Catechism of the Catholic Church §1713 — excerpt
Man is obliged to follow the moral law, which urges him "to do what is good and avoid what is evil" (cf GS 16). This law makes itself heard in his conscience.
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.