All men seek the good, or so they think.
St Thomas Aquinas refrains from defining “good” because it is so well known it doesn’t need a definition. He does say good is an analogical term, it has many meanings and they are used analogously. The two ways Aquinas describes the good are 1) something desirable, we desire what is good, 2) perfection, we seek what perfects us. The good is a final cause, which means that when we perform an action we do it seeking that final cause or result, which is something we perceive as good.
This is where the problem can come in, because there are also two ways something can be good. The first one is sometimes called the transcendental good (bonum) in philosophy because all things are good insofar as they are created by God. They have goodness just because they exist. But there is also a moral good, and this one depends on the person seeking it. For example, a hamburger is a good thing (transcendental good) and normally if you eat hamburger that is a good thing for you to do (moral good). But, if you just ate ten hamburgers it probably isn’t good for you to eat another one.
Whenever we make decisions, and choose an action we are seeking some good. We sin when we seek a lesser good, or a distortion of something good. When you lie to your teacher about why you didn’t do your homework (which is a sin) you are not seeking something bad, you are seeking a good result (i.e. the teacher not punishing you) but it is a distortion of the good. In the end being honest is much more valuable to you than escaping one punishment from your teacher.
So, we need to seek the higher goods, what is truly good, and those are not only moral goods, but they also happen to be the goods that perfect us. These are the virtues. When we seek Christ and live virtue, we perfect ourselves and get closer to our ultimate perfection, which is heaven. When we make bad choices, we choose lesser goods which actually dampens our intellect and will and makes it harder to choose what is good. This is why Christ says, “Very truly, I tell you, everyone who commits sin is a slave to sin.” (Jn 8:34) Prudence helps us to discern what is truly good and to understand what action we need to take in concrete situations. Prudence is the beginning of all virtue, because if the thing for which we are seeking with justice, fortitude, and temperance is actually a distorted good, then… well, it isn’t any of those virtues. To know what is good is the first step in knowing how to take good action toward it.