"Weirdly, although the beauty of physical theories is embodied in rigid mathematical structures based on simple underlying principles, the structures that have this sort of beauty tend to survive even when the underlying principles are found to be wrong. A good example is Dirac’s theory of the electron. Dirac in 1928 was trying to rework Schrödinger’s version of quantum mechanics in terms of particle waves so that it would be consistent with the special theory of relativity. This effort led Dirac to the conclusions that the electron must have a certain spin, and that the universe is filled with unobservable electrons of negative energy, whose absence at a particular point would be seen in the laboratory as the presence of an electron with the opposite charge, that is, an antiparticle of the electron. His theory gained an enormous prestige from the 1932 discovery in cosmic rays of precisely such an antiparticle of the electron, the particle now called the positron. Dirac’s theory was a key ingredient in the version of quantum electrodynamics that was developed and applied with great success in the 1930s and 1940s. But we know today that Dirac’s point of view was largely wrong. […] Yet the mathematics of Dirac’s theory has survived as an essential part of quantum field theory; it must be taught in every graduate course in advanced quantum mechanics. The formal structure of Dirac’s theory has thus survived the death of the principles of relativistic wave mechanics that Dirac followed in being led to his theory. So the mathematical structures that physicists develop in obedience to physical principles have an odd kind of portability. They can be carried over from one conceptual environment to another and serve many different purposes, like the clever bones in your shoulders that in another animal would be the joint between the wing and the body of a bird or the flipper and body of a dolphin. We are led to these beautiful structures by physical principles, but the beauty sometimes survives when the principles themselves do not."