Between 1880 and 1887, Heaviside developed the operational calculus (involving the D notation for the differential operator, which he is credited with creating), a method of solving differential equations by transforming them into ordinary algebraic equations which caused a great deal of controversy when first introduced, owing to the lack of rigour in his derivation of it. He famously said, “Mathematics is an experimental science, and definitions do not come first, but later on.” He was replying to criticism over his use of operators that were not clearly defined. On another occasion he stated somewhat more defensively, “I do not refuse my dinner simply because I do not understand the process of digestion.”

Oliver Heaviside