This chapter examines Descartes' non-Aristotelian account of the constitution of substance in light of criticisms on the part of some of his prominent contemporaries. The Meditations deploys a general method of inquiry about substances. Descartes explains that in order to know the nature of a mind, it is not necessary to have a complete enumeration of the modes that belong to a mind. The method is grounded on epistemic and metaphysical doctrines to the effect that the ideas to judge that there is an accident of such-and-such a sort are entirely accurate and complete representations of the dependence relations comprised in a substance. A functional-causal view of the nature of substance is proposed by other seventeenth-century philosophers, such as Locke. Malebranche is not alone in ascribing the model to Descartes. All fully determinate acts, or modes, of a thinking substance are unified by being determined in regard to the object.