Full Idea
My ability clearly and distinctly to understand one thing without another suffices to make me certain that the one thing is different from the other, since they can be separated from each other (at least by God).
Gist of Idea
If I can separate two things in my understanding, then God can separate them in reality
Source
René Descartes (Meditations [1641], §6.78)
Book Reference
Descartes,René: 'Discourse on Method/The Meditations', ed/tr. Sutcliffe,F.E. [Penguin 1968], p.156