Card paradox

The card paradox is a non-self-referential variant of the liar paradox constructed by Philip Jourdain. It is also known as the postcard paradox, Jourdain paradox or Jourdain's paradox.

Suppose there is a card with statements printed on both sides:

Trying to assign a truth value to either of them leads to a paradox.

The same mechanism applies to the second statement. Neither of the sentences employs self-reference, instead this is a case of circular reference. See Yablo's paradox for a variation of the liar paradox that does not even rely on circular reference.

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