Definition has been unpopular amongst philosophers since Wittgenstein's Philosophical Investigations. His invitation, to bring back words and concepts from their philosophical to their everyday use, his cautionary maxims against the danger of hasty generalisation, in which the human mind so effortlessly indulges, tended to be strictures on defining, certainly, but also on theorising as well. But ordinary language is not perfectly in order as it is. There is a genuine need, not only in science, for stipulative definitions, a need which Wittgenstein acknowledged and philosophers have long turned their backs on in linguistic philosophy. Theorising is once more the order of the day. There is a saying that if you can't define x in a single sentence, then use as many sentences as you need and then you have a theory, rather than a definition, of x. Theorising in philosophy tends to mean books, not articles. If we turn, for example, to two areas of current philosophical endeavour, Philosophical Logic/Philosophy of Language and Philosophy of Mind, we can see how true this is. In both fields, books and more books, theory upon theory, engulf the philosopher in ever-increasing tides. Where, for example, in Philosophical Logic, Russell's Theory of Definite Descriptions held sway for nearly half-a-century, theories of meaning and reference now supercede one another within a few years. The same is true of the Philosophy of Mind.