Why do we say "The spirit is willing but the flesh is weak"?

Meaning:

This expression can be taken to mean either that the person is too weak to resist a temptation presented, or too lacking in energy to complete a task

Background:

This statement expressing the difficulty of living up to ones own moral standards traces its origins to the New Testament Gospel of Matthew:

For example: I knew I shouldn't have eaten that second slice of cake - the spirit was willing but the flesh was weak.

It can also be used ruefully to express weariness or physical weakness; for example: I'd really like to play another set of tennis but I'm just out of energy - the spirit is willing but the flesh is weak.

"Watch and pray so that you will not fall into temptation. The spirit is willing, but the body is weak." - Mathew 26:41 (Bible: New International Version)

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