The cardinal virtues, upon which all the moral virtues hinge, are Prudence, Justice, Temperance and Fortitude. I find the symbolism of a bow and arrow to be most applicable.
I envision the bow, held firmly in hand, to stand for Justice. Its top end represents that which is due to God and its bottom end that which is due to others. The two ends are tied together with a string.
When the string is pulled back, it illustrates Temperance. When it is let go, it illustrates Fortitude. In the virtue of Temperance, we pull back and restrain ourselves. In Fortitude, we push forward and force ourselves.
If we lack courage, it may be because we have not practiced being temperate. By fasting and saying no to our indulgent nature, we are pulling the string tighter which enables the arrow to fly farther. A slight pull at Temperance will hardly generate enough force when it is needed for Fortitude.
Now, a bow’s purpose is to send an arrow to the target. Prudence, the most important of these virtues, is represented by the arrow; it is the intellectual decision on which direction we must go. By Prudence we discern if our actions are just, if they are done in temperance, and when fortitude is needed.
When we sin, we have missed the mark. However, when all four virtues work together, we are right on target.