Full Idea
My inability to grasp an infinite chain of successive causes without a first cause does not entail that there must be a first cause, just as my inability to grasp infinite divisibility of finite things does not make that impossible.
Gist of Idea
We can't prove a first cause from our inability to grasp infinity
Source
René Descartes (Reply to First Objections [1641], 106)
Book Reference
Descartes,René: 'Meditations on First Philosophy etc.', ed/tr. Cottingham,John [CUP 1986], p.87