basic constructions:
strong axioms
natural deduction metalanguage, practical foundations
type theory (dependent, intensional, observational type theory, homotopy type theory)
computational trinitarianism = propositions as types +programs as proofs +relation type theory/category theory
Burali-Forti’s paradox is a paradox of naive material set theory that was first observed by Cesare Burali-Forti. However, the paradox is not specific to material set theory and can be formulated in structural set theory or in type theory. When formulated in type theory, it is often called Girard’s paradox after Jean-Yves Girard.
Suppose that there were a set $Ord$ of all ordinal numbers. One could then prove that
There are many variations of the paradox, depending for instance on what precise definition of “well-ordered” (and “ordinal number”) one chooses.
As formulated in type theory by Jean-Yves Girard, the Burali-Forti paradox shows that the original version of Per Martin-Löf’s type theory, which allowed $Type:Type$, is inconsistent, in the precise sense that it contains (non-normalizing) proofs of false.
Moreover, by an adaptation of the proof, one can construct a looping combinator in this type theory, which implies the undecidability of type-checking.