This era also saw several theoretical steps which heralded the attempt to develop a general theory of feasible computability. The basic definitions of time and space complexity for the Turing machine model were first systematically formulated by Hartmanis and Stearns (1965) in a paper called “On the Computational Complexity of Algorithms”. This paper is also the origin of the so-called Hierarchy Theorems (see Section 3.2)  which demonstrate that a sufficiently large increase in the time or space bound for a Turing machine computation allows more problems to be decided.