Full Idea
According to the laws of true logic, we must never ask about the existence of anything until we first understand its essence.
Gist of Idea
Essence must be known before we discuss existence
Source
René Descartes (Reply to First Objections [1641], 108)
Book Reference
Descartes,René: 'Meditations on First Philosophy etc.', ed/tr. Cottingham,John [CUP 1986], p.88