Central to Descartes’ philosophy is a view about immutable essences and eternal truths. After mentioning a Platonist account of recollection in Meditation V, Descartes declares that the ideas we have of mathematical notions “are not my invention but have their own true and immutable natures” (AT VII, 64/CSM II, 44).Descartes claims that other important philosophical notions, such as God, mind, body, and human free will (AT VII, 68; AT VIII-2, 348; AT III, 383; AT VII, 433, respectively), also have immutable (...) natures or essences. Although Descartes says a good many things about this view, nowhere does he offer definitive doctrine on the matter, and in fact frequently confuses his reader with apparently inconsistent pronouncements about immutable natures and eternal truths. In this essay, I focus on the immutable natures and propose a solution to two of the main problems associated with Descartes’ position, the metaphysical status of immutable natures and the purported indivisibility of their existence as ideas in the mind. (shrink)