 The second key idea in the historical development of retrocausality in quantum mechanics occurs around the same time as Wheeler and Feynman’s absorber theory. French physicist Costa de Beauregard, a student of de Broglie, noticed a potential objection to the reasoning found in Einstein, Podolsky, and Rosen’s famous paper (1935) on the completeness of quantum mechanics (see the entry on  the Einstein-Podolsky-Rosen argument in quantum theory).  Now widely known as the EPR argument, They argue that quantum mechanics must be incomplete on the basis of the following assumption: no reasonable definition of reality could be expected to permit the reality of some system being dependent upon the process of measurement carried out on some other distant system which does not in any way disturb the first system. The “reasonable definition of reality” to which Einstein et al. refer is implicitly an assumption of relativistic “locality” (made explicit in Einstein 1948), which combines causal asymmetry with the Lorentz invariance of special relativity (more on this in a moment). Costa de Beauregard, however, was alert to a particular kind of unorthodox interpretation of this assumption which undermined its role in the EPR argument. His proposal was that two distant systems could “remain correlated by means of a successively advanced and retarded wave” (Costa de Beauregard 1953: 1634); that is, one system could influence, via an advanced wave, the state of the combined systems in their common past, which then, via a retarded wave, could influence the state of the distant system in a kind of “zigzag” through spacetime. This way, there could be a dependence between the two distant systems without any violation of Lorentz invariance. Thus, as Costa de Beauregard (1987b: 252) puts it, 