This concept raises four main questions:

1. 1. What exists?

2. 2. What modes of existence are there?

3. 3. Where does existence itself figure in Descartes’ ontology?

4. 4. Is existence a “real predicate” (Kant)?

As for the first two queries, Descartes’ official ontology includes three kinds of entities (substances, attributes, and modes) and two ways of existing (formally and objectively) (see being, formal versus objective). Formal existence is either necessary and eternal (God and his attributes) or contingent and temporal (created minds and bodies with their attributes and modes). Talk of the objective reality or existence of things in the mind is just Descartes’ Scholastic way of saying that ideas of them exist there formally (cf. AT VII 102, CSM II 74–75), while talk of the objective reality of ideas refers to their representational content (cf. AT VII 40, CSM II 28–29). A further type of entity is mooted in the Principles: the eternal truths “have no existence outside our thought” (AT IXA 22–24, CSM I 208–9). In his correspondence, however, Descartes insists that the eternal truths exist also in the divine intellect. Being no deceiver, God has implanted them in human minds as well (cf. AT I 145, CSMK 23), but whether Descartes is at bottom a Platonic realist who considers essences (the objects of eternal truths) to be universalia extra res rather than in rebus (cf. Schmalz 1991), or a conceptualist who reduces them to the objective reality of innate ideas (cf. Nolan 1997a), is controversial.

Similar puzzles about “existence outside our thought” arise regarding universals (cf. AT IXA 27, CSM I 212), those “common natures” (AT X 420, CSM I 45) or universal attributes (AT IXA 26, CSM I 211) among which Descartes includes existence (ibid.), and the “principal attributes” thought and extension (cf. AT IXA 25, CSM I 210–11). Insofar as they “reduce to substances outside our thought, and to mere words or names” within it (Nolan 1997b, 131), Descartes is a nominalist or a conceptualist concerning attributes, and it would be better to speak of a “substance – mode ontology” (ibid.) in which attributes reduce to modi cogitandi (in a sense reminiscent of the Scholastic ens rationis). This brings us to the third question.