Catechism of the Catholic Church §1757 — excerpt
The object, the intention, and the circumstances make up the three "sources" of the morality of human acts.
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.