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The development of theories of elementary particles as extended objects, from Thomson to Dehmelt and MacGregor, is reviewed. Most of the theories are inspired by the existence of particle spin. The arguments of their authors are contrasted to Pauli's assertion than spin is "a classically non-explainable two-valuedness," as well as to Dirac's theory of a point electron.

Superposition principle, as one of the basic principles in quantum mechanics, is essential for numerous quantum phenomena: interference, quantum beats, quantum interference, wave packets, florescence, Rabi oscillations, spin echo, superposition of spin states, etc. In all those cases a wave function of a quanton is a superposition of two or more eigenstates. Characteristic features of those phenomena are determined by relative phases of different components in the above superposition. But, the standard interpretation of quantum mechanics avoids to attribute physical meaning to phases as well as to relative phases of wave functions. As a consequence, attempts to physically explain and understand those phenomena often encounter paradoxes, difficulties and inconsistencies discussed for example by Feynman and Ballentine in the case of interference, by Klein et al. and Kaiser et al. in connection with wave packets, by Wigner in connection with spin state superposition, by Schrodinger and Brewer and Schenzle in relation to flourscence, etc.