Thomas Aquinas' Summa Theologica is a treatise dating to the High Middle Ages that was intended to instruct students in Christian doctrine. Aquinas himself was a 13th-century Dominican friar, and was deeply ingrained in the Neoplatonist teachings of pseudo-Dionysus (among other philosophers, secular and Christian alike).

His Summa Theologica (left unfinished owing to Aquinas' death) is a touchstone text in the history of both philosophy and Christianity for its ability to join both. Specifically, Aquinas does not take for granted the merits of studying church doctrine; he justifies his study by stating that Christianity and its study stands apart from the realm of science, and so constitutes a separate but greater pursuit. Aquinas quotes heavily from Scripture to support this claim.

Aquinas' quinque viae (Latin: "five ways") amounts to nothing sort of a reason-based justification for the existence of God (viz. arguments from motion--stating that God must be the "prime mover"--causation, contingency, degree, and a teleological argument"). The so-called "five ways" are not without critique from modern philosophers; however, the very fact that Aquinas sought a rational explanation for God was unique in contemporary Christian circles.

In brief, Aquinas is known for merging the studies of philosophy and doctrine, and (in related fashion) faith and reason. The latter dichotomy is exhibited, in addition to the quinque viae, by Aquinas' structure, wherein he lists a series of objections to each of his claims, followed by point-for-point responses.

The Summa is divided into three parts, the first devoted to Theology (the nature of God, Christ, and the Trinity), the second to Ethics (including human sins and virtues), and the third to the nature of Christ's incarnation and His relationship to mankind, as well as a discussion of eschatology (the end of the world). Aquinas' enduring legacy is as a comprehensive and multifaceted scholar, theologian, and philosopher.