Al Hazen (Ibn al-Haytham) (c. 965—1039)
It is not the person who studies the books of his predecessors and gives a free reign to his natural disposition to regard them favourably who is the real seeker after truth, but rather the person who in thinking about them is filled with doubts, who holds back his judgement with respect to what he has understood of what they say, who follows proof and demonstration rather than the assertions of a man whose natural disposition is characterized by all kinds of defects and shortcomings. A person who studies scientific books with a view to knowing the truth ought to turn himself into a hostile critic of everything that he studies ... He should criticize it from every point of view and in all its aspects. And while thus engaged in criticism he should also be suspicious of himself and not allow himself to be easy-going and indulgent with regard to the object of his criticism.
Critique of Ptolemy, translated by S. Pines in Actes X Congrès internationale d’historie des Sciences I (Ithaca, 1962) and cited by S. Sambursky in Physical Thought from the Presocratics to the Quantum Physicists (Hutchinson, London) 1974 p. 139.