“Deep within his conscience man discovers a law which he has not laid upon himself but which he must obey. Its voice, ever calling him to love and to do what is good and to avoid evil, sounds in his heart at the right moment… For man has in his heart a law inscribed by God… His conscience is man’s most secret core and his sanctuary. There he is alone with God whose voice echoes in his depths.”
– Gaudium et Spes, Vatican II document
Moral conscience, present at the heart of the person, enjoins him at the appropriate moment to do good and to avoid evil. It also judges particular choices, approving those that are good and denouncing those that are evil. It bears witness to the authority of truth in reference to the supreme Good to which the human person is drawn, and it welcomes the commandments. When he listens to his conscience, the prudent man can hear God speaking.
– Catechism of the Catholic Church. 1777
The Heart of Things 