Transitivity of identity
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The transitivity of identity is the logical principle that, if a=b, and b=c, then a=c.
For example, if you know that the Morning Star is the same thing as the Evening Star, and you know that the Evening Star is Venus, you can conclude that the Morning Star is Venus.
The transitivity of identity (like rules concerning identity generally) is not always valid in epistemological, modal, or otherwise opaque contexts. For example, if Oedipus knows that his wife is Jocasta, and you know that Jocasta is the mother of Oedipus, you may not conclude that Oedipus knows that his wife is his mother.
See also: transitive relation, identity