2 But some one will say, What can sin be? Is it a living thing? Is it an angel? Is it a demon? What is this which works within us? It is not an enemy, O man, that assails you from without, but an evil shoot growing up out of yourself. Look right on with your eyes, and there is no lust. [Keep your own, and] seize not the things of others, and robbery has ceased. Remember the Judgment, and neither fornication, nor adultery, nor murder, nor any transgression of the law shall prevail with you. But whenever you forget God, immediately you begin to devise wickedness and to commit iniquity.
Source: Catechetical Lectures (New Advent)