"The word “spirit” (pneuma (πνευμα)) refers to the human spirit as that part of man which gives him God-consciousness, or to the attitude or disposition of a man, as “that man’s spirit is good,” or to the Holy Spirit. The definite article appears before it in the Greek text. The absence of the Greek definite article emphasizes character.
If that were absent here, the reference would be to the disposition or attitude of a person. The presence of the article points to the Holy Spirit. It is the locative of sphere. That is, Paul exhorts to fervency in the Christian life which is engendered by the Spirit, not produced by the flesh (self-effort)."