But the history of Persian thought presents a phenomenon peculiar to itself. In Persia, due perhaps to semitic influences, philosophical speculation has indissolubly associated itself with religion, and thinkers in new lines of thought have almost always been founders of new religious movements. After the Arab conquest, however, we see pure Philosophy severed from religion by the Neo-Platonic Aristotelians of Islam, but the severance was only a transient phenomenon. Greek philosophy, though an exotic plant in the soil of Persia, eventually became an integral part of Persian thought; and later thinkers, critics as well as advocates of Greek wisdom, talked in the philosophical language of Aristotle and Plato, and were mostly influenced by religious presuppositions. It is necessary to bear this fact in mind in order to gain a thorough understanding of post-Islamic Persian thought. The object of this investigation is, as will appear, to prepare a ground-work for a future history of Persian Metaphysics. Original thought cannot be expected in a review, the object of which is purely historical; yet I venture to claim some consideration for the following two points:-- (a) I have endeavoured to trace the logical continuity of Persian thought, which I have tried to interpret in the language of modern Philosophy. This, as far as I know, has not yet been done. (b) I have discussed the subject of Ṣūfīism in a more scientific manner, and have attempted to bring out the intellectual conditions which necessitated such a phenomenon. In opposition, therefore, to the generally accepted view I have tried to maintain that Ṣūfīism is a necessary product of the play of various intellectual and moral forces which would necessarily awaken the slumbering soul to a higher ideal of life. Owing to my ignorance of Zend, my knowledge of Zoroaster is merely second-hand. As regards the second part of my work, I have been able to look up the original Persian and Arabic manuscripts as well as many printed works connected with my investigation. I give below the names of Arabic and Persian manuscripts from which I have drawn most of the material utilized here. The method of transliteration adopted is the one recognised by the Royal Asiatic Society. 1. Tārīḵẖ al-Ḥukamā, by Al-Baihaqī.--Royal Library of Berlin. 2. S̱ẖarḥi Anwāriyya, (with the original text) by Muḥammad S̱ẖarīf of Herāt. Royal Library of Berlin. 3. Ḥikmat al-‘Ain, by al-Kātibī. Royal Library of Berlin. 4. Commentary on Ḥikmat al-‘Ain, by Muḥammad ibn Mubārak al-Buḵẖārī. India Office Library. 5. Commentary on Ḥikmat al-‘Ain by Ḥusainī. India Office Library. 6. ‘Awārif al-Ma‘ārif, by S̱ẖahāb al-Dīn. India Office Library. 7. Mis̱ẖkāt al-Anwār, by Al-G̱ẖazālī. India Office Library. 8. Kas̱ẖf al-Maḥjūb, by ‘Alī Hajverī. India Office Library.