Catechism of the Catholic Church §1873 — excerpt
The root of all sins lies in man's heart. the kinds and the gravity of sins are determined principally by their objects.
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.