Book First. On the Sixth and Ninth Precepts of the Decalogue.

"Thou shalt not commit adultery." Ex. 20, 14.

"Thou shalt not covet thy neighbor's wife." Ibid. 20, 17.

The sixth precept of the Decalogue explicitly indeed prohibits adultery, or moechia, but implicitly any external lust whatever; but the ninth precept prohibits internal lust, namely that committed by depraved thoughts and desires. Therefore these two precepts, insofar as they are negative, prohibit all lust, both external and internal; but insofar as they are affirmative, they prescribe that chastity be preserved, suitable to each one's state.

In prohibiting moechia, this precept prohibits everything which is contrary to the ordered propagation of the human race: for all these things are reduced to moechia as to the principal act contrary to ordered propagation.

Lust

Within this book

Question First. On Chastity and Virginity.nn. 1–4
Question Second. On Lust in General.nn. 5–13
- Article First. On the Nature of Lust.nn. 5–10
- Article Second. On the Malice of Lust.nn. 11–13
Question Third. On Consummated Sins of Lust.nn. 14–47
- Article First. On Sins According to Nature.nn. 15–28
- Article Two. On Sins Against Nature.nn. 29–47
  - § 1. On Pollution.nn. 29–41
  - § 2. On Sodomy.nn. 42–43
  - § 3. On Bestiality.nn. 44–47
Question Four. On the Sins of Lust Not Consummated.nn. 48–63
- Article One. On Carnal Movements.nn. 48–50
- Article Two. On External Sins, Immodesty.nn. 51–60
- Article Three. On Internal Sins.nn. 61–63