"In God alone, essence, what He is, and existence, that he is, coincide."
Born in a village in Turkistan, the Persian philosopher and physician, Avicenna, became one of the most influential writers in both the Islamic and medieval Christian world. By the age of 18, he was already an accomplished physician and had absorbed the vast amount of philosophical knowledge that he later recorded in his encyclopedias. His later years were spent in the service of the ruler of Isfahan, whom he accompanied on his many journeys and military ventures. An important work distinguishing his "oriental" philosophy from the "occidental" philosophy of the Christian philosophers of Baghdad was lost when Isfahan was pillaged in 1043. Only three unconnected sections survive. The Oriental Philosophy itself has been lost completely. Fortunately, Demonstrations and Affirmations , a mature work, covering the whole of philosophy, survives in entirety.

Avicenna was as well known for his medical works as for his philosophical, with his Canon of Medicine serving as the foundation of medical learning in European universities for centuries. This was based mainly on Greco-Roman teaching, but incorporated some Arabic works and, to some extent, his own clinical experience. Unlike Averroës, he did not write commentaries, but he shared his desire to reconcile philosophy, which was the only true path to understanding, with Muslim teaching. His thinking was dualistic; mind and matter were separate. Only in God was being and existence one.