Full Idea
I am not free to think of God without existence, that is, a supremely perfect being without a supreme perfection.
Gist of Idea
I cannot think of a supremely perfect being without the supreme perfection of existence
Source
René Descartes (Meditations [1641], §5.67)
Book Reference
Descartes,René: 'Discourse on Method/The Meditations', ed/tr. Sutcliffe,F.E. [Penguin 1968], p.145