I often find the word 'individuation' used in philosophy of mind, i.e., "individuation of beliefs". Yet, I have a very vague idea of what 'individuation' means. Moreover, it seems that different philosophers use the word in different ways. The closest explanation of the aforementioned phrase I have seen is: "a way to taxonomize beliefs". But on what basis does this taxonomy rest?

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Individuation is the process of picking out individuals. We do this all the time in ordinary life. For example, if we were in a parking lot, we could individuate the cars in the lot. That is, from the group of objects in the lot, we could distinguish the individual cars. We wouldn’t have a difficult time figuring out, for example, whether there are five cars, or two cars or one car. We could distinguish each individual car. (Of course I haven’t said how we would do it, but it shouldn’t be hard to tell that story.) So there isn’t a problem of individuation for cars, at least in ordinary circumstances.

Your excellent question is how we individuate, that is distinguish, beliefs. Fred’s belief that the combined landmass of French Polynesia is roughly equivalent to the landmass of Rhode Island is a different belief from his belief that the total area (including the sea) of French Polynesia is roughly equivalent to the total area of Europe. Here we have individuated the beliefs by reference to the difference in the propositions which make up the content of the belief. But if we individuate beliefs by the propositions they are attitudes towards, it follows that beliefs about logically equivalent propositions are really the same belief. So Fred’s belief that if the boat sinks, everyone will drown, is the same belief as his belief that either the boat doesn’t sink, or everyone will drown. Contrast this with our car individuation task: Cars that look like separate cars really are separate. But here we have two beliefs that might, at least at first, appear to be separate beliefs, which, on this taxonomy, turn out to be different beliefs.

Some philosophers hold that mental states like belief are brain states. That is, they individuate beliefs such that if I’m in a particular brain state which, on their theory is the belief that p, then if we imagined a double of me on a twin-earth, who is physically identical to me, then, if he is in the same brain state, then he also has the belief that p. But some philosophers of mind, such as Hilary Putnam, and later Tyler Burge and others, have challenged this account, by arguing that two such individuals could have different beliefs, even while being in the same brain state. They conclude that we can’t individuate or taxonomize beliefs on the basis of brain states.

So to sum up: To individuate, we need a theory. We have a theory of what makes something a car, so we can distinguish cars in a parking lot. We’re less clear about what beliefs are, and different theories may individuate beliefs differently. If one theory of belief says there is one belief where another sees two, or two where another theory sees one, then those theories are in conflict, and the basis of taxonomy is still open.

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