However strong a castle may be, if a treacherous party resides inside (ready to betray at the first opportunity possible), the castle cannot be kept safe from the enemy. Traitors occupy our own hearts, ready to side with every temptation and to surrender to them all. (John Owen, Sin and Temptation)
Ever since that fateful day when Adam gave away the essence of his strength, men have struggled with a part of themselves that is ready at the drop of a hat to do the same. We don't want to speak up unless we know it will go well, and we don't want to move unless we're guaranteed success. What the Scriptures call the flesh, the old man, or the sinful nature, is that part of fallen Adam in every man that always wants the easiest way out. It's much easier to go down to the driving range and attack a bucket of balls than it is to face the people at work who are angry at you. It's much easier to clean the garage, organize your files, or cut the grass than it is to talk to your teenage daughter.
To put it bluntly, your flesh is a poser. And your flesh is not you. Did you know that? Your flesh is not the real you. When Paul gives us his famous passage on what it's like to struggle with sin (Rom. 7), he tells a story we are all too familiar with:
"I decide to do good, but I don't really do it; I decide not to do bad, but then I do it anyway. My decisions, such as they are, don't result in actions. Something has gone wrong deep within me and gets the better of me every time. It happens so regularly that it's predictable. The moment I decide to do good, sin is there to trip me up. I truly delight in God's commands, but it's pretty obvious that not all of me joins in that delight. Parts of me covertly rebel, and just when I least expect it, they take charge." (vv. 19-21, The Message)
Paul says, "Hey, I know I struggle with sin. But I also know that my sin is not me—this is not my true heart."