All is fair in love and war" and, I might add, in theoretical physics. As in those activities painful adjustments are often necessary. Until a few years ago physics has been so definitely tied to Newtonian mechanics that, for many persons, progress in physics has come to mean understanding of phenomena within that frame. So much has been accomplished within that frame that it is naturally tempting to believe that mathematical physics should be thus restricted and that a phenomenon is not "explained" unless it is fully described in terms of classical mechanics.
Recent trends have emphasized the movement away from this restricted point of view by development of a non-Newton-Jan scheme of quantum dynamics within the field of dynamics itself. To be sure it had previously been recognized that classical dynamics alone will not give us everything. I refer especially to the inclusion of statistical ideas to explain the second law of thermodynamics, and to the difficulties of * Four addresses delivered at a symposium held Thursday afternoon, December 9, I937.
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