This translation was originally published in Proceedings of the American Philosophical Society, 124, 323-38. [And then appeared as Section I.11 of Part I of Quantum Theory and Measurement (J.A. Wheeler and W.H. Zurek, eds., Princeton university Press, New Jersey 1983).] Contents Introductory Note 1. The Physics of Models 2. Statistics of Model Variables in Quantum Mechanics 3. Examples of Probabil