494. 14. Adulteries of the third and fourth degree are sinful evils according to the measure and nature of the intellect and will in them, whether they are committed in act or not. It can be seen from the discussion of them above (nos. 490-493) that adulteries of the reason or intellect, which are those of the third degree, and adulteries of the will, which are those of the fourth degree, are grave, consequently are sinful evils, according to the nature of the intellect and will in them. That is because a person is the person he is in consequence of his will and intellect; for from these two spring not only all actions which occur in the mind but also all actions which occur in the body. Who does not know that the body does not act on its own, but that the will does by means of the body, or that the mouth does not speak on its own, but that the thought does by means of the mouth? Consequently, if the will were to be taken away, instantly the action would cease, or if the thought were to be taken away, instantly the mouth's speaking would cease.

It is clearly apparent from this that adulteries committed in act are grave according to the measure and nature of the intellect and will in them. That these same evils are similarly grave even if not committed in act follows from these words of the Lord:

...it was said by those of old, "You shall not commit adultery." But I say to you that (if anyone) looks at (another's) woman so as to lust for her, (he) has already committed adultery with her in his heart. (Matthew 5:27-28)

To commit adultery in the heart is to do so in the will.

[2] There are many considerations which induce an adulterer to refrain from being an adulterer in act, while yet remaining so in will and intellect. For there are some who refrain from adulterous relationships as regards the act because they are afraid of the civil law and its penalties; because they are afraid of losing reputation and thus respect; because they are afraid of diseases resulting from such relationships; because they are afraid of being railed at by their wives at home and of having no peace in their lives on account of it; because they are afraid the husband or a relative will take revenge; so also because they are afraid of being beaten by the servants; because they are too poor, or too stingy; or because they are too feeble owing either to illness, or to their abusing themselves, or to age, or to impotence, and fear being disgraced on account of it.

If anyone refrains from adulteries in act for these and like reasons, and yet sanctions them in will and intellect, he is still an adulterer. For he nevertheless believes that they are not sins, and in his spirit makes them not unlawful in the eyes of God; and thus he commits them in spirit, even if he does not in body before the world. Therefore after death, when he becomes a spirit, he speaks openly in favor of them.