In the paper he coauthored with Podolsky and Rosen in 1935, Albert Einstein argued that quantum mechanics was not a complete description of reality by demonstrating that two eigenstates could exist on the same quantum particle at the same time. He then predicted the existence of local hidden variables (LHV), which postulated that a quantum system had to have all the possible measurement outcomes recorded before a measurement.
In 1964, the Bell inequalities were developed by John Bell, stating that any system violating the inequalities could not follow the LHV theory. With a thought experiment, he proved with a probabilistic argument that two electrons in an entangled spin system violated the inequalities, proving that Einstein had been incorrect. Thus, quantum mechanics is a complete description of reality. In 1981-82, French researcher Aspect verified Bell's thought experiment experimentally in his lab.