The "Summa Theologiæ" is divided into three great but unequal parts; for the second, much larger than the other two, is divided into two distinct sections.
The first part is a complete treatise on all existences, and especially on all intellectual existences, from that intelligence which is infinite in its nature as in its operations, to the intelligence which is bounded and severed by matter. It treats of God, of the Holy Angels, their qualities, and their abode, and of the Creation.
The first section of the second part contains a theory of man. It treats of happiness, as man's final object, of the passions, and of human acts, of the virtues in general, of sins, in their origin, nature, and effects.
The second section is closely allied to the first. It treats of the conditions of happiness and the moral laws, the three great virtues, Faith, Hope, and Charity. The impulse given to the soul by these three theological virtues communicates itself to the moral virtues as well; in treating of them afresh S. Thomas forms a universal theory of human duty.
The third part expounds the whole plan of Redemption. After having studied the work of Redemption in itself, S. Thomas studies it in its application to each individual. Thus he arrives at the theory of the Sacraments. But death did not give him time to finish this part of the work. It is interrupted where he treats on the fourth Sacrament, that of penance. An attempt has been made to complete it by various extracts from his other works, but one misses in this compilation the living hand of genius.
Before quitting this great subject, one word must be added on S. Thomas's method. It may be defined as geometry applied to theology. S. Thomas states, first of all, the theorem he is about to develop, or the problem which he proposes to solve. Then he considers the difficulties and solves them. He follows this up with a train of sustentations drawn from holy writ, tradition, and theological reason, and he ends by a categorical answer to all the objections which were made at the beginning. This order is invariably observed in every part of the work.
At the Council of Trent, on a table set in the midst of the council chamber, was placed the "Summa," alongside of the Holy Scriptures and the decrees of the popes. Well might Dante declare that the doctor inhabits a sphere above the reach of praise, or, with Lacordaire, exclaim, that "God alone can praise this great man in the eternal council of the Saints."
The "Summa Theologiæ" occupied the last nine years of our saint's life. The world was ignorant of the monument which was being raised in silence. Thomas preached, lectured, wrote as before.
About this time William of S. Amour republished his attack upon the religious orders, under the fresh title of "Collectiones S. Scripturæ;" our saint replied to it by issuing a fresh edition of his defence of the religious orders, and this silenced his foe.
During these nine years, Thomas visited several towns and convents of Italy. At Milan he wrote an epitaph on S. Peter Martyr. At Bologna he lectured with his usual success on theology.