The believer who would be truly spiritual must recognize the presence of the old nature within. It would be dangerous not to recognize a foe so near.
The old nature in the believer is that which is βbegotten of the flesh.β It is called, βthe flesh,β βthe old man,β βthe natural man,β βthe carnal mind.β
Just as βthey that are in the flesh cannot please Godβ (Rom. 8:8) so that which is of the flesh, in the believer, cannot please God. βThe flesh,β as we have already seen, is totally depraved. God calls it βsinful fleshβ (Rom. 8:3), warns that it seeks βoccasionβ to do wrong (Gal.Β 5:13), and declares that βthe works of the fleshβ are all bad (Gal. 5:19-21).
Nor is the old nature in the believer one which improves by its contact with the new. It is with respect to βthe fleshβ in the believer, even in himself that the Apostle declares that in it βdwelleth no good thingβ (Rom.Β 7:18), that it is βcarnal, sold under sinβ (Rom.Β 7:14), that it is βcorrupt according to the deceitful lustsβ (Eph.Β 4:22), that it is at βenmity against God,β and is βnot subject to the law of God,Β neither indeed can beβ (Rom. 8:7).
βThe flesh,β even as it remains in the believer after salvation, is that which was generated by a fallen begetter. It is the old Adamic nature. It is sinful in itself. It cannot be improved. It cannot be changed. βThat which is born [begotten] of the flesh is flesh,β said our Lord (John 3:6), and it is as impossible to improve the βold manβ in the believer as it was to make him acceptable to God in the first place.
The βold manβ was condemned and dealt with judicially at the Cross. Never once is the believer instructed to try to do anything with him or to make anything of him, but always to βreckonβ him βdead indeedβ (Rom.Β 6:11), and to βput him offβ (Col. 3:8-10). source