The conduct of the human pair immediately revealed this disharmony. Their covering portions of their divinely made bodies and thereafter their attempting to hide themselves from God were clear evidences of the alienation that had taken place within their minds and hearts. (Ge 3:7, 8) Sin thus caused them to feel guilt, anxiety, insecurity, shame. This illustrates the point made by the apostle at Romans 2:15, that God’s law was ‘written on man’s heart’; hence a violation of that law now produced an internal upheaval within man, his conscience accusing him of wrongdoing. In effect, man had a built-in lie detector that made impossible his concealing his sinful state from his Creator; and God, responding to the man’s excuse for his changed attitude toward his heavenly Father, promptly inquired: “From the tree from which I commanded you not to eat have you eaten?”​—Ge 3:9-11.

‘Doing by nature the things of the law.’ This did not mean that, since there was no comprehensive law code against which to measure their conduct, men during that period between Adam and Moses were free from sin. At Romans 2:14, 15, Paul states: “For whenever people of the nations that do not have law do by nature the things of the law, these people, although not having law, are a law to themselves. They are the very ones who demonstrate the matter of the law to be written in their hearts, while their conscience is bearing witness with them and, between their own thoughts, they are being accused or even excused.” Having been originally made in God’s image and likeness, man has a moral nature, which produces the faculty of conscience. Even imperfect, sinful men retain a measure of this, as Paul’s words indicate. (See CONSCIENCE.) Since law is basically a ‘rule of conduct,’ this moral nature operates in their hearts as a law. However, set over against this law of their moral nature is another inherited law, the ‘law of sin,’ which wars against righteous tendencies, making slaves of those who do not resist its dominance.​—Ro 6:12; 7:22, 23.

This moral nature and associated conscience can be seen even in Cain’s case. Although God had given no law regarding homicide, by the evasive way Cain responded to God’s inquiry, he showed that his conscience condemned him after he murdered Abel. (Ge 4:8, 9) Joseph the Hebrew showed God’s ‘law in his heart’ when he responded to the seductive request of Potiphar’s wife, saying: “How could I commit this great badness and actually sin against God?” Though God had not specifically condemned adultery, yet Joseph recognized it as wrong, violating God’s will for humans as expressed in Eden.​—Ge 39:7-9; compare Ge 2:24.