The toolkit of most philosophers features the notion of category mistakes. We can recognize clear cases (the three in "green ideas sleep furiously") with quasi-perceptual alacrity. But I, for one, usually feel a tinge of self-conscious awkwardness when I deploy it. This has to do, I think, with the fact that we do not professionally acquire the notion together with a minimally adequate account - of the kind that usually comes, for instance, with introductions to the use/mention distinction: a philosophical explication that helps to make the phenomenon prima facie intelligible. If you are like me, this excellent short, clearly focused monograph will help you fill this gap in a few hours of engaging reading.