One of the traditional three laws of thought, the other two being the laws of contradiction and excluded middle.
‘Everything is what it is and not another thing’, or (where ‘P’ is any proposition) ‘If P then P’. The English empiricist George Edward Moore (1873-1958) took the first quotation above as the motto for his book Principia Ethica (1903), attributing it to Bishop Joseph Butler (1692-1752).
In comparison to the other two laws of thought, little attention has been paid to this one.
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