[Letter to Editors]

Abstract

SIR JAMES JEANS proposes the finiteness of the velocity of light as a test case. I answer: Certainly this is a priori knowledge, but of a rather trivial kind. We know a priori that the velocity of light is not infinite, just as we know a priori that the velocity of light is not blue or hexagonal or totalitarian; it is not the sort of thing to which these, terms could apply. The alternatives “exceedingly large” and “actually infinite” concern only the abstract quantities which are the theme of pure mathematics; this is equally true of the alternatives “exceedingly small” and “actually zero”. No such alternatives exist for physical quantities defined in terms of observation. When an observer sets out to determine the velocity of light, an infinite result is not among the possible alternatives; and if he announces that he has found the velocity to be, not merely exceedingly large, but actually infinite, we know a priori that the announcement is untrue.