The flesh and the Spirit are used here by Paul to talk about two regimes or domains, if you will. Life in the Spirit, or according to the Spirit, is a life where the Spirit is in control and has come to reside with the individual who has accepted God’s gift of eternal life in Christ Jesus. To “walk by the Spirit” entails a genuine decision. However, this is not a decision involving a strong effort of the will to overcome enormous obstacles in order to get something accomplished. Rather, Christians are called to entrust themselves to the Spirit, to God’s activity, and simply to follow His guidance. It is just the opposite of all the characteristics of life, or existence, accord- ing to the flesh, which will eventuate in death.
Lesson 5: "Flesh Vs. Spirit", Galatians and Ephesians, Adult Teacher's Insights, p. 29.