two characteristics. First, in so far as the qualitative variety and the complexity of concrete things are considered, we find several distinct sciences each with its own special concepts and methods, though all are more or less inductive and experimental. But all qualities and complexities whatever that natural objects present, and all the changes that they undergo, appear to involve quantitative constants and configurations admitting of more or less precise determination and measurement. As soon indeed as the movements of sensible bodies were found to admit of exact description by the science of mechanics the hypothesis at once presented itself that, as Newton expressed it, “the other phenomena of nature might be deduced from mechanical principles.” And, as we all know, the hypothesis has been amply justified, though not indeed absolutely verified in every detail; mechanical explanation has therefore long been accepted as the ne plus ultra of what a scientific explanation can be. So much is this the case indeed that even the intractable problem of life is still generally regarded as only an outstanding difficulty and not as a veritable exception to the universality of mechanical laws.
We come now to the second characteristic. For long this mechanical theory was held to furnish us with the knowledge of the empirical reality which our sensible experience was supposed only obscurely to symbolise: it bore, in fact, the name of Natural Philosophy. But as its purely formal character became more apparent, and mathematical equations enabled it to dispense with the real categories of substance and cause, physicists themselves were the first to perceive
