 As in the case of the notion of an infinitesimal, the Dirac \(\delta\) function was eventually given a mathematically rigorous foundation. That was done within Schwartz’s theory of distributions, which was later used in developing the notion of a rigged Hilbert space. The theory of distributions was used to provide a mathematical framework for quantum field theory (Wightman 1964). The rigged Hilbert space was used to do so for quantum mechanics (Böhm 1966) and then for quantum field theory (Bogoluliubov et al. 1975).