In "Biblical Perspective" Gordon Snider writes:
If the law is not the problem (v. 16), then what causes me to act contrary to my own desires? Paul answered by distinguishing between I and sin that dwelleth in me. The former is the man as created by God, which in verse 15 made good decisions. The latter is the principle of sin that entered the human race at the Fall and holds controlling influence in the natural man.
Source: Studies in Romans: Adult Teacher's Insights, page 36.