17. For--the reason why walking by the Spirit will exclude fulfilling the lusts of the flesh, namely, their mutual contrariety.
      the Spirit--not "lusteth," but "tendeth (or some such word is to be supplied) against the flesh."
      so that ye cannot do the things that ye would--The Spirit strives against the flesh and its evil influence; the flesh against the Spirit and His good influence, so that neither the one nor the other can be fully carried out into action. "But" (Ga 5:18) where "the Spirit" prevails, the issue of the struggle no longer continues doubtful (Ro 7:15-20) [BENGEL]. The Greek is, "that ye may not do the things that ye would." "The flesh and Spirit are contrary one to the other," so that you must distinguish what proceeds from the Spirit, and what from the flesh; and you must not fulfil what you desire according to the carnal self, but what the Spirit within you desires [NEANDER]. But the antithesis of Ga 5:18 ("But," &c.), where the conflict is decided, shows, I think, that here Ga 5:17 contemplates the inability both for fully accomplishing the good we "would," owing to the opposition of the flesh, and for doing the evil our flesh would desire, owing to the opposition of the Spirit in the awakened man (such as the Galatians are assumed to be), until we yield ourselves wholly by the Spirit to "walk by the Spirit" (Ga 5:16, 18).

JFB.