Full Idea
He who in his sleep dreams of a winged man does not dream so without having seen some winged thing and a man. And in general it is impossible to find in conception anything which one does not possess as known by experience.
Gist of Idea
We can only dream of a winged man if we have experienced men and some winged thing
Source
Sextus Empiricus (Against the Logicians (two books) [c.180], II.058)
Book Reference
Sextus Empiricus: 'Against the Logicians', ed/tr. Bury,R.G. [Harvard Loeb 1997], p.267
A Reaction
This precisely David Hume's empiricist account of the formation of concepts. Hume's example is a golden mountain, which he got from Aquinas. How do we dream of faces we have never encountered, or shapes we have never seen?
Related Idea
Idea 2183 We can only invent a golden mountain by combining experiences [Hume]