The card paradox is a non-self-referential variant of the liar paradox

Liar paradox

In philosophy and logic, the liar paradox or liar's paradox , is the statement "this sentence is false"...

 constructed by Philip Jourdain

Philip Jourdain

Philip Edward Bertrand Jourdain was a British logician and follower of Bertrand Russell.He was born in Ashbourne in Derbyshire one of a large family belonging to Emily Clay and his father Francis Jourdain . He was partly disabled by Friedreich's ataxia...

. It is also known as the postcard paradox, Jourdain paradox or Jourdain's paradox.

The paradox

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

Front:	The sentence on the other side of this card is TRUE.
Back:	The sentence on the other side of this card is FALSE.

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

Paradox

Similar to Circular reasoning, A paradox is a seemingly true statement or group of statements that lead to a contradiction or a situation which seems to defy logic or intuition...

.

1. If the first statement is true, then so is the second. But if the second statement is true, then the first statement is false. It follows that if the first statement is true, then the first statement is false.
2. If the first statement is false, then the second is false, too. But if the second statement is false, then the first statement is true. It follows that if the first statement is false, then the first statement is true.

The same mechanism applies to the second statement. Neither of the sentences employs self-reference

Self-reference

Self-reference occurs in natural or formal languages when a sentence or formula refers to itself. The reference may be expressed either directly—through some intermediate sentence or formula—or by means of some encoding...

, instead this is a case of circular reference

Circular reference

A circular reference is a series of references where the last object references the first, resulting in a closed loop.-In language:A circular reference is not to be confused with the logical fallacy of a circular argument...

. See Yablo's paradox for a variation of the liar paradox that does not even rely on circular reference.

See also

• Liar paradox
  Liar paradox
  In philosophy and logic, the liar paradox or liar's paradox , is the statement "this sentence is false"...
  
  
• Yablo's paradox
• Self-reference
  Self-reference
  Self-reference occurs in natural or formal languages when a sentence or formula refers to itself. The reference may be expressed either directly—through some intermediate sentence or formula—or by means of some encoding...
  
  
• Circular reference
  Circular reference
  A circular reference is a series of references where the last object references the first, resulting in a closed loop.-In language:A circular reference is not to be confused with the logical fallacy of a circular argument...
  
  
• List of paradoxes