The spirit is willing, but the flesh is weak

However willing we may be to do a thing, our physical condition may prevent us from doing it.

The source is Matthew, xxvi, 41. Here is the story as told in the New English Bible:

Jesus then came with his disciples to a place called Gethsemane. He said to them, ‘Sit here while I go over there to pray.’ He took with him Peter and the two sons of Zebedee. Anguish and dismay came over him and, he said to them, ‘My heart is ready to break with grief. Stop here, and stay awake with me.’ He went on a little, fell on his face in prayer, and said, ‘My Father, if it is possible, let this cup pass me by. Yet not as I will, but as thou wilt.’

He came to the disciples and found the asleep ;and he said to Peter, ‘What! Could none of you stay awake with me one hour? Stay awake, and pray that you my be spared the test. The spirit is willing, but the flesh is weak.’