The "syntactic approach" requires much less logic and model theory to understand and use. This approach was developed in the mid-1970s by the mathematician Edward Nelson. Nelson introduced an entirely axiomatic formulation of nonstandard analysis that he called internal set theory (IST). IST is an extension of Zermelo–Fraenkel set theory (ZF) in that alongside the basic binary membership relation ∈, it introduces a new unary predicate "standard", which can be applied to elements of the mathematical universe together with some axioms for reasoning with this new predicate.