Physical science begins from the bottom and builds upwards. It analyzes phenomena into their elements, and thereupon seeks to combine these elements into structures that shall correspond to experience. In this business it never comes to a finish. Its analysis of the elements is provisional. Every element is hypothetical. Indeed it is plain in the nature of the case that no element can be ultimate. An element is a unit, and every empirical unit necessarily conceals in its bosom a plexus of which it is the unification. The very idea of unit requires for its complement a manifold of some kind. In hypothetical units, or ideal constructs that have for their purpose to lead to the discovery and arrangement of real phenomena, science abounds. Atoms, electrons, energy conceived as a substance by Ostwald, Spencer’s physiological units, are examples. The results achieved by science are never more than approximations in the sense that the units, the bricks with which the house is built, are liable to be rejected, and the constructions achieved are subject to revision. The point however which I wish to emphasize is that the scientist is satisfied of the truth, the reality of its partial results. Newton, for instance, in formulating the law of gravitation has, so to speak, marked off a strip of reality. The ground covered cannot be lost; when some natural law is enunciated, the proper conditions for its discovery and verification having been observed, a sure footing in reality has been gained, science standing to this extent on _terra firma_, though beyond the domain within which the law applies the phenomena may be heaving and billowing like the sea. Now the question I am intent upon is, Why is it possible for science to be content with partial acquisition? Why does it profess to know positively a part without knowing the whole? And why can ethics not take a step without an ideal of the whole?