Aquinas spends a lot of time in the Summa Theologica with what we might call psychology. In order to explain what the Christian life is like and how God works in people, he has to explain how people work. I've been fascinated by it for years, but also confused about all the various parts of the soul and how they interact. This year, though, something finally clicked, and it makes sense now.
Aquinas says that we have intellect, appetite (general desire, not just for food), and senses. This tripartite scheme seems to make sense and seems to cover all the actions of the soul (reasoning, imagining, seeing, wanting, disliking, etc.), but over the years, I've often been mixed up on exactly what he assigned to each of the three parts. It all came together last week when I noticed three things on one day: (1) that the appetite has a an intellectual part and a sensory part, (2) that both intellect and senses have "apprehensive powers," and (3) that he sometimes uses "Intellect" for a broader part of the soul and sometimes for a more specific part. Suddenly a nice, neat, balanced, medieval, four-part model became clear:
Appetitive Powers Apprehensive Powers
Intellect: Will Intellect (proper)
Senses: Sensitive Appetite Sensation
Think of the rows as two "parts" of the soul and the columns as two sets of powers. Each part intersects with each set of powers for four basic spheres of operation. (1) Sensation involves the five senses as well as other inner operations such as imagination. Through its operations, we experience pictures, sounds, etc.: basic "phantasms" that (2) the Intellect works with. Intellect, for instance, can compare mental pictures of cherries, a fire truck, and a stop sign that the eyes (and perhaps memory) provide and abstract the idea of red. Intellect also works with fundamental ideas and comes to conclusions, makes decisions, commands the body and other parts of the soul, and so on. (3) The Will (or "intellectual appetite") desires or rejects abstract ideas provided to it by the Intellect, and (4) the Sensitive Appetite desires or rejects concrete objects given to it by the senses.
This year's reading became much easier once I wrote out this Map of the Human Soul for myself. For instance, Aquinas points out that virtues only have to do with operations that have something to do with choice. I understand clearly now which parts those are: we can decide what to think and what to want, but we can't decide what to see, so virtues apply to the Intellect, the Will, and the Sensitive Appetite. But they have nothing to do with Sensation. We can decide which way to turn our heads and whether to close our eyes, but given the direction of the eyes, they just do their thing. Some people have better eyesight than others, but we don't credit a person with 20/20 vision as being a morally better person. Some people, on the other hand, make good decisions, aim for worthy goals, and control their appetites, and we see these good things as results of virtue.
As Aquinas explains it, the measure of all human things is reason. (God's ultimate, supernatural goals for us lie beyond human measure.) Right reason can and should control any operation about which we have a choice: the Intellect, the Will, and the Sensitive Appetite. (Psychology has learned since the thirteenth century about ways in which the intellect affects the senses, as well, but we'll leave that new knowledge aside for now.) It's easy to see how we can think about what to think about (although it's not always easy to do it--I have trouble setting aside thoughts of trouble at work). And it's easy to see how we can decide what to want: we do it every time we shop, pick a movie to see, or study possible vacation destinations, for instance. But it's harder to grasp the idea of reason ruling our sensitive appetite. The apple pie smells good, and I want it; exercise hurts, and I avoid it. Sadly, for many of us in our culture, the appetite has become exactly that mechanical. But the disciplined person with the virtue of temperance works hard at the thoughts that keep him from wanting to eat the extra dessert, from buying every song that sounds good, from kissing the pretty girl that happens to be married to someone else. And the disciplined person with the virtue of fortitude works hard at the thought patterns that keep him exercising when it hurts, that keep him working when it's lonely or boring, and that keep him searching for the splinter in the tiny finger even when his child is crying.
A high-school friend named Jim once took apart his car's engine in his garage. He laid out every part -- every strut, spring, and screw -- in nice, neat rows. When I asked him why he did it, he said he just wanted to know what was going on when he drove his car. Reading this part of Aquinas, I feel like Jim, except that instead of taking apart an engine, I'm letting Aquinas take apart my soul and lay it out in order. It's pleasant to know what's going on under my hood, and it certainly seems that this knowledge makes it a bit easier for me to think and act virtuously.
But maybe I'm just deciding to think that.