The apostle Paul claimed the Law is written on the 'fleshy tables' of the human heart (2 Cor. 3:3 KJV). What he meant are these 'shoulds' and 'shouldn'ts' are both instinctual and inescapable, part of our DNA. They are a psychological reality. We may justify our actions away, but deep down, we know when we've done something wrong.
Tullian Tchividjian, One Way Love:  Inexhaustible Grace for an Exhausted World
conscience, heart, law