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On the discussion related to the question here, there seemed to be many disagreements between users in regards to identity theory and its implications on whether psychophysical harmony needs explanation.

As a refresher,

Psychophysical harmony consists in the fact that experiences are correlated with physical states and with one another in strikingly fortunate ways.

For example,

C fibers firing cause pain. However, the intrinsic probability of this is super low. It’s just as intrinsically likely that C fibers firing would cause pleasure, or the taste of hummus. But the vast majority of possible psychophysical laws would involve disharmony between the mental and the physical.

One of the top comments in that question argued that this is not surprising under reductive materialism which seemed to link to an article about identity theory. So what exactly is identity theory?

From the SEP,

The identity theory of mind holds that states and processes of the mind are identical to states and processes of the brain.

What does identical mean here? Does it mean that it reduces to physical processes of the brain or that they are literally the same? If the latter, I’m having trouble making sense of this.

Secondly, a user by the name of @Dcleve in a discussion with @NotThatGuy argued that many test cases have shown this to be false: for example, a lot of our brain processes are unconscious. Would he be correct? In the article on SEP regarding the subject,

Some philosophers hold that though experiences are brain processes they nevertheless have fundamentally non-physical, psychical, properties, sometimes called ‘qualia’. Here I shall take the identity theory as denying the existence of such irreducible non-physical properties

So this denies that there are such thing as non physical properties. So what do identity theorists classify their qualia as? Do they just assert that these experiences are physical? But isn’t consciousness already defined as non physical? Any clarity would be appreciated.

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  • I wouldn't say they were "in a discussion" with me as much as I pointed out that the claims they're making are dubious, dishonest and personal attacks, and they responded with "I'm not, but you are".
    – NotThatGuy
    Commented Feb 20 at 6:46
  • True, added the link thanks! @NotThatGuy Commented Feb 20 at 7:05
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    Identity means extensional identity, they are literally the same but are accessed by different means and so have intensionally distinct descriptions. The prototypical examples of extensional identity are Frege's "the evening star is the morning star", Kripke's "water is H2O" and pop-culture's "Clark Kent is the Superman".
    – Conifold
    Commented Feb 20 at 7:52
  • You should know better than to ask what 'exactly' is anything in philosophy! Commented Feb 20 at 10:01
  • While I'm not a great fan of saying "Humans are just computers", I'm also a wizened old computer science teacher. Since some answers here are in "Human activity reduces to computer activity" camp, here's an answer of mine that shows this may be easily 12 different answers!!
    – Rushi
    Commented Feb 20 at 11:27

2 Answers 2

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Your questions look like as if you ask for an introduction to the main theories and controversies from the field of philosophy of mind.

Unfortunately, you do not write how much and from which sources you have informed yourself before. The paper you mention in the beginning has a strong bias as a religious apologetics. It is not an impartial source.

The entry identity from the Stanford online-encyclopedia introduces to the identity theory from philosophy of mind. It also points to further references. The field is broad and challenging :-)

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The closest analogy of what "identical" means here would probably be in computers:

The processing of a computer is "identical to" the states of the various components and the electricity flowing between them.

However, this doesn't mean that consciousness and the processing of a computer is the same thing or that computers/AI are conscious (because it's an analogy). Although it does raise some questions of exactly how parts of our brain combine to make consciousness, and how we'd figure out whether or not some future AI is conscious.


Since you mention reductive materialism, note that this is being compared to identity theory in the physicalism SEP article:

A third notion of reductionism is more metaphysical in focus than either the conceptual or theoretical ideas reviewed so far. According to this notion, reductionism means that the properties expressed by the predicates of (say) a psychological theory are identical to the properties expressed by the predicates of (say) a neurobiological theory — in other words, this version of reductionism is in essence a version of type physicalism or the identity theory.

That article provides 4 different versions of reductionism, and within identity theory there is type identity (one form of reductionism) and token identity. There is also the closely-related functionalism, which itself has a few different versions. Although, as the name suggests, this is about the function of mental states, rather than the physical implementation of them, which makes it a questionable alternative to theories about implementation. If someone wants to present a lay-person's understanding of all of that, they can go ahead.

These theories tend to leave a lot to be desired in terms of providing concrete physical examples (not analogies) of what would and would not meet the requirements of their theory. There may also not always be such a clear distinction between all these theories. As the SEP says, "it could be argued that functionalists greatly exaggerate their difference from identity theorists" and "the type-token distinction is not an all or nothing affair".

For me, the computer analogy offers a rather concrete understanding of what one might mean when one says mental states are "identical to" or "reduce to" physical states.

If we consider physicalism and look at things from the perspective of science, there doesn't really seem to be more than one way to interpret "your mental state reduces to some physical brain state", just like there doesn't really seem to be more than one way to interpret "the processing of a computer reduces to its physical state". Although you could ask what exactly this processing even is, whether processing of 2 computers can be "the same" and what exactly that means, whether the same processing can happen in non-computers, whether the physical medium of the processing even matters, how you know that processing is happening, etc. Where you land on the answers to these questions (with respect to minds) is what seems to differentiate most of the theories above.

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