In the Scholastic tradition, time is distinguished from duration. Whereas duration is an attribute of things, time is the measure of motion, that is, a mathematical quantity measuring the duration of a process. This measure is obtained by comparing it with the duration of a motion assumed to be uniform, such as that of a clock or of the Sun. Descartes upholds this distinction, although he argues that, despite time's being called “the measure of motion,” the duration involved in moving things is no different from that of things at rest (Principles of Philosophy I.57). “But in order to measure the duration of all things, we compare their duration with the duration of the greatest and most regular motions which give rise to years and days, and call this duration ‘time.’ Yet nothing is thereby added to duration, taken in its general sense, except for a mode of thought” (AT VIIIA 27, CSM I 212). Thus, duration is an attribute of things that is in the very things it is an attribute of; while time, when it is “distinguished from duration taken in the general sense and called the measure of motion, is simply a mode of thought” (27).

Moreover, Descartes, argues, the distinction between a particular substance and its duration is only a conceptual one, a distinction of reason (see distinction [real, modal, and rational]). The distinction between a substance and one of its attributes is one of reason if the substance is unintelligible without that attribute. “Such a distinction is recognized by our inability to form a clear and distinct idea of the substance if we exclude from it the attribute in question.” For example, Descartes explains, “since a substance cannot cease to endure without also ceasing to exist, the distinction between a substance and its duration is merely a conceptual one” (AT VIIIA 30, CSM I 214).

These distinctions are essential to a correct understanding of Descartes’ proof of the existence of God from a consideration of “the nature of time” in the Third Meditation (AT VII 49, CSM II 33) or, as reexpressed, in the Principles I.21, “from the fact that our existence has duration” (AT VIIIA 13, CSM I 200) (see cosmological argument).