The Summa Contra Gentiles is fairly straightforward, and reads in a similar way to a more contemporary work, so we will focus here on the Summa Theologica. The Summa Theologica, as well as the majority of Thomas’ writings there are generally Questions containing several Articles, which have the following general outline:
3. “On the contrary” (Sed contra)
4. “I respond that” (Respondeo)
5. Replies to the Objections
One way to quickly see Thomas’ stand on each question is to read through the Sed contra of each article within a Question (sometimes it helps to look at the beginning of the Respondeo as well if the answer isn’t clear enough in the Sed contra). If we take the second Question of the Summa, we might read it like this:
Question 2. The existence of God
1. Is the proposition “God exists” self-evident?
2. Is it demonstrable?
3. Does God exist?
Article 1. Whether the existence of God is self-evident?
On the contrary, No one can mentally admit the opposite of what is self-evident; as the Philosopher (Metaph. iv, lect. vi) states concerning the first principles of demonstration. But the opposite of the proposition “God is” can be mentally admitted: “The fool said in his heart, There is no God” (Psalm 52:1). Therefore, that God exists is not self-evident.
Article 2. Whether it can be demonstrated that God exists?
On the contrary, The Apostle says: “The invisible things of Him are clearly seen, being understood by the things that are made” (Romans 1:20). But this would not be unless the existence of God could be demonstrated through the things that are made; for the first thing we must know of anything is whether it exists.
Article 3. Whether God exists?
On the contrary, It is said in the person of God: “I am Who am.” (Exodus 3:14)
I answer that, The existence of God can be proved in five ways.
WE NOW KNOW THOMAS’ GENERAL VIEW OF THE QUESTION ON THE EXISTENCE OF GOD. (This is important, because many can become confused while reading Thomas’ objections, that are often so strong as to make one think that the objection is Thomas’ position)
There are different techniques to reading each article as well, besides simply reading straight through. Often, within each article, one finds it easier to read the Sed contra and Respondea first, and then read each Objection and its Reply.
Lets take Article 1 of the second Question in the Summa, and read it in this way:
Article 1. Whether the existence of God is self-evident?
No one can mentally admit the opposite of what is self-evident; as the Philosopher (Metaph. iv, lect. vi) states concerning the first principles of demonstration. But the opposite of the proposition “God is” can be mentally admitted: “The fool said in his heart, There is no God” (Psalm 52:1). Therefore, that God exists is not self-evident.
I answer that, A thing can be self-evident in either of two ways: on the one hand, self-evident in itself, though not to us; on the other, self-evident in itself, and to us. A proposition is self-evident because the predicate is included in the essence of the subject, as “Man is an animal,” for animal is contained in the essence of man. If, therefore the essence of the predicate and subject be known to all, the proposition will be self-evident to all; as is clear with regard to the first principles of demonstration, the terms of which are common things that no one is ignorant of, such as being and non-being, whole and part, and such like. If, however, there are some to whom the essence of the predicate and subject is unknown, the proposition will be self-evident in itself, but not to those who do not know the meaning of the predicate and subject of the proposition. Therefore, it happens, as Boethius says (Hebdom., the title of which is: “Whether all that is, is good”), “that there are some mental concepts self-evident only to the learned, as that incorporeal substances are not in space.” Therefore I say that this proposition, “God exists,” of itself is self-evident, for the predicate is the same as the subject, because God is His own existence as will be hereafter shown (3, 4). Now because we do not know the essence of God, the proposition is not self-evident to us; but needs to be demonstrated by things that are more known to us, though less known in their nature — namely, by effects.
NOW WE HAVE THOMAS’ ANSWER TO THE QUESTION AND CAN LOOK AT OBJECTIONS AND REPLIES TO THOSE
Objection 1. It seems that the existence of God is self-evident. Now those things are said to be self-evident to us the knowledge of which is naturally implanted in us, as we can see in regard to first principles. But as Damascene says (De Fide Orth. i, 1,3), “the knowledge of God is naturally implanted in all.” Therefore the existence of God is self-evident.
Reply to Objection 1. To know that God exists in a general and confused way is implanted in us by nature, inasmuch as God is man’s beatitude. For man naturally desires happiness, and what is naturally desired by man must be naturally known to him. This, however, is not to know absolutely that God exists; just as to know that someone is approaching is not the same as to know that Peter is approaching, even though it is Peter who is approaching; for many there are who imagine that man’s perfect good which is happiness, consists in riches, and others in pleasures, and others in something else.
Objection 2. Further, those things are said to be self-evident which are known as soon as the terms are known, which the Philosopher (1 Poster. iii) says is true of the first principles of demonstration. Thus, when the nature of a whole and of a part is known, it is at once recognized that every whole is greater than its part. But as soon as the signification of the word “God” is understood, it is at once seen that God exists. For by this word is signified that thing than which nothing greater can be conceived. But that which exists actually and mentally is greater than that which exists only mentally. Therefore, since as soon as the word “God” is understood it exists mentally, it also follows that it exists actually. Therefore the proposition “God exists” is self-evident.
Reply to Objection 2. Perhaps not everyone who hears this word “God” understands it to signify something than which nothing greater can be thought, seeing that some have believed God to be a body. Yet, granted that everyone understands that by this word “God” is signified something than which nothing greater can be thought, nevertheless, it does not therefore follow that he understands that what the word signifies exists actually, but only that it exists mentally. Nor can it be argued that it actually exists, unless it be admitted that there actually exists something than which nothing greater can be thought; and this precisely is not admitted by those who hold that God does not exist.
Objection 3. Further, the existence of truth is self-evident. For whoever denies the existence of truth grants that truth does not exist: and, if truth does not exist, then the proposition “Truth does not exist” is true: and if there is anything true, there must be truth. But God is truth itself: “I am the way, the truth, and the life” (John 14:6) Therefore “God exists” is self-evident.
Reply to Objection 3. The existence of truth in general is self-evident but the existence of a Primal Truth is not self-evident to us.
Of course, this is not the only technique for reading the scholastic style. I would certainly suggest reading the questions and articles more than once, and once familiar with the outline, one should try to read each article in order, for seeing the objections up front can give context to the problem.
Certainly it can help to read summaries (summas of the Summa) as I recommended HERE.
Hope this helps, and feel free to ask questions.