The Confessions of Al Ghazzali

..God at last deigned to heal me of this mental malady; my mind recovered sanity and equilibrium, the primary assumptions of reason recovered with me all their stringency and force. I owed my deliverance, not to a concatenation of proofs and arguments, but to the light which God caused to penetrate into my heart—the light which illuminates the threshold of all knowledge. To suppose that certitude can be only based upon formal arguments is to limit the boundless mercy of God. Some one asked the Prophet the explanation of this passage in the Divine Book: “God opens to Islam the heart of him whom He chooses to direct.” “That is spoken,” replied the Prophet, “of the light which God sheds in the heart.” “And how can man recognise that light?” he was asked. “By his detachment from this world of illusion and by a secret drawing towards the eternal world,” the Prophet replied..