They said it couldn’t be done, but the fool didn’t know it, so the fool went ahead and did it.

The saying circulated in U.S. print sources by 1919 as a short humorous maxim about how assumed impossibilities can be overturned. It later appeared in various venues (newspapers, trade publications, sermons) and was reused in Mary O’Hara’s 1941 novel, which helped spread it further.

The line jokes that certainty about what cannot be done often reflects shared belief rather than real limits; someone unaware of the supposed barrier may attempt it anyway and succeed, exposing the earlier “impossible” claim as premature.

They said it couldn’t be done, but the poor fool didn’t know it, so he went ahead and did it.

Everyone knew it was impossible, until a fool who didn’t know came along and did it.

People said it couldn’t be done; but the fool didn’t know it, so he went ahead and did it.