Full Idea
You say you cannot imagine that p, and therefore declare that p is impossible. Mightn't that be hubris?
Gist of Idea
Why pronounce impossible what you cannot imagine?
Source
Daniel C. Dennett (Elbow Room: varieties of free will [1984], §7.3)
Book Reference
Dennett,Daniel C.: 'Elbow Room - Free will worth wanting' [MIT 1999], p.170