One basic enterprise in philosophy going back to the ancient Greeks is to answer the question, What is it? It is reasonable to construe the answers to such questions as definitions, or what are nowadays called “real definitions,” since they typically go beyond the mere meanings of words and aim to specify the essences of things. Who, having read Plato's dialogues, can forget Socrates’ quest for definitions of things like “knowledge,” “justice,” or “virtue”? Aristotle and his Scholastic heirs continued and further developed this tradition. Aristotle distinguished different types of definitions, but definitions in the primary sense represent the essences of species. One defines an essence in terms of a genus and a differentia, as in the case of the species man, defined as rational animal. Since, on Aristotle's view, essences constitute the first principles of the sciences, definitions play a key role in his epistemology and in syllogistic reasoning, which is intended in part to systematize our knowledge of the natural world (see syllogism).

Descartes is very suspicious of this entire tradition, expressing three related criticisms of philosophical definitions: they obscure concepts that are already clear and distinct, tend to be vacuous, and fail to advance our knowledge. Concerning the first, he writes in the Principles I.10: “I have often noticed that philosophers make the mistake of employing logical definitions in an attempt to explain what was already very simple and self-evident; the result is that they only make matters more obscure” (AT VIIIA 8, CSM I 296; cf. AT III 597, CSMK 139). It helps to remember that for Descartes our simple ideas are innate. These include the ideas of thought, existence, and certainty, which one must grasp first in order to appreciate the force of the cogito. Elsewhere he notes that in order to understand these and other such concepts, we do not “have to rack our brains trying to find the ‘proximate genus’ and ‘essential differentia’ which go to make their true definition. We can leave that to someone who wants … to debate in the Schools.” All one must do is look within one's own mind. It is a mistake to “try to define what should only be conceived” (AT X 523–24, CSM II 417).

Descartes states his second criticism by mocking what he takes to be Aristotle's definition of motion: “A man who walks across a room shows much better what motion is than a man who says ‘It is the actuality of a potential being in so far as it is potential’” (AT III 597, CSMK 139). Definitions tend to be devoid of content, a jumble of words used without meaning.