The nuclear magnetic moment of lanthanum is 2.5 nuclear magnetons, consistent with the value of 2.8 nuclear magnetons determined from La III hyperfine structures. This work was conducted under the supervision of Professor G. Breit, and the author thanks him for his guidance. The author also acknowledges a Fellowship from the Royal Society of Canada and the support from the University of Wisconsin and the Department of Physics. The paper discusses the completeness of quantum mechanics in describing physical reality. Einstein, Podolsky, and Rosen argue that a complete theory must have an element corresponding to each element of reality. A physical quantity is real if it can be predicted with certainty without disturbing the system. In quantum mechanics, non-commuting operators mean that knowing one quantity precludes knowing another. Thus, either the wave function description is incomplete or the two quantities cannot have simultaneous reality. The authors argue that if the wave function is complete, then two non-commuting quantities cannot have simultaneous reality. However, they show that if the wave function is complete, then two non-commuting quantities can have simultaneous reality, leading to a contradiction. Therefore, the quantum-mechanical description of physical reality is not complete. The paper concludes that a complete theory must include additional elements beyond the wave function to account for simultaneous reality of non-commuting quantities.The nuclear magnetic moment of lanthanum is 2.5 nuclear magnetons, consistent with the value of 2.8 nuclear magnetons determined from La III hyperfine structures. This work was conducted under the supervision of Professor G. Breit, and the author thanks him for his guidance. The author also acknowledges a Fellowship from the Royal Society of Canada and the support from the University of Wisconsin and the Department of Physics. The paper discusses the completeness of quantum mechanics in describing physical reality. Einstein, Podolsky, and Rosen argue that a complete theory must have an element corresponding to each element of reality. A physical quantity is real if it can be predicted with certainty without disturbing the system. In quantum mechanics, non-commuting operators mean that knowing one quantity precludes knowing another. Thus, either the wave function description is incomplete or the two quantities cannot have simultaneous reality. The authors argue that if the wave function is complete, then two non-commuting quantities cannot have simultaneous reality. However, they show that if the wave function is complete, then two non-commuting quantities can have simultaneous reality, leading to a contradiction. Therefore, the quantum-mechanical description of physical reality is not complete. The paper concludes that a complete theory must include additional elements beyond the wave function to account for simultaneous reality of non-commuting quantities.