Because the doctor of Catholic truth ought not only to teach the proficient, but also to instruct beginners (according to the Apostle: As unto little ones in Christ, I gave you milk to drink, not meat), we purpose in this book to treat of whatever belongs to the Christian religion, in such a way as may tend to the instruction of beginners. We have considered that students in this doctrine have not seldom been hampered by what they have found written by other authors, partly on account of the multiplication of useless questions, articles, and arguments, partly also because those things that are needful for them to know are not taught according to the order of the subject matter, but according as the plan of the book might require, or the occasion of the argument offer, partly, too, because frequent repetition brought weariness and confusion to the minds of readers.
Endeavouring to avoid these and other like faults, we shall try, by God's help, to set forth whatever is included in this sacred doctrine as briefly and clearly as the matter itself may allow.
Most of Aquinas's proemia are very business-like and do little more than divide up a topic, but occasionally, as here, he explains the structure of what he is doing. I think it's notable, and not fully appreciated, that Aquinas does not state that the purpose of the work is to instruct beginners (incipientes) but to hand down (tradere) whatever belongs to the Christian religion in such a way as is appropriate (congruit) to the education or cultivation (eruditio) of beginners. The purpose of the work is not to fit some conception of what a beginner is, but to teach the Christian faith, keeping beginners in mind. (I think a very common problem today is the failure ever to avoid the mistake that Aquinas avoids right here at the beginning. Catechesis, for instance, is not about teaching what you think high school students, say, can understand; it's about teaching the Christian faith in such a way as takes into account that you are teaching high school students. If you want to make the liturgy more accessible, the goal should be to provide the liturgy, taking advantage of whatever would make it more accessible, not changing the liturgy to fit what you think is accessible. And so forth. In my view, this is actually a general problem with how modern society approaches education in any field, and you find similar errors far beyond any religious question.) And Aquinas very admirably tells us exactly what this means: he's going to cut down on the ever-multiplying branches of arguments, he's going to follow the order that comes from the subject-matter itself, not discussions about it, and he's going to cut out repetition that creates loathing (fastidium -- the word is quite strong) and confusion in the audience. Thus the goal is to include what's relevant, briefly (breviter) and distinctly (dilucide), to the extent the material allows. You can learn a lot about teaching beginners from Aquinas.
He then, in Part I, Question I, discusses what is involved in sacred doctrine (sacra doctrina), the topic of the work, specifically what insofar as they will establish the limits of what is being discussed.
Part I, Question II gives the general structure of the work's content. The primary purpose of sacred doctrine is to hand down acquaintance with God (cognitio Dei) not just in Himself but insofar as He is the source and end of things (principium rerum et finis earum) and especially of the rational creature. Thus we get the basic structure of the Summa; as Freddoso translates it:
Therefore, since our intention is to lay out this doctrine, we will deal first with God (Part 1); second, with the rational creature’s movement toward God (Part 2); and third, with Christ, who, insofar as He is a man, is our way of going to God (Part 3).
With the Prima Secundae we get another proemium:
Since, as Damascene states (De Fide Orthod. ii. 12), man is said to be made to God's image, in so far as the image implies an intelligent being endowed with free-will and self-movement: now that we have treated of the exemplar, i.e., God, and of those things which came forth from the power of God in accordance with His will; it remains for us to treat of His image, i.e., man, inasmuch as he too is the principle of his actions, as having free-will and control of his actions.
There are several important things here that are often overlooked. Part II is specifically about what it means for human beings to be made to the image of God. Being made to God's image implies having a ruling power in one's own right (per se potestativus) that works intellectually (intellectuale) and by free choice (arbitrario liberum), or, in other words, it means being a source of one's own works (suorum operum principium). Aquinas is usually read as downplaying exemplar causation, but he explicitly takes the relation between the first and second parts to be describing God as our exemplar cause and human beings insofar as God is our exemplar. How is God our exemplar, how are we in His image? God out of His power creates the world so that it proceeds from Him; and we likewise have a power that makes us the source of works.
