The view that our descriptive language of mental events ("x is in pain", "y believes that z", etc) refers to real events and is correct, but will correspond neatly to physical concepts and descriptions. In other words, the mind is "nothing but" physics, and will be completely describable in physical terms when neuroscience is finished. For example, to say that "x is in pain" will end up being describable by physics: "x's neurons 1234 to 5678 are firing".
II. What is "Reduction?"
When us humans learn about entities of type A, and also about entities of type B, because of the language and concepts we use might differ for each one, we may not realize at first that A entities are really a subtype of B entities. When we learn this, we then can go about replacing our original language and concepts for describing A entities with the language and concepts we use for describing B entities, as A entities are "nothing but" B entities. For example, we might use language and concepts referring to light, colors, etc, and then later learn that light is really nothing but a spectrum of the electromagnetic spectrum. So we then "reduce" the original language of color and light ("red", "bright", etc) to physics ("wavelength between 640nm", etc).
III. Two Types of Reduction
There are two main forms of physical reduction of the mind: behaviorism, and identity theory.
Behaviorism reduces talk of the mental to talk of behavior. The belief that it will rain is really just the behavior of grabbing an umbrella before going outside, etc.
Identity theory reduces talk of the mental to talk of the physical by saying that mental events are just another name for physical events. They are two names for one thing, like "Superman" and "Clark Kent" are two names for one person. For example, the mental state "the belief P is true" is just another name for the firing of a specific pattern of neurons. There is only one event, not two.
Mental descriptions are shorthand for types of behavior. To say "X is in pain" is just to say "X is wincing, screaming out, etc.", and that is all there is to a mental event. Since we can't know the contents of anyone else's mind, such as whether they are experiencing the actual feeling of pain or not, we should instead focus on what can be empirically verified.
Problems with Behaviorism:
1. Collapse of Logical Positivism - Behaviorism was part of the larger project of logical positivism, the view that all knowledge should be either tautological or empirically verifiable. But since the verifiability theory of the logical positivists is itself neither tautological nor empirically verifiable, it refutes itself. Logical positivism died out in the 1950s.
2. Holism of Psychology - Each form of behavior involves a network of other beliefs, all of which are necessary to explain the behavior. If Bob grabs an umbrella before he leaves the house, then this involves a huge network of other beliefs: the belief that it will rain, that an umbrella will repel water, that water falls down, etc. This huge network of beliefs cannot be reduced to just behavior, and will always involve some elements of the mental.
3. Awareness of Your Own Mind - To know you're in pain, you don't need to observe your own behavior in the mirror. You just feel pain, directly. The old joke about behaviorism: What does one behaviorist say to the other after sex? "That was good for you, how was it for me?"
4. Super Actors and Super Spartans - Imagine someone not in any pain at all but is a very good actor, who can convince you that he is in pain. Or imagine someone that is in extreme pain but does a very good job of not showing any sign of it. Behaviorism would have to say that the first person is in pain and the second one is not, when in fact the opposite is true.
Because of this, behaviorism has largely, though not completely, died out. Most reductive physicalists have moved over to identity theory.
V. Mental Descriptions as "Theory"
When we speak of mental states, such as "John believes that X", this is often thought of as an explanatory theory. For example, if John grabs the keys before he leaves, we would explain this behavior by saying "John believes that the car is in the garage" and "John desires to take the car." So mental descriptions postulate unobservable hypothetical entities to explain human behavior, which is exactly what a "theory" is. So when we speak of mental entities like beliefs, desires, and so forth, we are engaging in a theory. Most physicalist philosophies of mind accept this, and take different approaches to it. Behaviorism is the only theory that does not take mental language to be a type of theory, because behaviorism takes mental descriptions to be PRE-theoretic observations. Identity theory, on the other hand, accepts that mental descriptions are a type of theory and that the entities that are theorized to exist (beliefs, desires, etc) are identifiable with specific brain states.
VI. Identity Theory
The identity theory of mind states that the mind is the brain. They are just two different names for the same (one) thing, in the same way that "water" and "H20" are two different names for the same thing. So for example "the belief that P" is just another name for "firing of ABC neurons". They are just two different ways of describing the same one thing. There are two main versions of identity theory:
1. Occam's Razor: Defended by JJC Smart, who said that if there is no reason to postulate extra entities, such as mental entities, then we should favor the theory that postulates less extra entities, per Occam's Razor. So Smart arrives at identity theory via shaving off any extraneous entities.
2. Lewis/Armstrong: defended by David Lewis and David Armstrong, this version arrives at identity theory instead by describing a mental state as a particular kind of state, and then letting science discover what that particular state consists in. For example, "pain" is what, exactly? It is the mental state that causes wincing, screaming, and is caused by tissue damage, etc. Now, let's say that science discovers that "wincing, screaming, etc caused by tissue damage" is caused by brain state X. So, since pain = "wincing, etc", and "wincing, etc" = "brain state X", then by transitivity "pain" = "brain state X."
Problems for Identity Theory:
1. Multiple Realizability: If we accept identity theory, then we have to accept that aliens could never have the mental state "belief that P" since they have a different evolutionary history and don't have the "ABC" neurons that we have. And since "belief that P" is just another name for "firing of ABC neurons", then if you don't have ABC neurons, you can't possibly have "the belief that P."
Or imagine future artificial intelligences. We would have to say that they can never believe that P either, because they possess circuitry and not the "ABC" neurons that "belief that P" is just another name for.
The obvious solution presents itself: mental states can be realized by many different systems, and not just the specific neurons we possess. So a mental description is a description of an abstract concept ("X believes that P if 123"), rather than something concrete (firing of ABC neurons).This is, however, non-reductive physicalism.