Einstein tried 1905 to explain the photoelectric effect with an ad hoc hypothesis, but he has expressed also his problems with his own ad hoc light quantum hypothesis at the first times on the 81. Jahrestagung der Gesellschaft Deutscher Naturforscher und Ärzte 1909 in Salzburg.
Obviously, Einstein offered his malaise about the role of accidence in the new quantum physics. In the classical physics the causality has unrestricted validity. Therefore, for each action there must be a definite cause. The thinking on causality is governed our life, the thinking of cause and action. The Planck constant h puts again these reflections a great barrier. Nature is not more continuously, since the action quantum is not divisible. Moreover, the action quantum provides an insurmountable obstacle for our world description.
This negative vision is not true. In reality only the principle uncertainty that neither the positions, nor the velocities of particles are ever precisely known distinguish the microscopic physics from the classical physics. The definite inertial conditions of bodies of the classical physics cannot be assumed in microscopical physics. The causality is further valid in each region of physics: the interactions are caused by conserved elementary charges.