Full Idea
You can't prove that Praxiteles never made any statues on the grounds that he did not get from within himself the marble from which he sculpted them.
Gist of Idea
Our thinking about external things doesn't disprove the existence of innate ideas
Source
René Descartes (Reply to Fifth Objections [1641], 362)
Book Reference
Descartes,René: 'Meditations on First Philosophy etc.', ed/tr. Cottingham,John [CUP 1986], p.79