Peter Smith, of Logic Matters, has noticed a new *Stanford Encyclopedia of Philosophy* entry, Set Theory: Constructive and Intuitionistic ZF. Constructive and intuitionistic set theories result from the rejection of the law of excluded middle, and effectively restrict set theoretical ontology to potentially infinite sets:

The shift from classical to intuitionistic logic, as well as the requirement of predicativity, reflects a conflict between the classical and the constructive view of the universe of sets. This also relates to the time-honoured distinction between actual and potential infinity. According to one view often associated to classical set theory, our mathematical activity can be seen as a gradual disclosure of properties of the universe of sets, whose existence is independent of us. This tenet is bound up with the assumed validity of classical logic on that universe. Brouwer abandoned classical logic and embarked on an ambitious programme to renovate the whole mathematical landscape. He denounced that classical logic had wrongly been extrapolated from the mathematics of finite sets, had been made independent from mathematics, and illicitly applied to infinite totalities.

In a constructive context, where the rejection of classical logic meets the requirement of predicativity, the universe is an open concept, a universe “in fieri”. This coheres with the constructive rejection of actual infinity (Dummett 2000, Fletcher 2007). Intuitionism stressed the dependency of mathematical objects on the thinking subject. Following this line of thought, predicativity appears as a natural and fundamental component of the constructive view. If we

constructmathematical objects, then resorting to impredicative definitions would produce an undesirable form of circularity. We can thus view the universe of constructive sets as built up in stages by our own mathematical activity and thus open-ended. [SEP]

This article might interest our BA Seminar students, as well as students in Programming Languages who have recently encountered Curry-Howard Isomorphism — the correspondence between intuitionistic logic and CL_{K}.

–Paul

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