 Although level shifting of specific explanatory factors seems to work less easily within purely physical explanations, relocation attempts involving broad physical principles can sometimes avoid such difficulties. For instance, for centuries determinism was a basic background component of scientific explanations (apparently stochastic processes being explained away epistemically). Then, early in the 20th century physics was largely converted to a quantum mechanical picture of nature as involving an irreducible indeterminism at a fundamental level—apparently deterministic phenomena now being what was explained away. However, DeBroglie, Bohm and others (even for a time Einstein) tried to reinstate determinism by moving it back to an even deeper fundamental level via hidden variable theories. Although the hidden variable attempt is generally thought not to be successful, its failure is not a failure of principle.