Thomism is the philosophical and theological school which arose as a legacy of the work and thought of Thomas Aquinas (1225–1274), the Dominican philosopher, theologian, and Doctor of the Church.

In philosophy, Thomas's disputed questions and commentaries on Aristotle are perhaps his best-known works. In theology, his Summa Theologica is amongst the most influential documents in medieval theology and continues to be the central point of reference for the philosophy and theology of the Catholic Church. In the 1914 motu proprio Doctoris Angelici, Pope Pius X cautioned that the teachings of the Church cannot be understood without the basic philosophical underpinnings of Thomas's major theses:

The capital theses in the philosophy of St. Thomas are not to be placed in the category of opinions capable of being debated one way or another, but are to be considered as the foundations upon which the whole science of natural and divine things is based; if such principles are once removed or in any way impaired, it must necessarily follow that students of the sacred sciences will ultimately fail to perceive so much as the meaning of the words in which the dogmas of divine revelation are proposed by the magistracy of the Church.

Thomas Aquinas held and practiced the principle that truth is to be accepted no matter where it is found. His doctrines drew from Greek, Roman, Islamic and Jewish philosophers. Specifically, he was a realist (i.e. unlike skeptics, he believed that the world can be known as it is). He often affirmed Aristotle's views with independent arguments, and largely followed Aristotelian terminology and metaphysics. He wrote comprehensive commentaries on Aristotle, and respectfully referred to him simply as "the Philosopher".

He also adhered to some neoplatonic principles, for example that "it is absolutely true that there is first something which is essentially being and essentially good, which we call God, [...] [and that] everything can be called good and a being, inasmuch as it participates in it by way of a certain assimilation".

Aquinas says that the fundamental axioms of ontology are the principle of non-contradiction and the principle of causality. Therefore, any being that does not contradict these two laws could theoretically exist, even if said being were incorporeal.

Aquinas noted three forms of descriptive language when predicating: univocal, analogical, and equivocal.

Further, the usage of "definition" that Aquinas gives is the genus of the being, plus a difference that sets it apart from the genus itself. For instance, the Aristotelian definition of "man" is "rational animal"; its genus being animal, and what sets apart man from other animals is his rationality.