Formalized during a burst of intellectual activity during the 1920s, quantum theory had immediate success explaining experimental results in atomic and nuclear physics. However, physicists were aware that much work remained to clarify the conceptual foundations of the theory. In Lecture 3, Professor Bethe recalls work on the interpretation of the wave function, the Heisenberg Uncertainty Principle, and the Pauli Exclusion Principle. He shows how quantum theory forced discussion of issues such as determinism, physical observables, and action-at-a-distance.