Next, 'real' is what we may call a trouser-word. It is usually thought, and I dare say usually rightly thought, that what one might call the affirmative use of a term is basic--that, to understand 'x,' we need to know what it is to be x, or to be an x, and that knowing this apprises us of what it is not to be x, not to be an x. But with 'real' (as we briefly noted earlier) it is the negative use that wears the trousers.

Sense and Sensibilia (ed. 1964)