are identical, then X and Y cannot differ in their real properties.

So if a critic can show that a mental state has a property no brain state could have, or that a brain state has a property no mental state could have, then the identity fails.