Adultery in the third degree, committed by the reason or intellect, and adultery in the fourth degree, committed by the will, are serious offences and consequently sinful evils, depending on the nature of the intellect or will in them; this can be seen from the discussion of them above (490-493). The reason is that it is the will and the intellect that make a person human. For these two are not merely the source from which everything that happens in the mind arises, but also everything that happens in the body. Everyone knows that the body does not act of itself, but the will acts by means of the body. The mouth does not speak of itself, but thought speaks by means of the mouth. If therefore the will were taken away, action would instantly stop, and if thought were taken away, the mouth would instantly stop speaking. This makes it quite obvious that the seriousness of acts of adultery which take place depends upon the extent and nature of the involvement of the intellect and will in them. It is clear from the Lord's words that they are equally serious, even if not actually committed:

It was said by the men of old, you are not to commit adultery. But I tell you, if anyone looks upon another man's wife so as to desire her, he has already committed adultery with her in his heart. Matt. 5:27, 28.

To commit it in his heart means in his will.

[2] There are many reasons why an adulterer does not actually commit adultery although he does so in his will and intellect. For there are some who refrain from actually committing adultery for fear of the civil law and its penalties; for fear of losing reputation and so honours; for fear of contracting disease thereby; for fear of quarrelling with the wife at home and so having an unquiet life; for fear of vengeance being exacted by a husband or relative, and also for fear of being beaten by servants; by reason of poverty or of greed; by reason of weakness resulting from disease, self-abuse, age or impotence so as to cause embarrassment. If anyone refrains from actually committing adultery for these or like reasons, while approving it in his will and intellect, he is still an adulterer. For he none the less believes that such acts are not sins, and he treats them in his spirit as not unlawful in God's sight. He thus commits them in spirit, although not in the body for the world to see. After death therefore, on becoming a spirit, he speaks openly in favour of them.