The political revolution brought about by the Arab conquest marks the beginning of interaction between the Aryan and the Semitic, and we find that the Persian, though he lets the surface of his life become largely semitised, quietly converts Islām to his own Aryan habits of thought. In the west the sober Hellenic intellect interpreted another Semitic religion--Christianity; and the results of interpretation in both cases are strikingly similar. In each case the aim of the interpreting intellect is to soften the extreme rigidity of an absolute law imposed on the individual from without; in one word it is an endeavour to internalise the external. This process of transformation began with the study of Greek thought which, though combined with other causes, hindered the growth of native speculation, yet marked a transition from the purely objective attitude of Pre-Islamic Persian Philosophy to the subjective attitude of later thinkers. It is, I believe, largely due to the influence of foreign thought that the old monistic tendency when it reasserted itself about the end of the 8{th} century, assumed a much more spiritual aspect; and, in its later development, revivified and spiritualised the old Iranian dualism of Light and Darkness. The fact, therefore, that Greek thought roused into fresh life the subtle Persian intellect, and largely contributed to, and was finally assimilated by the general course of intellectual evolution in Persia, justifies us in briefly running over, even though at the risk of repetition, the systems of the Persian Neo-Platonists who, as such, deserve very little attention in a history of purely Persian thought.