Man's will is the choosing or deciding faculty of his soul. The soul's mind is the faculty by which we have conscious understanding. All conscious moral activity of man is active only through his facing and choosing, by an act of the will.

Then also there must be two options in order for any choice to be made. Free moral choice requires both conscious thought and the act of the free will. Our day is filled with conscious thought in the face of the many choices we must make.

Man was originally created in innocence, even though, as many theologians suspect, it lasted for only for a few days before man fell to the wiles of the Serpent's deception. Adam's innocence is seen in this verse. Genesis 2:25 And they were both naked, the man and his wife, and were not ashamed.

Initially, Adam had enjoyed God's presence, e.g., having "walked with God in the cool of the day" (Gen 3:8). At that time man's free will had not yet been stimulated by having to face a moral choice. But Adam soon was confronted with the first moral choice to be made by any human.

The Lord God commanded...

Gen 2:16-17 the Lord God commanded the man, saying, Of every tree of the garden thou mayest freely eat: 17But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die.

Then the Serpent contradicted the words of the Lord.

Gen 3:4-5 the serpent said unto the woman, Ye shall not surely die: 5For God doth know that in the day ye eat thereof, then your eyes shall be opened, and ye shall be as gods, knowing good and evil.

In the beginning Adam and Eve lived in pure innocence and in accord with God. But this fellowship was broken when the Serpent introduced the first negative possibility in contrast to God's words that had been spoken directly to them. Now, being confronted with divergent voices, Adam and Eve made a free will conscious moral choice for the very first time. Consider that Satan, the one who "sinneth from the beginning" (1John 3:8), lied by calling God a liar, saying "Ye shall not surely die" (v4). The Serpent then also tempted them with the idea they could "be as gods" (v5). Adam and Eve believed the Serpents lie that they could "be as gods" through "knowledge" to be gained by eating of the forbidden "Tree of the Knowledge of Good & Evil." They had food already so fruit was not the issue; it was the idea of self-godhood that was the most powerful lure. And, "they did eat."

Unlike the animals, which eat by instinct, it is by Adam's willful choice that all of us, as his progeny, have become morally conscious beings. This occurred when Adam and Eve were confronted by the two trees – "The Tree of Life" and "The Tree of the Knowledge of Good & Evil," and the Serpent's lies. Adam and Eve were told not to eat the fruit of "The Tree of the Knowledge of Good & Evil." Yet, they ignored "The Tree of Life" of which they were free to eat, choosing rather to believe the Serpent words. So they ate and took into themselves the Serpent's words as they partook of the "Tree of the Knowledge of Good & Evil."

That first free-will wrong moral choice made by Adam & Eve has had a life and death impact upon all humanity.

Romans 5:12 Wherefore, as by one man (Adam) sin entered into the world, and death by sin; and so death passed upon all men, for that all have sinned:

Adam's free moral choice to receive the words of the Serpent, by believing, was actualized in Eve reaching out and take of the forbidden fruit, and Adam also ate of it. Here we see the result of their choice. Gen 3:7 the eyes of them both were opened, and they knew that they were naked; and they sewed fig leaves together, and made themselves aprons. Their innocence was lost when they ate of the wrong tree. Man had now illicitly gained the knowledge of right and wrong and with that, moral consciousness. Since then, all mankind has a conscious knowledge by which their choices are framed, as evidenced by the fact that all mankind possesses an always active conscience.

Romans 2:14-15 For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves: 15 Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another;)

Thus, the sinful nature in man came by his eating of "The Tree of the Knowledge of Good and Evil,"  and that nature has taken root in the flesh bodies of all mankind.