The article by Prof. N. Bohr discusses the principles underlying the description of atomic phenomena in quantum theory, emphasizing the quantum postulate and its implications for causality. The quantum postulate attributes to atomic processes an essential discontinuity or individuality, symbolized by Planck's constant, which challenges classical physical ideas. This postulate implies a renunciation of causal space-time coordination, as any observation of atomic phenomena involves interaction with the observing agency, making an independent reality in the ordinary physical sense impossible. Bohr explores the complementary nature of space-time description and causality, noting that the definition of a physical system's state often conflicts with the possibility of observation. He discusses the limitations on the accuracy of measurements of position and momentum due to the uncertainty principle, and how these limitations affect the description of atomic phenomena. The article also delves into the correspondence principle, which links quantum theory to classical mechanics, and the development of matrix theory and wave mechanics. Matrix theory, introduced by Heisenberg, uses symbols to represent individual processes, while wave mechanics, developed by Schrödinger, provides a wave-based approach to atomic structure. Both methods have contributed to the quantitative interpretation of experimental results and the understanding of atomic phenomena. Finally, Bohr addresses the reality of stationary states, emphasizing that the concept of a stationary state involves a complete renunciation of time description, leading to an unambiguous definition of the energy of the atom. This concept is crucial for the conservation of energy and the stability of atomic states, despite the apparent paradoxes it raises in collision and radiation reactions.The article by Prof. N. Bohr discusses the principles underlying the description of atomic phenomena in quantum theory, emphasizing the quantum postulate and its implications for causality. The quantum postulate attributes to atomic processes an essential discontinuity or individuality, symbolized by Planck's constant, which challenges classical physical ideas. This postulate implies a renunciation of causal space-time coordination, as any observation of atomic phenomena involves interaction with the observing agency, making an independent reality in the ordinary physical sense impossible. Bohr explores the complementary nature of space-time description and causality, noting that the definition of a physical system's state often conflicts with the possibility of observation. He discusses the limitations on the accuracy of measurements of position and momentum due to the uncertainty principle, and how these limitations affect the description of atomic phenomena. The article also delves into the correspondence principle, which links quantum theory to classical mechanics, and the development of matrix theory and wave mechanics. Matrix theory, introduced by Heisenberg, uses symbols to represent individual processes, while wave mechanics, developed by Schrödinger, provides a wave-based approach to atomic structure. Both methods have contributed to the quantitative interpretation of experimental results and the understanding of atomic phenomena. Finally, Bohr addresses the reality of stationary states, emphasizing that the concept of a stationary state involves a complete renunciation of time description, leading to an unambiguous definition of the energy of the atom. This concept is crucial for the conservation of energy and the stability of atomic states, despite the apparent paradoxes it raises in collision and radiation reactions.