Since a power is defined by its end, Aquinas will then begin look at what the end of this power is (beatitude, union with God). He will then start looking at its exercise. Structure will be even more important for this part than the previous, and we get another major structural proemium at ST 2-1.6, in which he says that the point is to look at ways in which we achieve or fail to achieve this end; but practical knowledge (operativa scientia, knowledge having to do with our works), needs to examine particulars. What is required for moral examination (moralis consideratio) is to look at human acts both generally and in particular. This is the distinction the First Part of the Second Part (human action in general) and the Second Part of the Second Part (human action in particular); it's also why the Secunda Secundae is so much larger than the Prima Secundae -- looking at particulars is necessarily much more complicated than looking at general matters. The Prima Secundae will look at human action itself, ways we are acted upon that are relevant to our actions (passions), the sources of our actions either in the sense of what in us tends toward those actions (second nature like skill, virtue, and vice, with a focus on the latter two because they are more directly related to beatitude) or in the sense of sources outside ourselves that we need in order to act for our end of beatitude (law and grace).
That takes care of what in general is required to live as being in the image of God, but, again, we need to see particular. So what in particular do you have to do in order to act as the image of God (which, of course, is what we really need to know)? This brings us to one of the longest structural proemia in the Summa, and it is longer not because Aquinas is suddenly getting chatty but because he really does have to do more to explain why the book has the structure it does. Aquinas divides these particulars into two parts: particular virtues, which are common to all of us, and specific human states, which are different kinds of life. This is another thing that I think is often missed: only looking at the virtues gets us only a partial view of the particulars that are needed for moral life. We also need to look at our roles. I hadn't really thought of this before (and I am kicking myself for not recognizing it until writing this paragraph, because it is a point I have made in other contexts), but there is an old distinction between virtues and officia or offices that is quite clearly what St. Thomas has in mind here -- indeed, if you look at the Latin, when he actually talks about them, he relates status to officium and to gradus (rank), although he regards each of these as different. 'Officium' is often translated as 'duty', which is an OK translation if you understand by that a requirement for a role. One of the regular problems in modern virtue ethics, and it has been a problem since Hutcheson pointed out that Hume, the first major modern virtue ethicist, was guilty of it, is jumbling together virtues and offices. Everyone must be virtuous, but virtue does not and cannot express itself in the same way in everyone, because not everyone is living the same kind of life. Virtues we have in common have to be applied to our different places in life, and the tendency of virtue to particular kinds of actions in those contexts or roles or states of life is an officium. Some people are in charge, some are not; they need the same virtues but the requirements (duties or obligations, if you prefer) will be different. Some people are called to an active life, some to a contemplative life: same virtues, different offices expressing them due to different states of life. Some people are beginning in the virtuous life, some people are progressing in it, some people are virtuous; all the same virtue, but it's a sign of stupidity to demand the same things from each. In any case, you need to consider both virtues and officia, otherwise your moral examination is going to be flawed.
The second thing Aquinas takes into account is that when we look at moral particulars, there are several different ways to do it, and Christian theology in particular provides quite a few different ways to look at moral life. We can look at it in light of particular virtues, in light of particular vices, in light of particular gifts of the Spirit, in light of particular moral precepts (like the Ten Commandments), in light of the Beatitudes, etc. But, as Aquinas notes, if you treat all these individually you end up repeating yourself again and again, which, if you recall, is one of the things he explicitly said he was trying to avoid. So he says, he is going to go about things in the shorter and quicker way (compendiosior et expeditior), organizing it by virtue, and just adding the other things insofar as they relevant to each virtue. A major reason for doing this is that vices in particular are very unruly, and so whatever organization you pick, it needs to allow for the compendious and brief discussion of vices; but the most natural way to get the vices in order is to relate them to their opposed virtues. Then he concludes in his summing up:
Accordingly we may reduce the whole of moral matters to the consideration of the virtues, which themselves may be reduced to seven in number. Three of these are theological, and of these we must treat first, while the other four are the cardinal virtues, of which we shall treat afterwards (Question 47). Of the intellectual virtues there is one, prudence, which is included and numbered among the cardinal virtues. Art, however, does not pertain to moral science, which is concerned with things to be done, for art is right reason about things to be made, as stated above (I-II:57). The other three intellectual virtues, namely wisdom, understanding, and knowledge, agree, even in name, with some of the gifts of the Holy Ghost. Therefore we shall consider them while considering the gifts corresponding to those virtues. The other moral virtues are all in some way reducible to the cardinal virtues, as was explained above (I-II:61:3). Hence in treating about each cardinal virtue we shall treat also of all the virtues which, in any way whatever, belong to that virtue, as also of the opposite vices. In this way no matter pertaining to morals will be overlooked.
