Einstein grappled with the implications of quantum mechanics, recognizing its achievements but asserting its incompleteness. He believed that while quantum mechanics provided valuable insights, it could not fully explain the workings of the universe. Instead, he sought a comprehensive theory that would unite both his own theory of relativity and quantum mechanics.
This pursuit was driven by his desire for a deterministic framework where the randomness suggested by quantum mechanics would be resolved. For Einstein, a complete understanding of the universe needed a foundation that eliminated chance and provided a coherent picture of physical laws.