Thomas Aquinas called Aristotle simply the Philosopher, and built on him
A 13th-century Dominican friar took a pagan Greek thinker so seriously that he called him simply the Philosopher. By weaving Aristotle together with Christian theology, Thomas Aquinas produced a system that a pope in 1914 insisted Catholics could not do without. That system is called Thomism.
Aquinas lived from 1225 to 1274 and is honoured as a Doctor of the Church. His Summa Theologica ranks among the most influential works of medieval theology and remains the main reference point for Catholic philosophy and theology. In the 1914 document Doctoris Angelici, Pope Pius X went further, warning that his core theses were not open opinions but foundations, and that removing them would leave students unable even to grasp the meaning of church doctrine.
His guiding rule was to accept truth wherever it turned up, and he drew on Greek, Roman, Islamic and Jewish thinkers. He was a realist, convinced against the sceptics that the world can be known as it actually is. He wrote detailed commentaries on Aristotle and adopted much of his vocabulary, while also keeping some neoplatonic ideas, such as the view that God is being and goodness itself and that other things are good insofar as they share in it. The bedrock of reality, for Aquinas, was two principles: non-contradiction and causality. Anything consistent with both could in principle exist, even without a body.
He was careful about language. A word can be used univocally, with the same sense each time, as when milk describes what any female mammal produces. It can be analogical, keeping part of its meaning, as healthy applies both to a fit person and to a nourishing meal. Or it can be equivocal, switching meaning entirely, like bank for a river's edge and a lender, a slide modern philosophers call ambiguity.
At the centre is a definition of being as that which is. The first part points to quiddity, literally whatness, the essence or nature of a thing; the second to esse, its actual existence. A being is an essence that exists. Substances exist in their own right; accidents, like colour or shape, exist only in something else, so metaphysics mainly studies substances. In a living animal, the body is its matter and the soul its substantial form.
Source: Thomism