Note the last sentence, which is the essential point here: Et sic nihil moralium erit praetermissum, and thus nothing of morals will be missed. I think one thing that people often fail to recognize is that Aquinas is very explicitly picking one way to look at the moral life out of many legitimate ones, and he is guided by one very specific purpose, namely that this is the best way to cover everything without repeating yourself in a way that makes things unnecessarily tiresome for beginners. This is an action with trade-offs; covering everything without repeating yourself, although valuable for some things, is not always the best way to discuss a matter. One thing I've noted before, for instance, is that Aquinas's way of handling things leads to people not recognizing just how important the Beatitudes are for Aquinas (they are indeed in some sense more important than the virtues), because structurally they just get added to discussions of virtues and people are tempted to treat those discussions as mere appendices.
The real way to consider it is that moral life is like a tesseract, but we can only talk about it in slices or by projections. Different slice, different angle, and it can look very different despite being the same thing; different slices or projections are useful for different things. This means that you can go about discussing it different ways. You could talk about the moral life entirely in terms of the Beatitudes; this would have many advantages, including keeping the end directly in view. It's not the most straightforward way to talk about virtues and vices, since it would become difficult to keep track of how they all hold together, although you could in fact relate all virtues and vices to the Beatitudes. All of the other ways also have advantages and disadvantages. What you can't do -- at least without repeating yourself in a way that would quickly grow mind-numbing -- is cover the moral life completely by virtue and by vice and by Beatitude and by precept and by gift. The moral life is undeniably stunning in its infinite richness. What you don't want if you are trying to keep beginners in mind, however, is literally to stun them into blank-minded, eye-glazed educational comatoseness at the infinity of it. Since Aquinas has to cover the whole thing, he picks the one that's least likely to break the brains of beginners. Of course, even Aquinas's sketch of the virtue-projection of the moral life, one of his true claims to fame, is sometimes a bit mind-blowing. But there is more than one way to project that hypercube onto a flat surface; he is deliberately selecting one possible structure out of many, in order to achieve his specific purpose.
Part III of the Summa also has its structural proemium, of course:
Forasmuch as our Saviour the Lord Jesus Christ, in order to "save His people from their sins" (Matthew 1:21), as the angel announced, showed unto us in His own Person the way of truth, whereby we may attain to the bliss of eternal life by rising again, it is necessary, in order to complete the work of theology, that after considering the last end of human life, and the virtues and vices, there should follow the consideration of the Saviour of all, and of the benefits bestowed by Him on the human race.
I've been talking about features that Aquinas explicitly builds into his structure that have often missed, and there is an obvious one here, namely, one of the reasons why he has the huge double-epic Part II before Part III. Christ in His resurrection shows us the beatitude of undying life (beatitudinem immortalis vitae) and because of this, the last end of human life and virtues and vices naturally leads into the Christ and the benefits given by Him to humankind. In order to understand the beatitude Christ shows, you need to understand that human beings, as made to the image of God, are directed toward the ultimate end of beatitude, and in order to understand what good human beings receive from Christ, you have to understand what is good for human life (which we learn from seeing how the image of God is expressed in particular in the works of the virtues and vices). And this seems entirely right, and something that people often fail to grasp: our understanding of the good Christ has done for human beings is heavily limited by our understanding of what is good for human beings in the first place.
In any case, Aquinas's plan is to discuss first the mystery of the Incarnation, then the life of Christ as the Incarnate Word, then the sacraments as effects of the Incarnate Word, and no doubt ultimately to get to the ultimate benefits given to us by Christ -- heaven and salvation from hell and resurrection from the dead, as Reginald suggests in building the Supplement out of Aquinas's earlier writings. But, of course, he never finished and it was left incomplete. And so is all our good work on this mortal earth.