Full Idea
A triangle has a determinate nature, which I did not fabricate, and which does not depend on my mind. This is evident from the fact that various properties can be demonstrated regarding it, such as that its three angles are equal to two right angles.
Gist of Idea
A triangle has a separate non-invented nature, shown by my ability to prove facts about it
Source
René Descartes (Meditations [1641], §5.64)
Book Reference
Descartes,René: 'Discourse on Method/The Meditations', ed/tr. Sutcliffe,F.E. [Penguin 1968], p.143