the classical physics of the time,  which was largely deterministic.

Quantum mechanics introduced inherent  probabilities into the fundamental nature of the  

physical world, which was a significant departure  from the previously understood mechanics.

Albert Einstein, one of the most  respected physicists of his time,  

found the probabilistic interpretations of quantum  

mechanics difficult to reconcile with  his views on causality in the universe

His statement "God does not play dice with the  universe" has become emblematic of his discomfort  

with the idea that events at the quantum  level could occur without deterministic laws.

Despite his contributions to quantum theory,  

including the photoelectric effect,  which earned him the Nobel Prize,  

Einstein spent much of his later life  attempting to find a unified field theory  

that would provide a deterministic explanation  for the phenomena described by quantum mechanics.