INDETERMINISM IN QUANTUM PHYSICS AND IN CLASSICAL PHYSICS: PART II*

BEFORE entering into a more detailed discussion, I shall attempt, in this introductory section, to state my main points in outline. Quantum physics is now generally admitted to be indeterministic in the sense that it implies the impossibility of predicting certain kinds of physical events, however complete our initial information may be concerning the physical system in question ; given sufficiently precise initial information we may, however, predict the probability of these events, i.e. the frequency of their occurrence under sufficiently similar conditions. Classical physics, on the other hand, is usually taken to be deterministic in the sense that it implies the predictability, with any desired degree of precision, of every single physical event, on the basis of sufficiently precise initial information. In the present paper I propose to show that the opposition indicated here is misleading even although the prima Jade deterministic character of classical physics must be admitted. In spite of important differences, the situation in classical physics shows greater similarities to that in quantum physics than is usually believed. My thesis is that most systems of physics, including classical physics and quantum physics, are indeterministic in perhaps an even more fundamental sense than the one usually ascribed to the indeterminism of quantum physics (in so far as the unpredictability of the events which we shall consider is not mitigated by the predictability of their frequencies). The impossibility, implied by quantum physics, of predicting events of a certain kind is an impossibility of a peculiar character. If we assert of an observable event that it is unpredictable we do not mean, of course, that it is logically or physically impossible for anybody to give a correct description of the event in question before it has occurred; for it is clearly not impossible that somebody may hit upon such a description accidentally. What is asserted is that certain rational 1 Expanded version of a paper read before the Philosophy of Science Group of the British Society for the History of Science, at their first Ordinary Meeting on November 15th, 1948.