Albert Einstein to Max Born, 29 April 1924
After initial successes, quantum theory is in crisis in the early 1920s. For one thing, it is little more than an unclear mixture of fragments of classical physics and rules for their modification as applied to problems of atomic physics. For another, it fails in the attempt to understand the properties of complex atoms. The need for new foundations is becoming ever more apparent, and Einstein expects this from the unified field theory. He defends himself by saying that some of his colleagues want to give up basic principles of physics like determinism.