When Pope Leo XIII in 1879 issued his encyclical Aeterni Patris (on the restoration of Christian philosophy) urging that the philosophy of Thomas Aquinas be promoted and taught within the Roman Catholic Church, he ratified a view of Aquinas already held by many: that Aquinas is not only the supreme medieval and Roman Catholic theologian and philosopher but also one of the greatest philosophers of all time. Aquinas’s synthesis of biblical teachings with the philosophies of Aristotle and of Neoplatonism reaches its best-known expression in his final major work, the Summa Theologica. Despite its daunting size, intricate structure, and occasionally difficult ideas, this “summary of theology” was actually intended as an introductory text for students.

The summa, or summary form, was quite common in Aquinas’s time; many theologians and philosophers wrote in this encyclopedic, systematic style, and Aquinas himself had previously completed the Summa contra gentiles (c. 1258-1264; English translation, 1923). The Summa Theologica is divided into three principal parts (the second part has two sections): Prima Pars, Prima Secundae Partis, Secunda Secundae Partis, and Tertia Pars. Tertia Pars, left only about half done by Aquinas at his death (up through question 90), was supplemented by his followers with additional material based on one of Aquinas’s previously written philosophical commentaries. Each part is divided into treatises on various subjects, such as the “Treatise on Law” or the “Treatise on Man,” which are themselves divided into several numbered questions (extensive essays). Each question is further divided into articles. These articles deal with a single yes-or-no question, such as whether God exists. Aquinas begins each article with a series of theological and philosophical objections to the yes-or-no answer he has in mind. He follows these objections with a short statement, always beginning with “On the contrary” (Sed contra), presenting his answer, which is based on the authority of the...

(The entire section is 849 words.)