Full Idea
Existence can no more be separated from God's essence than its having three angles equal to two right angles can be separated from the essence of a triangle, or than the idea of a valley can be separated from the idea of a mountain.
Gist of Idea
Existence and God's essence are inseparable, like a valley and a mountain, or a triangle and its properties
Source
René Descartes (Meditations [1641], §5.66)
Book Reference
Descartes,René: 'Discourse on Method/The Meditations', ed/tr. Sutcliffe,F.E. [Penguin 1968], p.145