In 1267 Thomas Aquinas met William Moerbeke who was busy translating Aristotle directly from the Greek. Up to this point Latin scholars had only known Aristotle from translations of Arabic into Latin. The translations from Arabic presented Aristotle’s thoughts coloured by Islamic philosophy to Latin scholars.
From this point on, Thomas Aquinas based all of his own writings on the translations of Moerbeke of the original Greek texts. In this way he was able to lead Latin theology in a totally new direction: one that would become the basis to Western thought and Western Civilization.
The crucial point here is that whereas the teaching from the Islamic world was that men participated in one collective soul that they either followed or refused to follow, Thomas Aquinas taught that each man was an individual who had his own soul and free-will. As a consequence, each man has his own mind and is able to decide on his own actions whichever social class that man belonged to. Aquinas argued that it is because of this that slavery is wrong as it deprives a man of choice in his actions. (In Western Europe in the 13th century there was the system of feudal landlords and serfs, while in other parts of the world there were systems based on slavery).
The Principles of Creation 