The chapter discusses the completeness of the quantum-mechanical description of physical reality. It begins with an analysis of the nuclear magnetic moment of lanthanum, which is in agreement with previous findings. The main focus, however, is on the work by Einstein, Podolsky, and Rosen, which questions whether the wave function in quantum mechanics provides a complete description of reality. The authors propose a criterion for reality: a physical quantity is real if it can be predicted with certainty without disturbing the system. They argue that if this criterion holds, then either the wave function is incomplete or two non-commuting physical quantities cannot have simultaneous reality. They demonstrate that if the wave function is assumed to be complete, it leads to a contradiction when considering the interaction between two systems. Specifically, they show that measuring one quantity can leave the other system in different states, which contradicts the idea that the wave function should provide a complete description of the system's state. The authors conclude that the quantum-mechanical description of physical reality given by wave functions is not complete. They acknowledge that their criterion of reality might not be strictly restrictive enough, but they believe that a more complete theory is possible.The chapter discusses the completeness of the quantum-mechanical description of physical reality. It begins with an analysis of the nuclear magnetic moment of lanthanum, which is in agreement with previous findings. The main focus, however, is on the work by Einstein, Podolsky, and Rosen, which questions whether the wave function in quantum mechanics provides a complete description of reality. The authors propose a criterion for reality: a physical quantity is real if it can be predicted with certainty without disturbing the system. They argue that if this criterion holds, then either the wave function is incomplete or two non-commuting physical quantities cannot have simultaneous reality. They demonstrate that if the wave function is assumed to be complete, it leads to a contradiction when considering the interaction between two systems. Specifically, they show that measuring one quantity can leave the other system in different states, which contradicts the idea that the wave function should provide a complete description of the system's state. The authors conclude that the quantum-mechanical description of physical reality given by wave functions is not complete. They acknowledge that their criterion of reality might not be strictly restrictive enough, but they believe that a more complete theory is possible.