To solve the problem, Russell developed his type theory from 1903 to 1908, which allowed only a restricted use of class terms.
[3] After Quine, Arnold Oberschelp developed the first fully functional modern axiomatic class logic starting in 1974.
It is a consistent extension of predicate logic and allows the unrestricted use of class terms (such as Peano).
It presupposes in particular any number of axioms, but can also take those and syntactically correct to formulate in the traditionally simple design with class terms.
[6] The principle of abstraction (Abstraktionsprinzip) states that classes describe their elements via a logical property: The principle of extensionality (Extensionalitätsprinzip ) describes the equality of classes by matching their elements and eliminates the axiom of extensionality in ZF: The principle of comprehension (Komprehensionsprinzip) determines the existence of a class as an element: This mathematical logic-related article is a stub.