Quote: two of Frege's basic laws are excluded middle and reduction of implication; substitution rules are unstated

Quotation Skeleton

14. [a->(b->a)] says "The case in which a … cannot at the same time be denied and … [p. 31] "If a proposition a is a necessary … and if one of these, b, is in … of this latter one, c, alone". [Then uses these propositions, and unstated rules of substitution (according to the editor), to derive many others.]   Google-1   Google-2

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