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. Key points Theology is the most certain of all sciences because its source is divine knowledge (which cannot be deceived) and because of the greater worth of its subject matter, the sublimity of which transcends human reason. When a man knows an effect and knows that it has a cause, the natural desire of the intellect or mind is to understand the essence of that thing. This understanding is necessary for the perfection of the intellect. The existence of something and its essence are distinct (e.g., a mountain of solid gold would have essence, since it can be imagined, but not existence, since it is not in the world). More precisely, the being of something, and man's conception/imagination of such, are separate in all things—except for God, who is simple. Human reasoning alone can prove: the existence of God; His total simplicity or lack of composition; his eternal nature (i.e., He exists outside of time, as time is held to be a part of God's created universe); His knowledge; the way His will operates; and His power. However, although St. Thomas felt that human reason alone could prove that God created the universe, reason alone could not determine whether the universe