Now if I do - This verse is also a repetition of what was said in Romans 7:16-17.

It is no more I - My will is against it; my reason and conscience condemn it. But sin that dwelleth in me - the principle of sin, which has possessed itself of all my carnal appetites and passions, and thus subjects my reason and domineers over my soul. Thus I am in perpetual contradiction to myself. Two principles are continually contending in me for the mastery: my reason, on which the light of God shines, to show what is evil; and my passions, in which the principle of sin works, to bring forth fruit unto death.
This strange self-contradictory propensity led some of the ancient philosophers to imagine that man has two souls, a good and a bad one; and it is on this principle that Xenophon, in his life of Cyrus, causes Araspes, a Persian nobleman, to account for some misconduct of his relative to Panthea, a beautiful female captive, whom Cyrus had entrusted to his care: - "O Cyrus, I am convinced that I have two souls; if I had but one soul, it could not at the same time pant after vice and virtue; wish and abhor the same thing. It is certain, therefore, that we have two souls; when the good soul rules, I undertake noble and virtuous actions; but when the bad soul predominates, I am constrained to do evil. All I can say at present is that I find my good soul, encouraged by thy presence, has got the better of my bad soul." See Spectator, vol. viii. No. 564. Thus, not only the ancients, but also many moderns, have trifled, and all will continue to do so who do not acknowledge the Scriptural account of the fall of man, and the lively comment upon that doctrine contained in the seventh chapter of the Epistle to the Romans.

Now if I do that I would not,.... The same conclusion is formed here, as in Romans 7:17, not with any view to excuse himself from blame in sinning, but to trace the lusts of his heart, and the sins of his life, to the source and fountain of them, the corruption of his nature; and to ascribe them to the proper cause of them, which was not the law of God, nor the new man, but sin that dwelt in him.