“In the face of the lack of direct mathematical demonstration, one must be careful and thorough to make sure of the point, and one should make a perpetual attempt to demonstrate as much of the formula as possible. Nevertheless, a very great deal more truth can become known than can be proven.”
Richard P. Feynman, “The Development of the Space-Time View of Quantum Electrodynamics” (Nobel Lecture, December 11, 1965)