Catechism of the Catholic Church §2517 — excerpt
The heart is the seat of moral personality: "Out of the heart come evil thoughts, murder, adultery, fornication...." The struggle against carnal covetousness entails purifying the heart and practicing temperance:
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.
Also cites: Pastor Hermae, Mandate 2, 1: PG 2, 916