Descartes took eternal truths to include common notions or axioms, such as the proposition that nothing comes from nothing, as well as mathematical truths, such as that the radii of a circle are all equal. He claimed in the Principles of Philosophy I.49 (1644) that these eternal truths do not presuppose the existence of any object external to mind and that they “reside only within our mind” (AT VIIIA 23–24, CSM I 209).

However, Descartes also held in the Fifth Meditation (1641) that eternal truths concerning mathematical figures such as triangles derive from “true and immutable natures” that are themselves “eternal” and “not produced [effecta] by me or dependent on my mind [nec a mea menta dependet]” (AT VII 64, CSM II 45). This passage may seem to suggest that immutable natures are extramental objects akin to Platonic ideas. Indeed, there is the claim that the view in this Meditation is “thoroughly Platonic” and that Descartes “is the founder of modern Platonism” (Kenny 1970, 692–93; cf. the more “moderate Platonist” readings in Schmaltz 1991 and Rozemond 2008). Nonetheless, this Platonic reading seems to conflict with the “conceptualist” suggestion in the Principles that eternal truths are reducible to features of our own mind. On the basis of this suggestion, one commentator has concluded that “nothing is more contrary to Cartesianism than the realism of Platonic ideas and the exemplarism of essences” (Gueroult 1984, I 277).

In order to reconcile the conceptualism of the Principles with the position in the Fifth Meditation, there is the argument in the recent literature that the claim in the latter text that immutable natures do not depend on us indicates only that such natures are not constructed by us but rather are encoded in our innate ideas (as, for instance, in Chappell 1997 and Nolan 1997). The conclusion here is that “there is nothing … that commits Descartes to a transcendental realm of extra-mental objects” (Nolan 1997, 183–84).

Nevertheless, it seems that God's nature, at least, cannot be reduced to our innate idea of God given the conclusion in the Fifth Meditation that this nature is identical to God himself. Moreover, there is some question whether the identification of immutable natures of creatures with innate ideas can fully accommodate the claim that these natures are eternal.