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1 HistoryFirst there was Frege [Frege1879] which was sort of higher-order, but Russell showed Frege's system was inconsistent using Russell's paradox. Russell created (ramified) type theory to avoid Russell's paradox [Russell08]. Russell did not use λ-calculus to formulate type theory. Often λ-calculus is not used in formulations of higher-order logic. Church [Church40] introduced the form of higher-order logic as a theory in the simply typed λ-calculus (sometimes called “simple type theory”). 2 Cut-EliminationSchütte [Schutte60] gives a nice early discussion of issues regarding type theory, but with a “relational formulation” (not using λ-calculus). At this point cut-elimination was still an open problem. Cut-elimination was proven by Prawitz [Prawitz68] and Takahashi [Takahashi67] independently – though both without λ-calculus. Important: Andrews modified the technique to show cut-elimination for a nonextensional form of type theory with λ-calculus in [Andrews71]. Prawitz used the term “possible values“ in association with the technique for showing cut-elimintation. Andrews proved cut-elimination has a relationship to consistency of analysis [Andrews74a]. Due to this result combined with Godel's second incompleteness theorem, no proof of cut-elimination for higher-order logic can be formalized in higher-order logic (with infinity) – unless higher-order logic with infinity is inconsistent. 3 SemanticsHenkin gave a definition of a model for Church's type theory in [Henkin50] and proved completeness. Andrews later showed Henkin's original definition of model was incorrect (a form of extensionality failed for Leibniz equality) – see [Andrews72b]. Andrews added the condition that Henkin models should include an interpretation for equality (and called these “general models”). See also [Andrews72a] for more about semantics. The semantics with (and without) different forms of extensionality are discussed in [BBK04]. Brown's thesis [Brown2004a] discusses many semantic issues in a more general framework than Church's type theory (in which logical constants may or may not be present). 4 Simply Typed λ-calculusNote that this is not higher-order logic. This is about simply typed lambda calculus. Church's type theory is a theory in the simply typed lambda calculus. Hindley's book [Hindley97] gives a nice introduction. Huet's unification is vital [Huet75] for automated reasoning in systems which use simply typed λ-calculus. 5 OverviewsSee [Leivant94] (without λ-calculus) and [Andrews00c] (with λ-calculus) for nice discussions.
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