It is the mark of an educated mind to be able to entertain a thought without accepting it.
Aristotle (384-322 BC) Greek philosopher
(Spurious)
First attributed to Aristotle in Lowell L. Bennion, Religion and the Pursuit of Truth (1959). Unfound as such.
Sometimes thought (though I feel it's a bit of a stretch) a misinterpretation of Nicomachean Ethics 1.3.4 (1094b): "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."
More discussion of this quotation: