It is well known that Descartes replaced the Aristotelian essence/property (proprium)/accident distinction with that between principal attributes and modes (cf. AT VIIIA 25–26, CSM I 210– 12). Among the questions this raises are (1) whether the change is more than terminological; (2) how essence differs from existence (i) in general, (ii) in finite substance, and (iii) in infinite substance; (3) whether, and in what sense (or senses), Descartes is an essentialist; and (4) how essences or definitions are known. Questions regarding the ontology of Cartesian essences are treated elsewhere (see true and immutable nature and universal).

The first question may be answered via the second. How essence differs from existence in general is straightforward enough: the essence of a thing is the answer to the question “What is it?” (Quid est?) as opposed to “Is it?” (An est?). The difference can be illustrated by a mathematical example from the Fifth Meditation:

Here two properties are said to follow from the essence of a triangle, loosely defined in the Principles as “a [plane] figure made up of three [straight] lines” (AT VIIIA 28, CSM I 212). The triangle's true and immutable nature (AT VII 68, CSM II 47) or essence is contrasted chiefly with its existence but also with “invented” natures represented by “factitious” ideas (cf. AT VII 38, CSM II 26); in the Principles (where the language of “the Schools” for which the work was written figures more prominently), genus and propria are distinguished from accidentia.