"In the wake of the death knell of philosophy and rationalism sounded by the Sunni theologian, al-Ghazali (d. 1111), the story went, Muslim rationalism fell into a deep slumber. Philosophers were persecuted; miracles became the sole explanation of natural phenomena; angels were taken to be the cause of celestial motions; and Muslims could no longer do science because they denied causality. It is therefore not surprising, so the audience was told, that no Muslim made any contribution to the world of knowledge from this point onwards. And so what we are witnessing in the Muslim world today is only the natural outcome - indeed continuity - of a history of the persecution of reason and of a lack of intellectual curiosity that stretches back several centuries.
But I am afraid that this is not how things happened. To begin with, al-Ghazali's attack in his famous Tahafut al-falasifa (The Incoherence of the Philosophers) is not aimed at reason or philosophy (despite how the title sounds). He takes issue, as he clearly outlines in his multiple introductions to the work and in the main body, only with metaphysicians, insofar as they use faulty logic - incoherent formal syllogisms, arguments with internal contradictions, premises that are unrelated to the conclusions, etc. - to argue about matters that pertain to the Muslim creed (such as the issues of bodily resurrection, that God knows particulars only in a universal way, etc.). Beyond these creedal matters, al-Ghazali is in fact rather explicit in stating that on matters pertaining to scientific demonstrations, Muslims should not argue with philosophers.
Indeed he goes so far as to claim that when a scientific demonstration contradicts a hadith (Saying of the Prophet), it is more suitable to reject the latter as unsoundly transmitted; similarly, when any other scriptural proof text fails to conform to the demonstrated conclusions of reason, the former must be interpreted allegorically. In other words, there is practically no doubt that al-Ghazali gives authority to reason over transmitted sciences. His other works, including those on law and legal theory, include sentiments such as "reason is the source of transmitted knowledge" and sharp attacks against the blind imitation (taqlid) of authority."
There is no Golden Age of Islam and al-Ghazali Did Not Kill Rationalism in the Muslim World‏ http://www.opendemocracy.net/openindia/asad-q-ahmed/islam%E2%80%99s-invented-golden-age
Tahafut al-falasifa (The Incoherence of the Philosophers) A translation of Al Ghazali
http://www.ghazali.org/works/taf-eng.pdf