Al-Ghazali, through his investigations, showed that both certainty of sense-perceptions (e.g. though the shadow of a stick that seems to imply that the stick is moving when it is not) and certainty of alleged intellectual truths (i.e. The possibility of judging an alleged fact in opposing and diverse manners) could be questioned. Turning to dreams, al-Ghazaali illustrates that wakefulness is simply a higher consciousness of the dream state. Might there not be, therefore, (he questions), a state beyond that of habitual living that denotes a higher consciousness to that of life itself, hence, nullifying whatever beliefs we might cherish in this mundane world of ours? A hadith supports his supposition: "The people are dreaming, (but) when they die, they become awake." The Sufis call this a special mystic state of ecstasy when we have withdrawn into ourselves and are distinct from our senses.

Similarities and Differences between Al-Ghazali and Descartes

Similarities are striking in that both Descartes and al-Ghazali questioned the veracity of intellectual and sensory knowledge. Both set about examining their cognitive structures, although in reverse directions. Descartes commenced with presuming that all knowledge were questionable and he set about erecting a secure foundation on which he could base his beliefs, whilst al-Ghazali commenced by asserting belief then investigating how he could 'cut the ground out' from under that belief (and he did so by demonstrating that both empirical and intellectual knowledge could be spurious).

A striking similarity in both philosophical structures was the concept of an external Judge or force. To Descartes, that was a deceptive God or a demon that could have distorted our reality. To al-Ghazali that was a Judge who may have had an alternative perspective on much of our intellectual certitudes.

Both Descartes...

To Descartes, however, that led to thoughts of whether reality itself could be trusted (was it not as dream?), whereas al-Ghazlai went on to promote a higher state of consciousness external to this mundane reality that could disprove all of our certainties; just as wakefulness is a state superior to dreams, so too could a higher more mystic level exist beyond this state called 'life'.
Both philosophers believed in God. Both theorists also claimed the world of ideas to be superior to that of senses. To Descartes, it was the theory that sensory existence owed its basis to an innate repository of 'ideas', whereas to al-Ghazali it was a mystic state of higher consciousness that superimposed on, and was far superior to sensual reality:

Someone 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 recognize that light?" he was asked. "By his detachment from this world of illusion and by a secret drawing toward the eternal world," the Prophet replied.

However, whereas al-Ghazali labeled other theologies and philosophies disparate from Sufism as misleading and anti- or ultra-religious, Descartes was more accepting (though skeptical, perhaps) of other direction of thought.

Sources

Descartes, R. Meditations on first philosophy: with selections from the Objections and Replies. New York: Cambridge University Press, 1996

Samsudin, M.Z. Al-Ghazali: Skepticism and Denial of All Knowledge. Psychology, Religion, & Politics. 2008. Available at: http://zakisamsudin.blogspot.com/2008/08/al-ghazali-skepticism-and-denial-of-all.html