This is a sourcebook that draws upon the 400 years of transition from ancient Greek philosophy to the medieval philosophy of Islam and the West. Philosophy was then often written in the form of commentaries on the works of Plato and Aristotle. Many ideas wrongly credited to the Middle Ages derive from this period, e.g. that of impetus in dynamics and intentional objects in philosophy of mind. The later Neoplatonist commentators fought a losing battle with Christianity, but inadvertently made Aristotle acceptable to Christians by ascribing to him belief in a Creator God and human immortality. They also provided a panorama of up to 1000 years of preceding Greek philosophy, much of it otherwise lost. They serve as the missing link essential for understanding the history of Western philosophy.