DIRAC, P.A.M. "Quantum Theory of Localizable Dynamic Systems", American Physical Society, in the Physical Review, 73/9, 1 May 1948. Original printed wrappers. Very good condition. $250

Abstract from AIP's great PROLA website: "A dynamical system is called localizable if its wave functions can be expressed in terms of variables, each referring to physical conditions at only one point in space-time. These variables may be at points on any three-dimensional space-like surface in space-time. A general investigation is made of how the wave function varies when the surface is varied in any way. The variation of the wave function is given by equations of the Schroedinger type involving certain operators Hn(u) which play the role of Hamiltonians. The commutation relations for these operators are obtained. The theory works entirely with relativistic concepts and it provides the general pattern which any relativistic quantum theory must conform to, provided the dynamical system is localizable."

AND: "Dirac was also original in his conception of the role of probability in quantum mechanics. He thought that probabilities entered into the description of quantum phenomena only in the determination of the initial state (still described in terms of p s and q s), and not necessarily in the behavior of an isolated system. But, as Bohr had said at the Solvay Conference in 1927, isolated systems were unobservable. Dirac then assumed that the state of the world was represented by its wave function ...; and that it changed abruptly during a measurement, whereupon nature made a choice.” -- Complete Dictionary of Scientific Biography, vol. 17, Charles Scribner's Sons, 2008, pp. 224-233.