 Max Jammer (1974, pp. 166–181) locates the development of the EPR paper in Einstein’s reflections on a thought experiment he proposed during discussions at the 1930 Solvay conference. (For more on EPR and Solvay 1930 see Howard, 1990 and Ryckman, 2017, pp. 118–135.) The experiment imagines a box that contains a clock set to time precisely the release (in the box) of a photon with determinate energy. If this were feasible, it would appear to challenge the unrestricted validity of the Heisenberg uncertainty relation that sets a lower bound on the simultaneous uncertainty of energy and time. (See the entry on the  Uncertainty Principle  and also Bohr 1949, who describes the discussions at the 1930 conference.) The uncertainty relations, understood not just as a prohibition on what is co-measurable, but on what is simultaneously real, were a central component in the irrealist interpretation of the wave function. Jammer (1974, p. 173) describes how Einstein’s thinking about this experiment, and Bohr’s objections to it, evolved into a different photon-in-a-box experiment, one that allows an observer to determine either the momentum or the position of the photon indirectly, while remaining outside, sitting on the box. Jammer associates this with the distant determination of either momentum or position that, we shall see, is at the heart of the EPR paper. Carsten Held (1998) cites a related  correspondence with Paul Ehrenfest  from 1932 in which Einstein described an arrangement for the indirect measurement of a particle of mass m using correlations with a photon established through Compton scattering. Einstein’s reflections here foreshadow the argument of EPR, along with noting some of its difficulties.