In his Summa Theologica, Thomas Aquinas offers a closely reasoned and orderly Treatise on the Virtues. Aquinas drew heavily from the philosopher Aristotle, but also imported theological considerations that moved beyond the ancient Greek philosopher.
Aristotle was concerned with (among other things) what has been referred to as the function argument, that is, virtue refers to the ability of something to function well. The virtue of the horse is running, the virtue of the knife is cutting. When the matter of function is applied to the human person, one asks what characteristics allow a person to live well. This not only refers to the qualities necessary for an individual in the performance of his or her job, but also to one's station in life as a husband or wife, etc. Aquinas utilizes St. Augustine's definition of virtue as a habit "by which we live righteously, of which no one can make bad use, which God works in us, without us" (Augustine, On Free Will).
Thomas' concerns are two-fold: First, he considers the qualities necessary that define the good, that is the virtuous life, and second, the good assumes a particular end or a purpose for human life. What is that end? What is that purpose? Aquinas argues that there is an ultimate end, a ratio bonitatis, which can only be achieved by an ultimate goodness. Aquinas is well aware of the fact that human beings have competing and conflicting understanding of what constitutes goodness, but taking his cue from Aristotle that virtue is what benefits the polis, Aquinas reasons that not all accounts of goodness are created equal. Thus one must not only work through what the good is, one must also delineate the virtues or habits that contribute to the good, which has as its end eudaimonia (often translated "happiness," but better rendered as "flourishing").
Aquinas divides the virtues into three categories: intellectual, moral, and theological.