Multiple realizability - Why can we just categorize mental states?

Question

I am unable to understand why we can label mental states. Intuitively I would think that no two mental states are equal. My mental state when I was happy 2 weeks a ago was not the same as when I was happy today. The state of happiness was not identical, nor can I fathom how any two mental states I have can ever be identical.

From my understanding of type-identity theory I also do not understand why this kind of labeling is even necessary. If mental states are equal to brain states, then unless there is some proof that total brain states repeat throughout a persons life, I don't see why this would even be relevant.

Because of this I cannot understand how multiple realizability can be a thing either.

I know zero neurobiology though, so I apologize if all of this is obvious stuff.

Answer 1

The same way we can label a group of animals as cats, though no two cats are identical. They share enough characteristics that we group them together for purposes of convenience and discussion, then distinguish how they differ if and when that is relevant.

This is imperfect. Category errors are certainly possible. Different people will draw the lines in different places. If you need finer understanding, you need to discuss in greater detail. But for many purposes (I am a better cat owner than I am a dog owner), the more general statement is entirely sufficient.

The universe is fractal, fuzzy, and poorly defined. Accepting some generalizations and imperfections is how we normally deal with that.

(Question from the MIT freshman quiz: "Define the universe. Give three examples.")

Answer 2

This goes to Wittgenstein's notion of 'family resemblances': things that we categorize as having an essential common feature may instead merely have multiple overlapping points of similarity with no specific feature common to all. There are no characteristics that are identical; there is only a loose but recognizable trend in a given direction. And moreover, these categories are public (social) in nature. We only make them because it's useful as a matter of communication and coordination with others.

In other words, mental states like anger, happiness, lust, greed, loyalty, etc., are distinguished not because they are (or are not) specific brain states, but because other people need words to describe our inner mental state in order to understand and predict our actions. Anger is that which leads to certain kinds of actions; happiness that which leads to other kinds of actions; lust that which leads to still other kinds of actions… We cannot know the mind of another, and so we look at their behavior and make a best guess; and likewise, we tell people that we are angry or happy or lustful so that they can know what to expect from us.

We label mental states because we have to; living with others demands it.