Article I: On the Principles of Morality

296. Theologians call the principles or sources of morality all those things from which it comes about that some action is good, or evil, or indifferent. For just as the natural goodness of a thing rises from the fullness of its being, or from the collection of all those things which are required for its integrity, so moral goodness rises from the collection of all those things which befit a human act according to right reason; but moral malice rises from the defect of that integrity (St. Thom., 1.2, q. 18, a. 1).

But the integrity of a human act coalesces from object and from circumstances. Hence, so that a right judgment may be made concerning some act, it must be compared with the rule, both as to object and as to circumstances. But one circumstance, namely the end, which on account of its peculiar influence on morality demands for itself a peculiar consideration. Wherefore three principles of morality are counted, namely object, circumstances, and end. Now more about each.