Descartes asserts that all that there is that is directly cognisable to us, by mind or by eye, are modifications of two substances: res cogitans and res extensa, mental and corporeal.

How does he himself concieve of time in his scheme? Does he say?

For when I think, I can think without space - I close my eyes; but I still think in time; for I have a thought now, and then, then: so thinking on thinking, serially and consecutively; and then, in parallel; that is reflectively.

Is time then a modification of res cogitans?

Or reflecting, is res cogitans a modification of time?

Or must we say: time has res extensa?

This might be supposed given how we model time - through the concept of space - the timeline or spacetime; but in itself, as it is experienced; as I experience it, or you; it doesn't appear to have the sense of spatiality that space has:

To go left in time, or right; or to go back in time, but not to turn time back; is a sense that time lacks; but not space, it's front or back.

Or in the way a body placed in space has space because it occupies place; can time have height, or breadth or depth? Even when time has its depths...

What's Descartes actual position on this?

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    Descartes official account of time comes from the Principles, where he holds that time is just a "mode of thinking". Thus, while divisibility of space arises in spacial extensions, divisibility of time arising in temporal extensions is modal rather than substantial. Here is a link to a paper which describes Descartes view much better than I am able : analytica.inf.br/analytica/diagramados/168.pdf
    – nwr
    Sep 10 '15 at 22:40
  • I had a look at this paper a few weeks ago, but it is not easy reading for me at this stage of my learning. Section 3 is probably the most relevant.
    – nwr
    Sep 10 '15 at 23:01

The issue is complicated because Descartes treats time in two distinct but equally fundamental ways, duration and mental time. In both manifestations time is not a substance however, neither res cogitans nor res extensa, it is an attribute.

It is mental time which is merely a "mode of thought" referred to in Principles I.57, by which duration is measured. It is dependent on motion (e.g. of clocks), but motion is independent of it. In contrast, duration (the enduring of any res cogitans or res extensa) is independent of motion, but motion is dependent on it. The distinction is somewhat similar to Bergson's (mechanical) time vs. duration, i.e. external vs. internal time. However, Descartes is not as dismissive of mental time as Bergson, it is not an artificial construct but an innate idea of a true and immutable nature, the idea of measuring. While it is a mode of thought it is not a mental creation but a fundamental attribute on par with duration.

See Lloyd's Descartes on time and Gorham's Descartes on Time and Duration

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