Here is the text of the Wikipedia entry on category mistake.
A category mistake, or category error is a semantic or ontological error by which a property is ascribed to a thing that could not possibly have that property. For example, the statement “the business of the book sleeps eternally” is syntactically correct, but it is meaningless or nonsense or, at the very most, metaphorical, because it incorrectly ascribes the property, sleeps eternally, to business, and incorrectly ascribes the property, business, to the token, the book.
The term “category mistake” was introduced by Gilbert Ryle in his book The Concept of Mind to remove what he argued to be a confusion over the nature of mind born from Cartesian metaphysics. It was alleged to be a mistake to treat the mind as an object made of an immaterial substance because predications of substance are not meaningful for a collection of dispositions and capacities.
This seems to be quite misleading to me in several ways.
As Ishani just pointed out to me, that a is necessarily not F doesn’t mean that saying a is F is a category mistake. If I said that 3 was half of 5, I’d be ascribing to 5 a property it necessarily doesn’t have, but the mistake is arithmetic, not mathematical.
And it isn’t clear that predication is at all central to category mistakes. Ryle’s introduction of the concept largely involves people asking mistaken questions, not making mistaken assertions.
The last sentence is also odd. It is a little tricky to say just what Ryle’s argument is here against Descartes. On the one hand, Ryle does end up concluding that to have a mind just is to have the right kinds of dispositions. And he does say that we shouldn’t identify the having of those dispositions with any kind of substance, either physical or non-physical. But is that how he argues against Descartes? I would have thought that the identification of the mind with the dispositional properties was after the conclusion that Descartes had made a category mistake, so this wasn’t Ryle’s argument that Descartes had indeed made a category mistake. But what then is the argument against Descartes here?
Ryle at times in chapter one of Concept of Mind says that he is outlining his argument, not making it, so maybe I’m going wrong in reading the order of Ryle’s conclusions. Perhaps, that is, he really does conclude first that mental talk is talk about dispositions, and from that infer that Descartes made a subtle category mistake.
I think this is all rather interesting, because even those philosophers who are neither Rylean nor Cartesian (i.e. most philosophers) should still be interested in Ryle’s claim about logical grammar. Is Ryle right that Descartes makes a category mistake in treating the mind as a substance? Jack Smart thought he was not – if the mind is the brain then the mind is a substance. So the question cuts across physicalism/dualism lines to some extent. Perhaps some property dualists could agree that Ryle is right to accuse Descartes of a category error, while insisting that mental properties are distinctive. (Actually, Ryle’s relation to property dualism is complicated I think, because of his avowed anti-reductionism. But that’s for another post or two.)
Anyway, it would be good if all this stuff about category mistakes could be sorted out, then we could fix the Wikipedia entry!