By NIELS BOHR Institute for Theoretical Physics, University of Copenhagen
The significance of physical science for philosophy does not merely lie in the steady increase of our experience of inanimate matter, but above all in the opportunity of testing the foundation and scope of some of our most elementary concepts. Notwithstanding refinements of terminology due to accumulation of experimental evidence and developments of theoretical conceptions, all account of physical experience is, of course, ultimately based on common language, adapted to orientation in our surroundings and tracing of relationships between cause and effect. Indeed, Galileo's programme to base the description of physical phenomena on measurable quantities has afforded a solid foundation for the ordering of an ever larger field of experience.
In Newtonian mechanics, where the state of a system of material bodies is defined by their instantaneous positions and velocities, it proved possible, by the well-known simple principles, to derive, uniquely from the knowledge of the state of the system at a given time and of the forces acting upon the bodies, the state of the system at any other time. A description of this kind, which evidently represents an ideal form of causal relationships, expressed by the notion of determinism, was found to have still wider scope. Thus, in the account of electromagnetic phenomena, in which we have to consider a propagation of forces with finite velocities, a deterministic description could be upheld by including in the definition of the state not only the positions and velocities of the charged bodies, but also the direction and intensity of the electric and magnetic forces at every point of space at a given time.
A new epoch in physical science was inaugurated by Planck's discovery of the elementary quantum of action, which revealed a feature of wholeness inherent in atomic processes going far beyond the ancient idea of the limited divisibility of matter. Indeed, it became clear that the pictorial description of classical physical theories represents an idealization valid only for phenomena in the analysis of which all actions involved are sufficiently large to permit the neglect of the quantum. While this condition is amply fulfilled in phenomena on the ordinary scale, we meet, in experimental evidence concerning atomic particles, with regularities of a novel type, incompatible with deterministic analysis. These quantal laws are determining for the peculiar____________________