 Computability Theory is a subfield of contemporary mathematical logic devoted to the classification of functions and sets of natural numbers in terms of their absolute and relative computability and definability-theoretic properties. This subject is closely related in both origin and content to the study of recursive functions. This is reflected by the fact that computability theory was known as recursive function theory (or simply recursion theory) from the time of its inception in the 1930s until the late 1990s. It is also reflected in the formulation and proof of the so-called Recursion Theorem which provides a fundamental link between recursive definability and the sort of self-referential constructions which are at the core of many methods in computability theory (see  Section 3.4).