Summa Theologica, Pars Prima

St. Thomas's greatest work was the Summa, and it is the fullest presentation of his views. He worked on it from the time of Clement IV (after 1265) until the end of his life. When he died, he had reached Question 90 of Part III (on the subject of penance). What was lacking was added afterwards from the fourth book of his commentary on the Sentences of Peter Lombard as a supplementum, which is not found in manuscripts of the 13th and 14th centuries. The Summa was translated into: the Indo-European language of Greek (apparently by Maximus Planudes around 1327) and Armenian; many European languages; and Chinese.

The structure of the Summa Theologiae is meant to reflect the cyclic nature of the cosmos, in the sense of the emission and return of the Many from and to the One in Platonism, cast in terms of Christian theology: The procession of the material universe from divine essence; the culmination of creation in man; and the motion of man back towards God by way of Christ and the Sacraments.

The structure of the work reflects this cyclic arrangement. It begins with God and his existence in Question 2. The entire first part of the Summa deals with God and his creation, which reaches its zenith in man. The First Part, therefore, ends with the treatise on man. The second part of the Summa deals with man's purpose (the meaning of life), which is happiness. The ethics detailed in this part are a summary of the ethics (Aristotelian in nature) that man must follow to reach his intended destiny. Since no man on his own can truly live the perfect ethical life (and therefore reach God), it was necessary that a perfect man bridge the gap between God and man. Thus God became man. The third part of the Summa, therefore, deals with the life of Christ.

In order to follow the way prescribed by this perfect man, in order to live with God's grace (which is necessary for man's salvation), the Sacraments have been provided; the final part of the Summa considers the Sacraments.