Einstein now argues that there are only two options to explain this result.
Firstly, both photons could communicate during the measurement whether they are measured diagonally or vertically/horizontally and adopt the correspondng polarisations. But Einstein objects: That would be a "spooky action at a distance," completely incomprehensible to physics.
Secondly, the photons could carry the information whether they are absorbed or pass through for every possible polarisation measurement. Naturally this information could be correlated between both photons. But then the claim of quantum mechanics that it is pure chance whether, for example, a vertically polarised photon passes through a diagonally positioned filter would be incorrect. Quantum mechanics would be incomplete.