System U

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In mathematical logic, System U and System U are pure type systems, i.e. special forms of a typed lambda calculus with an arbitrary number of sorts, axioms and rules (or dependencies between the sorts). They were both proved inconsistent by Jean-Yves Girard in 1972.[1] This result led to the realization that Martin-Löf's original 1971 type theory was inconsistent as it allowed the same "Type in Type" behaviour that Girard's paradox exploits.

Formal definition[edit]

System U is defined[2]:352 as a pure type system with

  • three sorts ;
  • two axioms ; and
  • five rules .

System U is defined the same with the exception of the rule.

The sorts and are conventionally called “Type” and “Kind”, respectively; the sort doesn't have a specific name. The two axioms describe the containment of types in kinds () and kinds in (). Intuitively, the sorts describe a hierarchy in the nature of the terms.

  1. All values have a type, such as a base type (e.g. is read as “b is a boolean”) or a (dependent) function type (e.g. is read as “f is a function from natural numbers to booleans”).
  2. is the sort of all such types ( is read as “t is a type”). From we can build more terms, such as which is the kind of unary type-level operators (e.g. is read as “List is a function from types to types”, that is, a polymorphic type). The rules restrict how we can form new kinds.
  3. is the sort of all such kinds ( is read as “k is a kind”). Similarly we can build related terms, according to what the rules allow.
  4. is the sort of all such terms.

The rules govern the dependencies between the sorts: says that values may depend on values (functions), allows values to depend on types (polymorphism), allows types to depend on types (type operators), and so on.

Girard's paradox[edit]

The definitions of System U and U allow the assignment of polymorphic kinds to generic constructors in analogy to polymorphic types of terms in classical polymorphic lambda calculi, such as System F. An example of such a generic constructor might be[2]:353 (where k denotes a kind variable)


This mechanism is sufficient to construct a term with the type , which implies that every type is inhabited. By the Curry–Howard correspondence, this is equivalent to all logical propositions being provable, which makes the system inconsistent.

Girard's paradox is the type-theoretic analogue of Russell's paradox in set theory.


  1. ^ Girard, Jean-Yves (1972). "Interprétation fonctionnelle et Élimination des coupures de l'arithmétique d'ordre supérieur" (PDF). Cite journal requires |journal= (help)
  2. ^ a b Sørensen, Morten Heine; Urzyczyn, Paweł (2006). "Pure type systems and the lambda cube". Lectures on the Curry–Howard isomorphism. Elsevier. ISBN 0-444-52077-5.

Further reading[edit]