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Full Idea
Leibniz's Law underwrites the substitutivity of identity and this is a principle not long dispensable in any form of reasoning.
Gist of Idea
Substitutivity, and hence most reasoning, needs Leibniz's Law
Source
David Wiggins (Sameness and Substance Renewed [2001], 2.7)
Book Ref
Wiggins,David: 'Sameness and Substance Renewed' [CUP 2001], p.46
A Reaction
Thus the modern fashion of deriving our metaphysics from our logic. Presumably we can derive it from our epistemology too, or even from our intuitions, if we thought they were good enough as evidence.