What is the Flesh?
flesh, temporal
When Paul says, "the flesh lusts against the spirit, and the spirit against the flesh," Galatians 5:17 King James Version 17For the flesh lusteth against the Spirit, and the Spirit against the flesh: and these are contrary the one to the other: so that ye cannot do the things that ye would. Study at Blue Letter Bible what does he mean by flesh? This is what I hope to clearly answer.
The word for flesh in Greek is Strong's G4561 · Greek σάρξ sárx Definitionflesh (as stripped of the skin), i.e. (strictly) the meat of an animal (as food), or (by extension) the body (as opposed to the soul (or spirit), or as the symbol of what is external, or as the means of kindred), or (by implication) human nature (with its frailties (physically or morally) and passions), or (specially), a human being (as such) Derivationprobably from the base of G4563 (σαρόω); KJV usagecarnal(-ly, + -ly minded), flesh(-ly) More at Blue Letter Bible . It's translated this way for every occurrence except two. Those two refer to sacred animals, so they aren't important to our study. This means every other use of the English word is the same Greek word.
Even with that, what does it mean? Strong's G4561 · Greek σάρξ sárx Definitionflesh (as stripped of the skin), i.e. (strictly) the meat of an animal (as food), or (by extension) the body (as opposed to the soul (or spirit), or as the symbol of what is external, or as the means of kindred), or (by implication) human nature (with its frailties (physically or morally) and passions), or (specially), a human being (as such) Derivationprobably from the base of G4563 (σαρόω); KJV usagecarnal(-ly, + -ly minded), flesh(-ly) More at Blue Letter Bible essentially means the part of the body that is soft tissue. By definition, it's a physical thing: it's your body. Your flesh is your body.
If fleshly desires are just desires of my body, then is eating a fleshly desire? Well, yes, but it's not a sin. After all, Jesus ate and was without sin. It could be a sin, though, if food is lifted higher than God.
Paul says in 2 Corinthians that, "the things which are seen are temporal, but the things which are not seen are eternal." 2 Corinthians 4:18 King James Version 18While we look not at the things which are seen, but at the things which are not seen: for the things which are seen are temporal; but the things which are not seen are eternal. Study at Blue Letter Bible He is saying that material things (like the flesh) are inherently less important because they don't last as long.
So now we have a framework for thinking about desires. Desires of the flesh aren't as important as desires from the spirit because the flesh is playing the short game. This is how the flesh and spirit are contrary to each other: they have different goals.
This is also why Paul says, "Live in the Spirit and you won't fulfill the desires of the flesh." Galatians 5:16 King James Version 16This I say then, Walk in the Spirit, and ye shall not fulfil the lust of the flesh. Study at Blue Letter Bible If you have an eternal outlook, you won't be thinking of the temporal consequences. Our bodies have a timer, but our spirit doesn't. Our spirit will live on forever, but our flesh will decay and die.
With all of that said, what is sin?
It's putting now above later. It's choosing our temporal pleasure above God's eternal plan. Righteousness is not sacrificing eternity at the expense of today; it's sacrificing today on the altar of eternity, and God is the only One who holds eternity.