Then I find the law: to desirously will in me to do the proper, that in me the bad is present. For I delight in the law of the God according to the inner man, but I see a different law in my members making war against the law of my mind and leading me into captivity to the law of the sin [nature], the one being in my members (Romans 7:21–23).
As Paul sought to live out the Christian life by means of law, he discovered that this was not possible due to the presence of the sin nature. He therefore came to understand a principle—a law—governing the operation of the sin nature. When we seek justification through law, the sin nature brings us into captivity to its desirous will, producing in us the very things we do not wish to do. This principle of law is analogous to the physical laws of nature: for every action, there is an equal and opposite reaction; in a closed system, total momentum remains constant before and after interaction; and the direction of an induced current is such that it opposes the change that produced it. In the same way, living by law results in captivity to the sin nature and the working out of what is wrong, rather than the good we desire to do.