These quotes from Studies in Perfectionism by Benjamin B. Warfield explain the teaching so prevalent today regarding the two natures of the Christian: the "carnal" nature and the "new Spiritual" nature.
[The teaching] ... familiar to us among "the Brethren" ... divides the Christian man into two ineradicably antagonistic "natures," the "fallen nature" and the "new nature".
It is not only hard for a fallen man to do good, we are told, but impossible. This is not altered by his "new birth." The "new birth" does not change his "fallen nature." It only puts into him, by its side, a "new nature." Henceforth he has two natures in him, one of which can only sin, and the other of which cannot sin. The man himself -- whatever the man himself, apart from his two natures, may be; he is apparently conceived as bare will -- sits up between these two natures and turns over the lever as he lists, to give the one nature or the other momentary control.
The two natures, we are told, have absolutely no effect on one another. "The carnal nature in the Christian is utterly evil, and is never mixed with any good." "The new nature has no effect whatever upon the carnal nature. It is utterly distinct from it and cannot mingle with it, any more than God can have sin in His nature." It does not "change the character of the evil that the carnal nature is capable of." Apparently the carnal nature of man is never in any way changed or modified; from all that appears it remains in him forever and forever just badness and unalloyed badness. At least nothing is said to relieve that situation. Salvation does not consist in its eradication. It consists in the dominance in the life of "the new nature" existing by its side.
This "new nature" is identified, now, with the indwelling Spirit. It is sometimes spoken of, no doubt, as "the God-begotten nature"; but it is more frequently and properly treated as just the indwelling Spirit Himself, and it is because it is the indwelling Spirit Himself that it cannot sin. "It is impossible for the Spirit of God to be anything but good and well-pleasing to God." "The sinless and invincible Spirit of God has taken up His dwelling in us," we read further, "and has made it possible for us to permit Him to win the victories over the temptations that assail."
It is disappointing to learn from this statement that when "the invincible Spirit of God" takes up His dwelling in us, all that He does is "to make it possible for us to permit Him" (an odd clause that!) to win victories for us. He is not "in full control" of us, it seems. It would indeed be truer to say, that He is only at our disposal. Everything is after all in our own control. "A Christian possessed of the indwelling Spirit of God," we read with sad eyes, "may choose to walk after the flesh." That is no doubt because he is possessed of rather than by the Spirit of God. At any rate it belongs ineradicably to "the Christian" to turn on the old carnal nature, or the new Spiritual nature, as he may choose, and let it act for him.
Who this "Christian" is who possesses this power it is a little puzzling to make out. He cannot be the old carnal nature, for that old carnal nature cannot do anything good -- and presumably, therefore, would never turn on the Spirit in control. He cannot be the new Spiritual nature, for this new Spiritual nature cannot do anything evil -- and this "Christian" "may choose to walk after the flesh." Is he possibly some third nature? We hope not, because two absolutely antagonistic and non-communicating natures seem enough to be in one man. The only alternative seems, however, to be that he is no nature at all -- just a nonentity: and then we do not see how he can turn on anything.
In the pamphlet, Once-for-all Reality, George objected to this teaching on the two natures, but for very different reasons.
George's objection was that this teaching produced an attitude of resignation in believers that "the normal Christian life" was characterized by the constant "fight" between the "old man" and the "new man" (like two dogs) thus putting the believer in "a state of defeat" and "paralysis". George says,
"It is easy to identify these wretched people because they have no joy, they are always battling and at a standstill. But this paralysis is not the normal Christian life. Horatius Bonar says clearly in a booklet that the Christian has only one nature, not two. What Romans 6 teaches is that the old man, the old nature, was crucified with Christ; it is not alive. We must believe this if we are to possess our inheritance" (page 18-19).
George never states that this view of the two natures is erroneous. He simply says that the old nature was crucified with Christ. Apparently, George still believes that we have two natures. Without any explanation He starts talking about the need to possess our souls -- "The land is our soul; the Lord has given it to us, but unbelief will keep us from possessing it" (page 19).
So is the soul the "old man", the "new man" or some other "entity'? In all probability it is some other "third" entity, since George believes that man is comprised of three parts: body, soul, and spirit. The soul is perhaps that "third" entity that releases the grace of God by an act of "once-for-all and at once" faith. Without its activity of "possessing", there can be no "power, vitality and victory".
Obviously, George is infected by the teaching of the two natures, because it is difficult to know what the nature of the "third" entity that gives control to one or the other nature.
See the related article Is God's Grace Suspended Until We Act? to understand how the human will is essential to George's teaching on perfectionism.