The spirit is willing, but the flesh is weak

Meaning: Even though we may want to say "No" to temptation (doing something bad), our body may be unable to resist it.

Note: spirit (noun): the non-physical part of a person (including emotions and character); the soul | willing (adj.): ready or happy to do something | flesh (noun): the physical part of a person; the body; the outer surface of the body

Origin: This proverb is based on a verse in the Bible (Matthew 26:41): "Watch and pray, that ye enter not into temptation; the spirit indeed is willing but the flesh is weak."