Because we can choose freely, we can be of ourselves and so are responsible for what we make of ourselves. The ability to make free choices would be useless, however, if we could not know which choices are good ones. But we can; judgments distinguishing good choices from bad ones are called “conscience.” As an essential prerequisite of moral good and evil, conscience is a basic existential principle, just as free choice is.
Conscience is one’s last and best judgment concerning what one should choose. Genuine judgments of conscience are derived from sound moral principles. One’s knowledge of these principles and of their application develops. This development is the formation of conscience. It is based on natural law and assisted by the teaching of the Church.