spirit is willing but the flesh is weak, the

the spirit is willing, but the/(one's) flesh is weak

One has the desire or intention to change do something beneficial, but one lacks the resolve or motivation to do it. Every year I resolve to eat better and do more exercise; and every year, inevitably, the spirit is willing, but the flesh is weak. We all know someone who has the best intentions in the world to get their lives on the right track, whose spirit is willing, but their flesh is weak.
See also: but, flesh, spirit, weak

spirit is willing but the flesh is weak, the

One would like to undertake something but hasn't the energy or strength to do so. For example, Another set of tennis? The spirit is willing but the flesh is weak. Today often used as a rueful admission of weariness or other physical weakness, this idiom was first recorded in the New Testament (Matthew 26:41), where Jesus tells his disciples: "Watch and pray, that ye enter not into temptation: the spirit indeed is willing, but the flesh is weak." A modern equivalent is I would if I could but I can't.
See also: but, flesh, spirit, willing