The proof of the existence of God is founded upon the argument in Plato, _Timæus_ 28, and Aristotle, _Metaphysics_ 12. 7, and was later on used by Albertus Magnus and others. In the first place a distinction is made between the possible, which may be only potential, and the real. For the possible to become real it is necessary that there should be an effective cause. The world is evidently composite, and so cannot itself be the first cause, for the first cause must be single and not multiple: therefore the world evidently proceeds from a cause other than itself. The immediate cause may itself be the result of another preceding cause, but the series of causes cannot be infinite, nor can they return as a circle upon themselves, therefore if we trace back we must ultimately reach an _ens primum_, itself uncaused, which is the cause of all, and this first cause exists of necessity, but not by a necessity caused by anything other than itself. It must be single and unchangeable, free from all accidents, absolute, perfect, and good, and the absolute _intelligentia_, _intelligibile_, and _intelligens_. In itself it possesses wisdom, life, insight, will, power, beauty and goodness, not as acquired or external qualities, but as aspects of its own essence. It is the first will and the first willing, and also the first object of will. It is the end of all philosophy to know this first Cause, which is God, because as He is the cause of all, all can be understood and explained by understanding and knowing Him. That the first Cause is single and one and the cause of all agrees with the teaching of the Qur´an, and al-Farabi freely uses Qur´anic phraseology in perfect good faith, supposing that the Aristotelian doctrine corroborates the doctrine of the Qur´an. The most curious part of al-Farabi’s work is the way in which he employs the terminology of the Qur´an as corresponding to that of the neo-Platonists, so that the Qur´anic pen, tablet, etc., represent the neo-Platonic, etc. It may be questioned whether, even in al-Farabi, philosophy really does fit in with Qur´anic doctrine, but the divergence was not yet sufficiently marked to compel attention.