In this paper an outline of a metaphysical conception of modern science is presented in which a fundamental distinction is drawn between scientific principles, laws and theories. On this view, ontologicalprinciples, rather than e.g. empirical data, constitute the core of science. The most fundamental of these principles are three in number, being, more particularly (A) the principle of the uniformity of nature, (B) the principle of the perpetuity of substance, and (C) the principle of causality.