Some context first, then. The Summa Theologia is in three parts. The Prima Pars ('First Part' for you non-Latin speakers) deals with God and his work in creation. The Prima Secundae and Secunda Secundae (Parts 2a and 2b) deal with humankind and their work in ethics and in attaining the final human end in God. The Tertia Pars (Third Part) deals with Christ and his work, including a discussion of the sacraments.
The whole of the Summa is based around questions. Aquinas starts with a question, breaks it down into sub-questions or 'articles', and then addresses each in turn. He does this in a style that was popular in universities of the time: someone proposes an answer to the question which runs contrary to the 'right' answer, giving reasons for it; someone quotes an important authority disagreeing; then someone explains why the authority was right and the first argument was wrong. Normally this would be done in a debating context, but in the Summa it's really just Thomas arguing with himself. So each question goes like this:
Question: is theological position X the case?
Point: Yes it is, for these reasons: 1) A, 2) B, 3) C
On the other hand: Authoritative person Y says Z, which contradicts the above points.
Reply: This is why points A, B, and C were wrong and authoritative person Y was right.
Let's look at an example from the Prima Pars which, in discussing creation, gets into some questions about what human life was like in Eden before the Fall. Question 102 addresses 'The Garden of Eden, the scene of innocence' and breaks this question down into four component parts:
- Whether Paradise is a physical place
- Whether it is a suitable place for human occupation
- What man was put in Paradise for
- Whether he should have been made in Paradise
Article 1: Is Paradise a physical place?
Point: It would seem not. 1. Bede says that it reaches 'as far as the lunar sphere', which can't be true of any earthly place - not only does the earth not go up that high, but there's a region of fire below the moon that would burn it. 2. Genesis says that it's the source of four rivers, but we know that those four rivers have their sources all over the place. 3. No one has found it yet. 4. The tree of life was there, but that's something spiritual, so Paradise must be a spiritual rather than a physical place. 5. If Paradise was physical, so must its trees have been, but God creates trees on the third day but only plants the garden of Eden after the six days are finished.
On the other hand: Augustine says, there are three general opinions about Paradise; one would have Paradise understood only in a physical sense, another only in a spiritual sense; the third takes Paradise in both ways, and I confess this is the opinion I favour.
Reply: Scripture describes Paradise in the form of a historical narrative, so we must read it as history and believe in Paradise as a physical place. It must have been in the East, because it would have been in the noblest spot in the earth, and the East is on the right side of the heavens as Aristotle tells us, and the right side is noble than the left. 1. Bede didn't mean that Paradise literally reached the lunar sphere, but that it had a really temperate climate, like the heavenly bodies. Duh. 2. We know that some rivers go underground and then reappear elsewhere - the rivers really do have their source in Paradise, and the places we consider to be their sources are actually where they reemerge after going underground. 3. We haven't found Paradise yet because it's either behind some big mountains or in the middle of the sea somewhere. 4. The tree of life both material and spiritual, like the rock in the desert that signified Christ. 5. Either we could go with Augustine, who said that only the 'seminal virtues' of the plants were created on the third day and they didn't start growing till afterwards, or we could go with other people who say that when Genesis says God planted the garden of Eden, it was just pointing out that this had happened at the same time as all the other plants were created.