Recent years have shown a revival of interest in the conceptual problem raised by the incorporation of quantum mechanics into physics. The problem is an old one, but the use of both waves and particles as visualizations of electrons has presented it in a new and acute form. Several observations based on the methods of quantum mechanics can be made. (1) The time development of an isolated system is completely determinate, as in classical mechanics. (2) A precisely defined quantum mechanical state is an eigenstate of some operator described in classical terms. The production of such a state requires a suitable large scale apparatus. (3) If the state is not precisely defined as an eigenstate, quantum statistical mechanics must be used. (4) The idea of measurement is of doubtful applicability in the analysis of idealized experiments, since the process of measurement destroys the identity of the object (an electron) whose properties are measured. Quantum mechanics may be viewed strictly as a method of describing, in classical terms, the behavior of large scale apparatus, although the use of visualizations with a properly conditioned intuition is probably essential to progress in the analysis of new phenomena.