He accepted as genuine the _Theology of Aristotle_ which had been put into circulation by Naymah of Emessa, and, we are told, revised the Arabic translation. The _Theology_ was an abridgment of the last three books of Plotinus’ _Enneads_, and presumably al-Kindi compared this with the text of the _Enneads_, corrected the terminology and general sense in accordance with the original, and evidently did so without any suspicion that it was not a genuine work of Aristotle. The _Theology_ had not been long introduced to the Muslim world, and it is certain that the use of it made by al-Kindi was a main cause of its subsequent importance. Endorsed by him it not only took an assured place in the Aristotelian canon, but became the very kernel of the teaching developed by the whole series of _falasifa_, emphasizing the tendencies already marked in the commentary of Alexander of Aphrodisias. The influence of the _Theology_ and of Alexander appear most clearly in the treatise “On the Intellect” which is based on the doctrine of the faculties of the soul as described in Aristotle’s _de anima_ II. ii. Al-Kindi, developing the doctrine as presented by the neo-Platonic commentators, describes the faculties or degrees of intelligence in the soul as four, of which three are actually and necessarily in the human soul, but one enters from outside and is independent of the soul. Of the three former one is latent or potential, as the knowledge of the art of writing is latent in the mind of one who has learned to write; the second is active, as when the scribe evokes from the latent state this knowledge of writing which he desires to put into practice; the third is the degree of intelligence actually involved in the operation of writing, where the knowledge now quickened into activity guides and directs the act. The external faculty is the “Agent Intellect” (_`aql fa``al_) which proceeds from God by way of emanation and which, though acting on the faculties in the body, is independent of the body, as its knowledge is not based upon perceptions obtained through the senses.