This is the last of a three‐part series. The first essay in the series described Dirac’s generalizations of quantum mechanics leading to his theory of the electron. The second essay surveyed evaluations of Dirac’s theory by other physicists, especially by Bohr who used Dirac’s speculations about negative energy electrons as evidence for the failure of quantum mechanics at nuclear dimensions. This essay shows how the material reality of positrons vindicated quantum mechanics and opened new paths for physics.

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