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Full Idea
With the principle of extensionality anything true of one propositional functions will be true of every coextensive one.
Gist of Idea
Extensionalism means what is true of a function is true of coextensive functions
Source
Bernard Linsky (Russell's Metaphysical Logic [1999], 6.3)
Book Ref
Linsky,Bernard: 'Russell's Metaphysical Logic' [CSLI 1999], p.97