These are just a few of the inconsistencies of the philosophers pointed out by al-Ghazali in the course of his disputation with regard to the eternity of the world and they could be mentioned here only very briefly, considering the space at our disposal. One further point of criticism may, however, be added for its importance in the history of modern philosophy. Prior to its origination, the philosophers hold, the world must have either been possible (mumkin), or impossible (mumtanig ), or necessary (weijib). It is impossible that it should have been impossible; for that which is impossible in itself is never brought into existence. Again, it is impossible for it to have been neces-sary in itself, for that which is necessary in itself is never deprived of existence. It follows then that the existence of the world must have always been possible in itself, otherwise it would never have come to be. This possibility cannot inhere in possibility itself, nor in the agent, nor in no-substratum, for the possible is that which is in the process of becoming actual. Hence the subject of possibility is some substratum which is susceptible of possibility, and this is matter. Now, this matter cannot be considered to have been originated. If it had been originated, the possibility of its existence would have preceded its existence. In that case possibility would have existed in itself, but possibility existing in itself is unintelligible. Hence matter is eternal and it is only the passing over of the forms to matter which is originated.
In rebutting this highly sophisticated argument of the philosophers al-Qhazali points out in Kantian fashion that possibility like impossibility is a purely subjective notion to which nothing need correspond in reality. If possibility requires an existent to correspond to it, so would impossibility require something to correspond to it, but avowedly there is no existing thing in concrete reality to which impossibility may be referred. Hence possibility like impossibility is merely a concept; the assumption of an existing substratum to which this concept may be related is to have a metaphysical jump from mere thought to actual existence and is to commit as we understand now an ontological fallacy.
Taken from here: http://ghazali.org/articles/hmp-4-30.htm#attack