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Discussing Aristotle and Aquinas refers to discussing ancient Greek philosophy and medieval Christian philosophy and theology.
Aristotle as an ancient Greek philosopher explained most of his metaphysical and epistemological issues in terms of causation. He distinguishes between four different causes and emphasizes the "efficient cause" to explain his concept of God (the Unmoved Mover), who initiates the existence of the Substance (World) without interfering with the particulars in it. Without the "Unmoved Mover," the Substance could not have existed.
Aristotle also emphasizes the "final cause" (purpose) and views every aspect of life as having an end (Telos). Even morals are explained in terms of character Virtue leading to flourishing and highest life (Eudaimonia).
Unlike Plato, who believes the Forms (properties) are distinct from objects, Aristotle sees the Forms as eminent in the objects, which explains his view of knowledge. He maintains that knowledge can be attained through abstracting the concepts from the objects.
Aquinas follows the same path as Aristotle. However, he conceives of God not only as an Unmoved Mover, but as the highest being (emanation from God). Another difference is his view of creation. Aquinas does not believe in the "material cause", the stuff prior to the existence of the Substance. Instead, he thinks the Substance was created from "Ex nihilo" (void).
Aquinas also follows Aristotle in his view that knowledge is gained through "sense perception" by "active intellect". He then differs from Augustine, who sees knowledge through "illumination."
Aquinas combines the Aristotelian teleological morals but adds to it the medieval Christian idea of "unity with God", "moral natural laws" etc.