Like many philosophers, Descartes draws a distinction between the existence of a thing and its essence or nature. Even if an entity does not exist, presumably there is still a fact about what the entity would be like if it did exist. Sometimes Descartes speaks of an essence as an idea that is before a mind when it is thinking of an object (AT VIIIA 27–28, CSM I 212–13; AT IV 348–50, CSMK 279–81). Descartes also speaks of the essences as things that exist independently of our ideas of them – for example, when he discusses existing objects and the essences that constitute them (AT IV 348–50, CSMK 279–81). For Descartes, an essence or nature can exist in thought, and it can also exist in reality or in re.
Descartes introduces the notion of a true and immutable nature in the Fifth Meditation. One of the central goals of this meditation is to provide an additional argument for the existence of God, and Descartes sets up that argument by engaging a discussion of truths about triangles and other geometrical figures (see ontological argument). More specifically, he notices that there are “ideas of things which even though they may not exist anywhere outside me still cannot be called nothing” (AT VII 64, CSM II 44). The reason they cannot be called nothing is that “various truths can be demonstrated of them” – for example, that the three angles of a triangle add to two right angles, and that the longest side of a triangle subtends the triangle's greatest angle. Such truths “are not my invention” for we “clearly recognize [them] whether we want to or not” (AT VII 64, CSM II 44–45). There is something that we are picking out when we demonstrate a result about a triangle, an entity that puts constraints on what we can truly think about triangles and what we cannot. Triangular figures may not exist, but the fact that we recognize truths about them entails that triangles are “not merely nothing” but “have their own true and immutable natures” (AT VII 64–65, CSM II 44–45).