In the history of human civilization as a whole and in that of Islamic civilization in particular, there have been outstanding figures who played decisive roles in forming or changing various aspects of human life. Some of them are still surrounded by a mass of historical data containing contradictory judgments about them. Since their activities took place many centuries ago, modern scholars are often short of enough information to judge their real impact. It is difficult to come to a fair conclusion. In the history of Islamic civilization, Khāja Nasir al-Din Tusi (597- 1201/672-1274) was a unique scholar, in one of the most crucial periods, whose real role and personality are still obscure. His age was full of such harsh socio-political events in all parts of the Islamic world that some regarded it as the worst period in human history. The intercontinental Mongol invasion and the collapse of all Islamic powers and states at the same time as the abolishment of Ismāʻili fortresses and the destruction of the Abbasid caliphate created a bloody and unstable condition for all Muslims including Tusi. In this complicated atmosphere, one can hardly rely on historical data which are conflicting and controversial.

Considering these difficulties, this paper attempts to assess Tusi’s role as a person who witnessed the situation but stood apart from it, preventing himself from assimilating to his age. Holding the chain of Islamic thought, Tusi played an active role in linking pre-Mongol civilization to the post-Mongol world.