Frequently misquoted, but still relevant:
It Is the Mark of an Educated Mind to Entertain a Thought Without Accepting It
Aristotle, Nicomachean Ethics, 1 1094a24-1095a
“It is right that we ask [people] to accept each of the things which are said in the same way: for it is the mark of an educated person to search for the same kind of clarity in each topic to the extent that the nature of the matter accepts it. For it is similar to expect a mathematician to speak persuasively or for an orator to furnish clear proofs!