The article defines the problem of the nature of ideas in Descartes’s philosophy according to the ontology of substances. First, it illuminates Descartes’s relation to antecedent theories of ideas (as platonic forms or as corporal images) and demonstrates that, in opposition to them, Descartes conceives ideas as modes of thinking substance. Then, it develops two possible explanations of his theory. The first one understands an idea as a complex consisting of perception and its necessary internal object, which enables the representation (...) of external objects. The second one understands it as an act with intrinsic representative structure. By using Arnauld’s analysis of Descartes’s texts, it demonstrates that in accordance with the second model, an idea is a singular mode of thinking substance, which should at the same time be understood as an act of mind and as a representation of objects. (shrink)