"That which is born of the flesh is flesh, and that which is born of the Spirit is spirit" (John 3:6); "The flesh lusteth against the spirit, and the spirit against the flesh; and these are contrary the one to the other" (Galatians 5:17). These and similar passages clearly show that there are two distinct and diverse springs of action in the Christian. These are frequently referred to as "the two natures in the believer."
What is a Nature?
In ordinary terms "nature" expresses what we have by our origin, as well as the qualities that are developed in us by growth. We speak of a lion's nature (ferocity), a vulture's nature (scavenger), and a lamb's nature (gentleness). Now the Christian has experienced two births, and is subject to two growths. Two sets of moral qualities belong to him: the one as born of Adam, the other as born of God.
"Born of the Flesh"
In contemplating what we are as men, we must distinguish sharply between what we are by God's creation, and what we became when Adam sinned. At that moment, man did not lose any component part of his being: he still consists of "spirit and soul and body." Nor was a fourth part added to man's being at the Fall—that which entered man's being was sin, which has defiled every part of his person.
"Born of the Spirit"
At the moment of conversion, a new, spiritual life is given, which produces a distinct moral change in the believer. This communication of Divine life to the soul is viewed in the New Testament under various figures. It is likened to the implanting of an incorruptible "seed" in the soul (1 Peter 1:23; 1 John 3:9); to a cleansing of the heart, a "washing of water by the Word" (Titus 3:5; Ephesians 5:26); to a renovation of the will, or a writing of God's Law in the mind (Hebrews 8:10). The figure of the seed conveys the idea of a subsequent growth; the washing of water suggests a process of cleansing only begun; while that of God writing His Law in our minds displays the durability and permanence of His work of grace. It is from this new life or nature, imparted by the Spirit, that all spiritual life proceeds.
Who is Responsible?
Having thus considered, very briefly, the two natures in the Christian, we must now distinguish between them and the person in whom they reside. Deeds belong to the individual and not to his nature. How many Christians today speak of "the flesh" in such a way as to escape responsibility for their sins. If evil deeds by a Christian were excusable because the flesh still remains within him, could not every sinner on earth excuse himself in the same way?
It is the person who sins, and is the sinner; it is the man who needs to be forgiven and justified; it is the man who is responsible to walk not in the flesh but in the Spirit. It is the same person all through. It is the man who is born again, and not a nature. True, at the new birth he receives a new life, so that he now has two natures, and his responsibility is to mortify the old while feeding, strengthening, and being governed by the new. The "flesh" is in no way improved by the presence of the "spirit," any more than weeds are bettered by planting flowers in their midst. My responsibility lies in making no provision for the flesh, and acting according to the dictates of the spirit.
—Condensed from The Two Natures by A.W. Pink.