 Building hierarchies is a method to circumvent both the set-theoretic, semantic and epistemic paradoxes. Russell’s original solution to his paradox—as well as Tarski’s original solution to his undefinability of truth problem—was to build hierarchies. In Russell’s case, this led to type theory. In Tarski’s case, it led to what is now known as Tarski’s hierarchy of languages. In both cases, the idea is to stratify the universe of discourse (sets, sentences) into levels. In type theory, these levels are called types. The fundamental idea of type theory is to introduce the constraint that any set of a given type may only contain elements of lower types (that is, may only contain sets which are located lower in the stratification). This effectively blocks Russell’s paradox, since no set can then be a member of itself.