The seeker after truth is not one who studies the writings of the ancients and, following his natural disposition, puts his trust in them, but rather the one who suspects his faith in them and questions what he gathers from them, the one who submits to argument and demonstration, and not the sayings of a human being whose nature is fraught with all kinds of imperfection and deficiency. Thus the job of the man who investigates the writings of scientist, if learning the truth is his goal, is to make himself an enemy of all that he reads, and applying his mind to the core and margins of its content, attack it from every side. He should also suspect himself as he performs his critical examination of it, so that he may avoid falling into either prejudice or leniency.
Abu Ali Ibn al-Haytham (Alhazen)
Cited at https://www.reddit.com/r/math/comments/1hpa1s/what_is_your_favourite_quote_by_a_mathematician/cawp3vj/
More detail at http://www.firstscientist.net/
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