Oh, I was speaking of the work that John Bell did back in the day.
So Einstein was always a hater of quantum mechanics because he did not believe that “God would play dice”, or in other words that there was such thing and ‘total randomness’ in the universe. The scientific theory that was in competition with this was the Heisenberg Uncertainty Principle, which was one of governing importance in quantum mechanics and hence in chemistry, genetics, biology, etc.
So he set out to prove that this was not so. However, after proposing a theory it was refuted by John Bell, who extended the theory to say that at some level, everything is probabilistic and thus nothing can be totally determined. There is still some debate over hidden variable theories and loopholes in the EPR paradox refutation, but largely people of science take quantum mechanics and its assumptions to be true because of their massive supporting evidence.
I however hold to some form of hidden variable theory. Even though you cannot KNOW an electrons momentum and direction simultaneously, and thus the level of uncertainty, this doesn’t mean that the electron does not possess these properties as a matter of truth. This is a philosophical/religious argument rather than a scientific one though I am afraid.