Full Idea
When I think that I exist now, and recollect that I existed in the past, and when I conceive various thoughts, the number of which I know, then I acquire the ideas of duration and number which I can thereafter transfer to all the other objects I wish.
Gist of Idea
I can learn the concepts of duration and number just from observing my own thoughts
Source
René Descartes (Meditations [1641], §3.44)
Book Reference
Descartes,René: 'Discourse on Method/The Meditations', ed/tr. Sutcliffe,F.E. [Penguin 1968], p.123