The Apostle Paul refers to "the spirit" and "the flesh" in several of his letters. I specifically refer (in this post) to his letters to the Galatians and to the believers in Rome. He admonishes them to walk after the spirit--which brings life and peace, rather than walk after the flesh--which brings death. (Romans 8:13)
For years, I saw "spirit" capitalized in my English version of the Bible, and I thought Paul was always referring to the Holy Spirit. I didn't realize that in the Greek, spirit is never capitalized. When I thought spirit always referred to the third person of the trinity, I was sometimes left with a cloudy understanding.
From what I presently understand, sometimes Paul is referring to the Holy Spirit when he uses the term spirit, but often he is referring to the forever aspect of us which is being made into the image of Jesus as we trust and "walk after" Him.
The flesh is that physical part of us which is dying. It is not evil, it is just independent and will go its own way, do its own thing, unless something is done about it. It gets hungry and says, "Give me something to eat." It gets lonely and says, "Somebody hold me." It gets bored and says, "Give me something fun to do." It sees something it likes to look at and says, "I want to see that." On and on its demands go. If we follow its demands, if we do not subject every one of its requests to the understanding Jesus has given us, and ask the question, "What is pleasing to Him?" we will follow the flesh to our own destruction.
At various stages in life, the flesh demands different things. Our children are subject to forces of the flesh that we sometimes forget we dealt with as youngsters. Depending on our gender, the desires vary somewhat.
As long as we live, we are trapped in this flesh; we have to deal with it. Paul gives us the Christian answer to our dilemma. "Now those who belong to Christ Jesus," he states, "have crucified the flesh with its passions and desires." (Gal 5:24 NAS)
When our flesh puts a force on us to do something, to feel something, to think something, to gaze at something, we have the responsibility to prayerfully examine and crucify, in our own mind (or die to) what is not Christlike. We do not have to give the flesh what it wants.
Getting a better understanding of these two concepts has been life-changing for me, something to sink my teeth into! It's real and exciting, and it works! It's not spiritual-pie-in-the-sky, empty terminology, but it is a way of life that makes everything change for the better! It makes Jesus's teachings, as well instructions from the other New Testament writers come to life! Every situation, every thought, every word, every attitude, every deed must be examined in the light of this reality.
While we never escape feeling the forces of the flesh in this life, we can say with Paul, "I [will] die daily." And we can share in the promise that "if [we live] by the [s]pirit [we] are putting to death the deeds of the body, [and we] will live." (Rom 8:13 NAS)
Thanks be to God who has made a way for us to become His sons and daughters! Do I realize what that means?! If the Bible is true, and we really can become heirs of God, it's too amazing to fully comprehend, but I'm certainly going to keep pursuing the path!