The Confessions of Al Ghazzali (Deliverance from Error) (tr. Claud Field, 1909)

Their wordsKnow then, my brothers (may God direct you in the right way), that the diversity in beliefs and religions, and the variety of doctrines and sects which divide men, are like a deep ocean strewn with shipwrecks, from which very few escape safe and sound.

Their wordsI then understood that all forms of knowledge which do not unite these conditions (imperviousness to doubt, etc.) do not deserve any confidence, because they are not beyond the reach of doubt, and what is not impregnable to doubt cannot constitute certitude.

Their wordsTo suppose that certitude can be only based upon formal arguments is to limit the boundless mercy of God.

Their wordsIt was plain to me that, in order to discover where the professors of any branch of knowledge have erred, one must make a profound study of that science; must equal, nay surpass, those who know most of it, so as to penetrate into secrets of it unknown to them.

Their wordsIt is therefore a great injury to religion to suppose that the defence of Islam involves the condemnation of the exact sciences. The religious law contains nothing which approves them or condemns them, and in their turn they make no attack on religion.

Their wordsIn a word, truth does not cease to be true because it is found among them. Such, however, is the tendency of weak minds: they judge the truth according to its professors instead of judging its professors by the standard of the truth.

Their wordsI saw that Sufism consists in experiences rather than in definitions, and that what I was lacking belonged to the domain, not of instruction, but of ecstasy and initiation.