Contribution of Al Ghazali to Islamic Philosophy and Theology has been immense. Al Ghazali`s study of philosophy deeply affected all he afterwards did, and indeed the whole subsequent course of Islamic theology. As Al Ashari, by combining Mutazilite and Hanbalite views, overcame the first wave of Greek influence, so by bringing together philosophy and theology Al Ghazali overcame the second wave, that is, the philosophical movement culminating in Avicenna. His attitude was far from purely negative, however. On the one hand, he argued powerfully against Neoplatonism in `The Inconsistency of the Philosophers,` and after this there was no further philosopher of note in the eastern Islamic world. On the other hand, he was carried away by admiration for Aristotelian syllogistic logic, wrote several books about it and thus introduced it to other theologians and jurists for whom the books of the philosophers were inaccessible or technically difficult. In brief, al Ghazali defended the central Sunnite dogmas by the far superior Neoplatonic (including Aristotelian) methods and concepts which he had learnt from Avicenna and others.

In theology, al Ghazali`s contribution lies in his introduction of philosophical methods. He produced one work, `The Golden Mean in Belief`, which, though shorter, is similar to the Right Guidance of al Juwayni. There is an introduction dealing with the nature and importance of syllogism, and some of the arguments are given syllogistic form, but otherwise the slight differences between this and al Juwayni`s work are comparable to those between al Juwayni and al Baqillani.

Since al Ghazali`s maturity was devoted to the practice of Sufism, it is important to try to assess the contribution of this aspect of his achievement to the development of Islamic thought. It must not be supposed that prior to al Ghazali there was a complete rift between the Sufis and the Ulema. Much of the Sufistic movement was close to the main body of Sunnism. One thing that al Ghazali did, however, was to bring about a greater degree of fusion. For most of his predecessors, it would seem, Sufism began where the canonical practices laid down in the Sharia left off. A large part of al Ghazali`s work, however, consisted in showing the inner, one might almost say "Sufistic", meaning of the canonical duties binding on every Muslim. The beginning of the truly Sufistic life was the faithful observance of all these duties. Only on this foundation could one proceed to the higher mystical states which were the special province of Sufism. Of these states al Ghazali was able to write from a measure of personal experience. This made it clear to all that the states did not entail heresy, but could be combined with the faithful observance of the Sharia.

Thus al Ghazali was able to show that canonical duties and theological formulations of doctrine did not just stand in any external relation to man but could be linked up with his deepest life. This almost certainly contributed to a genuine revival of religion, without which theology might have faded away. Al Ghazali also engaged in speculation about the nature of the mystical experiences and their relation to the Prophet`s experience of revelation, but his speculations were not followed up by later theologians, and do not seem to have been influential in any way.

His conversion to Sufism came in the aftermath of his realisation of the limitations of theology. He believed that it was not by theological learning but by the Sufistic life of moral uprightness and closeness to God that man attained to heaven. Theology still remained, however, as a necessary safeguard for true belief. The Golden Mean was possibly not completed till after his conversion. His great work `The Revival of the Religious Sciences` contains a creed for memorizing and a short exposition of general Sunnite doctrine. In a later summary of the `Revival known as The Forty` he speaks of The Golden Mean with qualified approval, as better than most books of its kind. Finally, it has been known to scholars that about a fortnight before he died he finished an essay entitled `The Restraining of the Commonalty from the Science of Theology`, and in this essay his theological position is still Asharite. Thus he never really ceased to be an Asharite theologian, though he came to think theology in general less important than he had once done.

His study of Ismailism was probably rather different from that of philosophy and theology, and it is doubtful whether he expected to learn much from it. In view of the political threat from Fatimid propaganda, however, he was requested by the Abbasid Caliph to write a critique of the intellectual or theological aspects of this propaganda. This he must have done between February 1094 and his departure from Baghdad in 1095. He also wrote one or two smaller books on particular points. How influential these were is difficult to say, but they doubtless contributed to the defeat of Ismailism.

Ghazali thus became the new champion of Islam, demanding the strict observance of its forms and finding in them a means of approach to God. He cast off the extravagances of the mystics and though he heard words which cannot be uttered, he made no attempt to repeat them. Ghazali discovered that the motive of religion is love to God and he won over Islam to his view. Man`s perfection and happiness consist in trying to practise the qualities of God and in adorning himself with the real nature of His attributes. Put in other words, the consciousness of the unity and universality of God, when it has penetrated man`s feelings and mastered his spirit, makes him act according to the will of the one God. There was a belief that every hundred years a restorer of religion was sent into the world. Ghazali was acclaimed as the restorer of his age, the great renewer, the `Proof of Islam`. Henceforward mysticism, rooted on the pillars of Islam, was part of religion.