The pre-Islamic Era
In the course of its long history philosophy has always been an inseparable part of the Iranian culture and its presence can be felt manifestly in the various periods of its history. However, during the pre-Islamic era philosophy was an integral part of religion and like the other great Asian civilizations like India, China, and Japan – as contrary to the trend in ancient Greece – philosophical texts were not separate from religious texts. Therefore, since it was only in Greece that, following the 6th century BC, philosophy gradually came to be separated from religion it would be groundless to expect the same phenomenon to have taken place in the other civilizations – including the Iranian civilization – and it would, thus, be rather irrational to compare the exceptional situation that occurred in Greece with other civilizations. This is especially important in the case of Iran since many scholars have, after searching in vain in the pre-Islamic history of Iran, for philosophical works like the “Al-Shefā” of Ibn Sinā (Avicenna) and the “Hekmat-e Eshrāq” of Sohravardi, either concluded that all the philosophical and other scientific books belonging to ancient Iran have been destroyed, particularly by the invaders, or they have ended up concluding that no philosophical thought existed in pre-Islamic Iran. However, both these notions are incorrect and invalid and beyond all doubt, as mentioned above, the search for discovering the philosophical thoughts pertaining to ancient Iran should be made among the surviving religious books and texts.
It is also important to note that the ancient Greeks considered Iran as the land of philosophy and according to some Western historians of Greek philosophy like F. Cornford even Plato had been influenced by the Mazdakite thought in the formulation of his “Theory of Forms” about incorporeal beings and the angelic realms. Similarly, another Greek philosopher, Parmenides, too can be said to have had some interaction with the philosophical thought of Iran. Moreover, it is popularly known that Plotinus had voluntarily joined the Roman army simply in order to be sent to the East so that in this manner it could become possible for him to interact with the Iranians and to gain an acquaintance with their philosophy. Interestingly, in ancient Greece and in the first century of the Christian Era, even “Zoroaster” was basically known as a philosopher rather than a prophet.
During the Sassanid period and even in the first couple of centuries following the advent of Islam in Iran a number of books were written in the Pahlavi language that could, although, in a way be considered as religious books that dealt with various aspects of the Mazdayasnian religion, were also philosophical books since they dealt with philosophical thoughts and ideas. Among these books mention must be made of the “Bundahishn”, the “Dādestān-e Dinik”, the “Dānāy-e Minug-e Kherad, and parts of the “Dinkard”, which contain important discussions on philosophy and cosmology. These texts as well as other Zoroastrian books are also important from the viewpoint of practical philosophy or ethics, and particularly the “Gathas” that are replete with Zoroastrian ethical teachings. In the area of political philosophy, too, some books like the “Khodāynāmeh” had been translated into Arabic and had influenced a number of Islamic scholars. The tradition of philosophical thought pertaining to ancient Iran and the Sassanid period dealt with such issues as the division of the world into light and darkness, the war between good and evil, the incorporeal nature of the angels, the relationship between the material world and the non-material world or “this” world and the incorporeal world, the significance of man’s deeds in determining his final destiny, the relationship between time and the universe, and a number of other philosophical issues. Similarly, some philosophical thoughts and views were translated from the Greek and Indian philosophical thoughts into the Pahlavi language, including a treatise on “Logic” by Aristotle, which was later on translated into Arabic from the Pahlavi version and not from Syriac or Greek.
Besides the Zurvanistic version of Zoroastrianism from the Sassanid period, Manichaeism, too, comprised important philosophical thoughts and although most of the Manichaean books and texts have been destroyed over the ages and from what has survived and from the writings of the later scholars, it is evident that some of the Manichaean philosophical thoughts, and particularly its views on cosmology and spiritual knowledge (shenākht) have been critically analyzed by the philosophers of the East and the West.
 Nasr , Seyed Hossein ” Iran Entry ” The Great Islamic Encyclopedia . Ed. Kazem Musavi Bojnourdi.Tehran: The Center of Great Islamic Encyclopaedia , 1989-, V.10 , pp.606 – 607