One of my graduate professors was Mark Woodward, a leading scholar of Islam. He is co-author of a World Religion text from Pearson that I regularly use, and co-author of a book "Defenders of Reason in Islam" which partly informs this post.
I'd like to consider three of the main thinkers in Medieval Islamic Philosophy, how the most influential of the three embraced mysticism, and parallels between this and the Medieval Christian thinker Thomas Aquinas.
The Persian philosopher, Avicenna, lived from c980-1037. Having a remarkable intellect, he studied various subjects such as medicine, logic, and philosophy. Having memorized the Quran, he also worked to memorize Aristotle's Metaphysics, but struggled to understand its meaning. Happening upon a copy of Al-Farabi's commentary on Aristotle's Metaphysics, Avicenna said this work illuminated the subject for him.
Although he is considered the first great Islamic philosopher, it is important to note that his philosophy contrasts with Islamic teaching. Beginning with the assumption that Aristotle represents "the height of human rationality," Avicenna sought to reconcile Aristotle and Islamic theology. This led to emanation theology sometimes compared to Neo-Platonism. This view denies creation ex nihilo and instead argues that all being has existed from eternity, and distinctions in being are due to their proximity to the first cause.
By way of contrast, Al-Ghazali (1055-1111) began as a philosopher and ended by rejecting philosophy and embracing mysticism. He has been called the second most important person in Islamic history (the first is Muhammad). His book, "The Incoherence of the Philosophers," argues against the possibility of knowledge of the world through reason, and instead affirms the viewpoint of Occassionalism, or Nominalism, which argues that only particulars exist, causation only appears to exist, and each moment is created and upheld by God.
If we are to know anything, it must be revealed to us by God. It was through a particularly strong mystical experience that Al-Ghazali turned from his "rationalist philosophy" and became a Sufi mystic and joined the Ash'arite school of theology. Of particular concern to Al-Ghazali is the claim by philosophers that the material world is eternal, and the denial of creation ex nihilo. This doctrine must be accepted on authority of the prophets, and Al-Ghazali supports this claim by arguing that human reason cannot come to know this truth.
Averroes (1126-1198), born in Cordova, Spain, lived after Al-Ghazali and therefore during the decline of Islamic Philosophy. His book, "The Incoherence of the Incoherence," is an argument is support of the Aristotelian/Islamic synthesis of earlier philosophers. One of the ways he supports philosophical inquiry is by pointing out the diversity of opinion about how to understand verses from the Quran.
He accepted the eternality of the material world, and even argued in support of this from Quranic verses that seem to indicate pre-creation material objects. His philosophical argument relies on Aristotelean ideas of causation, and refuting Al-Ghazali's Ash'arite theology of occasionalism.
The history of these three philosophers demonstrates an important pattern that occurred in other theistic religions. The background assumption is that Greek philosophers like Plato, but especially Aristotle, represent the height of human rationality. There is then an attempt to combine this with theology both for apologetic purpose and for systematic unity in human thought. When Aristotle is seen to conflict with theism, as Al-Ghazali saw with respect to the doctrine of creation, then either Aristotle (representing human reason) is embraced or human reason is rejected in favor of mysticism.
Something very simliar happened in the life of the Christian philosopher/theology Thomas Aquinas (1225-1274). Known as the great systematizer of Medieval Christian thought, Aquinas sought to reconcile Roman Catholic theology with Aristotle. He relied on Islamic thinkers like Avicenna and Averroes. However, toward the end of his life he reported a mystical vision through which he concluded that all his previous work was insufficient.
By way of conclusion I'd like to suggest two lines of thought. One has to do with the identification of Aristotle with "human rationality." Aristotle, and various kinds of Aristotelianism, is one worldview (let's call it a kind of Greek Dualism). It doesn't get a privileged place nor can it be said to be "the height of human reason" unless it can prove the basic beliefs on which it rests. Specifically, that the material world has existed from eternity.
Finally, each of these thinkers (Plato, Aristotle, Avicenna, Al-Ghazali, Averroes, Aquinas) said that knowing God is the good. Nevertheless, each of them defines "God" differently, and claim that this knowledge is attained through direct perception/contemplation of God. These different definitions have been quickly skimmed over in the philosophy of religion, and attempts are made by theists to say that Plato or Aristotle basically "got it right." For instance, when a contemporary skeptic, Anthony Flew, converted to Aristotelianism, theists hailed this as a move to belief in God. I'd like to suggest that dualistic beliefs about the nature of God are logically contrary to the beliefs about God held by theists so that both cannot be correct. If human reason cannot help us decide between theism and Greek dualism, then it also cannot help us decide between competing mystical visions or competing revelations from prophets, and we are left to skepticism.