The following brief passage is from The Alchemy of Happiness by the Persian philosopher and mystic Muhammad al Ghazali (c.1058 – 1111 CE). Al Ghazali was a very brilliant philosopher, jurist and theologian who advanced quickly to a very high position in the religious system of the day.  In his late thirties, though, he suffered a spiritual crisis (some have said, a nervous breakdown) and he withdrew from his teaching, made arrangements for his family, and went on a lengthy pilgrimage to Mecca.  On his return, and now following the path of the Sufis, he spent some years in relative seclusion, and although invited, did not return to the prominent positions he had once enjoyed.  Nevertheless, he had a profound and widespread influence not only on Islam, reviving a religion that Al Ghazali felt had become tradition bound and moribund and winning acceptance for Sufism within the faith, but also on the Christian west, where his writing stimulated, among others, Thomas Aquinas.  The excerpt is quoted in A Jesuit Among the Sufis by Fr. Paul Jackson SJ. 

You must certainly have heard the following saying of the Sufis: “Knowledge is a veil along this Way,” and you must have rejected it.  Don’t reject this saying, for it is true. If you become absorbed and engrossed in the objects of sense or any type of knowledge acquired by way of things perceived, your inner eye will be veiled.

The heart can be compared to a tank, while the senses can be compared to five drains by means of which water reaches the tank from outside. If you want pure water to arise from the bottom of the tank, the procedure is to empty all the water from it, as well as the sludge produced by this water; seal off all the drains so that no more water might come in; and deepen the bottom of the tank until pure water appears in the tank itself.  As long as the tank is filled with the water which comes from outside, it is not possible for water to rise up from within. In the very same way this knowledge which emanates from the heart cannot be obtained until it is cleared of whatever has come to it from outside.