Source: Deliverance from Error (al-Munqidh min al-Dalal) by Al-Ghazali (translated by R.J. Mccarthy, S.J.), pp. 64-66. Please note that the title given is of my own and not included in the actual work.
Man is formed of a body and a heart–and by the “heart” I mean the essence of man’s spirit which is the seat of the knowledge of God, not the flesh which man has in common with corpse and beast; that his body may have a health which will result in its happiness, and a malady in which lies its ruin; that his heart, likewise, may have a health and soundness–and only he will be saved “who comes to God with a sound heart” (26:89), and it may have a malady which will lead to his everlasting perdition to the next life, as God Most High has said: “In their hearts is a malady” (2:9-10); that ignorance of God is the heart’s deadly poison, disobedience to God its incapacitating malady, knowledge of God Most High its quickening antidote, and obedience to Him by resisting passion its healing remedy; that the only way to treat the heart by removing its malady and regaining its health lies in the use of remedies, just as that is the only way to treat the body.
Remedies for the body effectively procure health because of a property in them which men endowed with intellect cannot perceive by virtue of their intellectual resources, but rather it must be the object of blind obedience to the physicians who learned it from the prophets, who, because of the special attribute of prophecy, came to know the special properties of things. In a similar fashion it became necessarily evident to me that the reason for the effectiveness of the remedies of the acts of worship, with their prescriptions and determined quantities ordained by the prophets, cannot be perceived by means of the intellectual resources of men endowed with intellect. On the contrary, they must be the object of blind obedience to the prophets who perceived those qualities by the light of prophecy, not by intellectual resources.
Moreover, just as medicaments are composed of mixtures of elements differing in kind and quantity, some of them being double others in weight and quantity, and just as the difference of their quantities is not without a profound significance pertaining to the kind of the properties, so, likewise, the acts of worship, which are the remedies of hearts, are composed of actions differing in kind and quantity, so that a prostration is the double of a bowing, and the morning prayer is half as long as the afternoon prayer. This difference is not without a profound significance which pertains to the kind of the properties knowable only by the light of the prophecy. Very stupid and ignorant would be the man who would wish to discover in them a wisdom by means of reason, or who would suppose that they had been mentioned by chance, and not because of a profound divine significance in them which requires them to be such because of the special property in them. And just as in medicaments there are basic elements which are their chief ingredients and additional substances which are their complements, each of them having a special effect on the workings of their basic elements, so, likewise, supererogatory prayers and customary practices and complements for perfecting the effects of the principal elements of the acts of worship.
In general, then, the prophets (peace be upon them!) are the physicians for treating the maladies of hearts. By its activity reason is useful simply to acquaint us with this fact, to bear witness to prophecy by giving assent to its reality, to certify its own blindness to perceiving what the “eye” of prophecy perceives, and to take us by our hands and turn us over to the prophets as blind men are handed over to guides and as troubled sick men are handed over to sympathetic physicians. To this point reason can proceed and advance, but it is far removed from anything beyond that except for understanding what the physician prescribes.