Given the distinction between philosophy and theology, one can then distinguish between philosophical and theological sources and influences in Aquinas’ work. As a philosopher Thomas is emphatically Aristotelian. His interest in and perceptive understanding of the Stagyrite [i.e. Aristotle] is present from his earliest years and did not await the period toward the end of his life when he wrote his close textual commentaries on Aristotle. When Thomas referred to Aristotle as the Philosopher, he was not merely adopting a façon de parler of the time. He adopted Aristotle’s analysis of physical objects, his view of place, time and motion, his proof of the prime mover, his cosmology. He made his own Aristotle’s account of sense perception and intellectual knowledge. His moral philosophy is closely based on what he learned from Aristotle and in his commentary on the Metaphysics he provides a cogent and coherent account of what is going on in those difficult pages. Quite often deep insight into Thomas’ philosophical thought can be gained from a close attention to the ways in which he comments upon and interpretively clarifies difficult passages in Aristotle that can be otherwise very obscure.
But to acknowledge the primary role of Aristotle in Thomas’s philosophy is not to deny other philosophical influences. Augustine is a massively important presence. Boethius, Pseudo-Dionysius, and Proclus were conduits through which he learned Neo-platonism. There is nothing more obviously Aristotelian about Thomas than his assumption that there is something to be learned from any author and not only mistakes to be avoided. He definitely adopted many features from non-Aristotelian sources.
This has led some to suggest that what is called Thomistic philosophy is an eclectic hodgepodge, not a set of coherent disciplines. Others, struck by the prominence in Thomas of such Platonic notions as participation, have argued that his thought is fundamentally Platonic, not Aristotelian. Still others argue that that there is a radically original Thomistic philosophy which cannot be characterized by anything it shares with earlier thinkers, particularly Aristotle.
The recognition that Thomas is fundamentally an Aristotelian is not equivalent to the claim that Aristotle is the only influence on him. It is the claim that whatever Thomas takes on from other sources is held to be compatible with what he already holds in common with Aristotle. And, of course, to draw attention to the sources of Thomas’s philosophy is not to say that everything he holds philosophically can be parsed back into historical antecedents, or that he never disagrees with his sources, Aristotle in particular.