What was the original Catch-22?

In Joseph Heller’s 1961 novel of the same name, it is the catch that prevents a U.S. Air Force pilot from asking to be grounded on the basis of insanity.

A man “would be crazy to fly more missions and sane if he didn’t, but if he was sane, he had to fly them.

If he flew them he was crazy and didn’t have to; but if he didn’t, he was sane and had to.”