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 aligns 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 argue that a complete theory must have an element corresponding to each element of reality, meaning that if a physical quantity can be predicted with certainty without disturbing the system, it should have a corresponding element in the theory. They propose a criterion for reality: if a physical quantity can be predicted with certainty without disturbing the system, then it has a corresponding element of reality. To illustrate this, they consider a particle with a single degree of freedom and its wave function. If the wave function is an eigenfunction of the momentum operator, the momentum has a definite value. However, if the wave function is an eigenfunction of the coordinate operator, the coordinate does not have a definite value and can only be determined through measurement, which alters the system's state. The authors then introduce the concept of non-commuting operators, where the knowledge of one operator precludes the knowledge of another. They show that if the wave function is assumed to be complete, it leads to a contradiction. Specifically, they demonstrate that if two systems interact and then separate, measuring one system can leave the other in different states, each corresponding to a different wave function. This contradicts the idea that the wave function provides a complete description of reality. In conclusion, the authors argue that the quantum-mechanical description of physical reality given by wave functions is not complete. They leave open the question of whether a complete theory exists but believe it 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 aligns 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 argue that a complete theory must have an element corresponding to each element of reality, meaning that if a physical quantity can be predicted with certainty without disturbing the system, it should have a corresponding element in the theory. They propose a criterion for reality: if a physical quantity can be predicted with certainty without disturbing the system, then it has a corresponding element of reality. To illustrate this, they consider a particle with a single degree of freedom and its wave function. If the wave function is an eigenfunction of the momentum operator, the momentum has a definite value. However, if the wave function is an eigenfunction of the coordinate operator, the coordinate does not have a definite value and can only be determined through measurement, which alters the system's state. The authors then introduce the concept of non-commuting operators, where the knowledge of one operator precludes the knowledge of another. They show that if the wave function is assumed to be complete, it leads to a contradiction. Specifically, they demonstrate that if two systems interact and then separate, measuring one system can leave the other in different states, each corresponding to a different wave function. This contradicts the idea that the wave function provides a complete description of reality. In conclusion, the authors argue that the quantum-mechanical description of physical reality given by wave functions is not complete. They leave open the question of whether a complete theory exists but believe it is possible.