This is because no human being can be considered right with God by [obeying] the requirements of a law. [Note: It is difficult to determine the exact sense in which "law" is used in these verses. It may be law generally, or the law of Moses specifically. See Bruce, pages 52-58]. For the knowledge of what sin is comes from the law of Moses.

For I do not [really] understand what I am doing; I practice what I do not want to and I hate what I do. [Note: This highly controversial section (verses 14b-25) is here viewed as the struggles of the apostle Paul after his conversion, and by extension, of all Christians. See Bruce, pages 150ff; Murray, pages 255ff; Lard, pages 236ff].