The full question and response can be found online here. In this question, I've included everything; but on future questions, I'll probably truncate it to discuss what interests me.
Aquinas begins the Summa with a question about theology itself -- whether it is necessary. He raises two objections to the idea that it is necessary.
Article 1. Whether, besides philosophy, any further doctrine is required?Objection 1. It seems that, besides philosophical science, we have no need of any further knowledge. For man should not seek to know what is above reason: "Seek not the things that are too high for thee" (Sirach 3:22). But whatever is not above reason is fully treated of in philosophical science. Therefore any other knowledge besides philosophical science is superfluous.
Objection 2. Further, knowledge can be concerned only with being, for nothing can be known, save what is true; and all that is, is true. But everything that is, is treated of in philosophical science--even God Himself; so that there is a part of philosophy called theology, or the divine science, as Aristotle has proved (Metaph. vi). Therefore, besides philosophical science, there is no need of any further knowledge.
This is an important question -- it is as important question today as it was in the thirteenth century. Today, underneath the mantle of philosophy we would include science, and so if we were considering this first proposition today, it would be even more compelling -- Do we really need anything beyond philosophy, science, and technology?
Consider the relationship between science and religion and it certainly seems that science no longer believes that any sort of theology is necessary -- the big bang explains physics, and Darwin explains biology. But these are merely physical explanations. One can assert that the big bang occurred from reason, but that reason cannot answer why the big bang occurred. Science is ultimately tautological; it can explain rules within its system of definitions, but it cannot go beyond it.
I think Aquinas shows us a central problem with Science -- Science assumes that everything can be answered by reason alone. just as philosophers did. You'll find that if you take this approach, you can get pretty far -- Science does have a lot to offer. But it cannot answer things beyond a defined beginning; it can get to the big bang, but can offer nothing before it. It can get to natural selection, but it cannot tell you why life began. Science ultimately becomes trapped in its own tautologies.
At the beginning of Aquinas's great enterprise is an entirely different proposition. Reason is not man's highest function, and there are things beyond what reason can get to. Reason, in other words, has limits. But beyond those limits there must be something; some purpose. Aquinas finds it in God, in the drama of man's salvation, in Scripture, and this shapes his answer:
On the contrary, It is written (2 Timothy 3:16): "All Scripture, inspired of God is profitable to teach, to reprove, to correct, to instruct in justice." Now Scripture, inspired of God, is no part of philosophical science, which has been built up by human reason. Therefore it is useful that besides philosophical science, there should be other knowledge, i.e. inspired of God.I answer that, It was necessary for man's salvation that there should be a knowledge revealed by God besides philosophical science built up by human reason. Firstly, indeed, because man is directed to God, as to an end that surpasses the grasp of his reason: "The eye hath not seen, O God, besides Thee, what things Thou hast prepared for them that wait for Thee" (Isaiah 66:4). But the end must first be known by men who are to direct their thoughts and actions to the end. Hence it was necessary for the salvation of man that certain truths which exceed human reason should be made known to him by divine revelation. Even as regards those truths about God which human reason could have discovered, it was necessary that man should be taught by a divine revelation; because the truth about God such as reason could discover, would only be known by a few, and that after a long time, and with the admixture of many errors. Whereas man's whole salvation, which is in God, depends upon the knowledge of this truth. Therefore, in order that the salvation of men might be brought about more fitly and more surely, it was necessary that they should be taught divine truths by divine revelation. It was therefore necessary that besides philosophical science built up by reason, there should be a sacred science learned through revelation.
Aquinas thus argues that beyond reason must come revelation from a creator, from a being greater than ourselves. Theology is therefore a science which deals in that revelation, which Aquinas finds in Scripture and the church. Aquinas also ties in the question of man's purpose to a question of salvation -- central to his understanding of man is that man is a fallen being, in need of salvation from man's creator. Aquinas therefore presupposes that his reader has a Christian worldview -- he does not presume to prove theology from first principles; he supposes therefore that any reader of his works has gotten at least that far. Aquinas viewed the works of the Greek philosophers as a kind of preparation for Christianity; doubtless he would turn his readers in that direction should they need training in reason (Aquinas finds that one can reason to God's existence, but having gotten that far, one needs God to reveal to man things which man cannot understand through reason alone). A person coming to read the Summa therefore is best prepared if that person has already made two humble assumptions -- that man in not God, and neither can Man, on his own, understand God.
In dealing with the objections, Aquinas disposes of the first question through a further analysis of the text of Sirach:
Reply to Objection 1. Although those things which are beyond man's knowledge may not be sought for by man through his reason, nevertheless, once they are revealed by God, they must be accepted by faith. Hence the sacred text continues, "For many things are shown to thee above the understanding of man" (Sirach 3:25). And in this, the sacred science consists.
Thus, the first book of the Bible that is mentioned to the reader of the Summa is not, as one might expect, one of the Gospels; but rather a book that is part of the Jewish wisdom literature. Sirach was the last book in the Old Testament of the Vulgate, and is from an original Hebrew text that was translated into Greek for edification of Greek- speaking readers; in other words, of the Jews of the diaspora. Sirach is one of the Deuterocanonical works; it comes from the Septuagint; and is therefore found in Catholic and Orthodox Bibles, but generally not in the Protestant bibles, except where they are included as Apocrypha. Aquinas makes great use of the Deuterocanonicals in the Summa. This is not surprising because the wisdom literature of the Septuagint forms one of the great bridges between the Greek world and the Hebrew world. Aquinas has as his philosophical mission the desire to bring the world of the Greek philosophers into the reasoning of the Church; in that his works form a kind of hinge between reason and revelation, it is not surprising that he uses earlier bridges or hinges to make his case.
I also often wonder if part of Luther's desire to exclude the Deuterocanonicals was not simply to get rid of the doctrine of purgatory, but also to attempt to banish much of the thought of Aquinas. If the Summa is not built on Scripture, one can disregard it. Rather than facing Aquinas head on, one can simply avoid him entirely by undercutting his work from below.
Aquinas deals with the second objection to the question, which is a technical one, by differentiating between theology and what is today called philosophy of religion. The theological work performed in philosophy of religion is not dealing with revelation, therefore it is not theology proper, as he defines it.
Reply to Objection 2. Sciences are differentiated according to the various means through which knowledge is obtained. For the astronomer and the physicist both may prove the same conclusion: that the earth, for instance, is round: the astronomer by means of mathematics (i.e. abstracting from matter), but the physicist by means of matter itself. Hence there is no reason why those things which may be learned from philosophical science, so far as they can be known by natural reason, may not also be taught us by another science so far as they fall within revelation. Hence theology included in sacred doctrine differs in kind from that theology which is part of philosophy.