“To acquire knowledge is neccessary at all times.”

Source: Guide for the Perplexed (c. 1190), Part III, Ch.23
Context: The words of God are justified, as I will show, by the fact that Job abandoned his first very erroneous opinion, and himself proved that it was an error. It is the opinion which suggests itself as plausible at first thought, especially in the minds of those who meet with mishap, well knowing that they have not merited them through sins. This is admitted by all, and therefore this opinion was assigned to Job. But he is represented to hold this view only so long as he was without wisdom, and knew God only by tradition, in the same manner as religious people generally know Him. As soon as he had acquired a true knowledge of God, he confessed that there is undoubtedly true felicity in the knowledge of God; it is attained by all who acquire that knowledge, and no earthly trouble can disturb it. So long as Job's knowledge of God was based on tradition and communication, and not on research, he believed that such imaginary good as is possessed in health, riches, and children, was the utmost that men can attain; this was the reason why he was in perplexity, and why he uttered the... opinions, and this is also the meaning of his words: "I have heard of thee by the hearing of the ear; but now mine eye seeth thee. Wherefore I abhor myself, and repent because of dust and ashes" (xlii. 5, 6); that is to say, he abhorred all that he had desired before, and that he was sorry that he had been in dust and ashes; comp. "and he sat down among the ashes" (ii. 8) On account of this last utterance, which implies true perception, it is said afterwards in reference to him, "for you have not spoken of me the thing that is right, as my servant Job hath."

Source: Sustainable History and the Dignity of Man (2009), p.28

As quoted in Courage: the heart and spirit of every woman : reclaiming the forgotten virtue (2001) by Sandra Ford Walston

"On Medicine, (c. 1020) http://www.fordham.edu/halsall/source/1020Avicenna-Medicine.html
Context: The knowledge of anything, since all things have causes, is not acquired or complete unless it is known by its causes. Therefore in medicine we ought to know the causes of sickness and health. And because health and sickness and their causes are sometimes manifest, and sometimes hidden and not to be comprehended except by the study of symptoms, we must also study the symptoms of health and disease. Now it is established in the sciences that no knowledge is acquired save through the study of its causes and beginnings, if it has had causes and beginnings; nor completed except by knowledge of its accidents and accompanying essentials. Of these causes there are four kinds: material, efficient, formal, and final.

4 October 1746
Letters to His Son on the Art of Becoming a Man of the World and a Gentleman (1774)

As quoted by Mark Damazer in "In Our Time's Greatest Philosopher Vote" at In Our Time (BBC 4) http://www.bbc.co.uk/radio4/history/inourtime/greatest_philosopher_celeb.shtml

[Bakerian lecture.―On the statistical and thermodynamical relations of radiant energy, Proceedings of the Royal Society of London. Series A, 83, 560, 1909, 82–95, 10.1098/rspa.1909.0080] (p. 82)