Second Question: On Conscience as It Is the Rule of Moral Acts

205. Principle. Conscience is the rule of human acts in such a way that we never licitly act against it when it commands or prohibits; but we always licitly act according to it if it has the due qualities.

It is proved: to operate against conscience includes an evil will, for it is nothing other than to do that which is judged to be prohibited by God or to omit a precept (Rom. XIV, 23). Hence the axiom: Whatever is done against conscience builds unto Gehenna. This must be understood of conscience commanding or prohibiting, but not of conscience counseling or permitting, because the latter does not obligate. On the contrary, to act according to conscience is to do what we judge to be commanded by God or at least permitted. But to do what we judge to be commanded or permitted by God of itself includes the will to conform itself to the law and will of God. Therefore it is licit per se; namely, provided that that judgment of conscience have the due qualities, concerning which we are now going to speak.

But they are two: on the part of the object truth and rectitude must be present; on the part of the subject or assent, certitude. For conscience is the rule and measure of human acts. But every measure must be adequate, that is, true and right, not falsified; and certain, not ambiguous. Hence conscience must be true and right on the part of the object, and certain on the part of the subject.