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.These three principles set basic constraints (...) on the methodology of both empirical and theoretical science. The uniformity principle is central to the empirical aspect of science, suggesting a methodology consisting in the attempt to discover empiricallaws, while the causality principle is central to the theoretical aspect of science, suggesting the postulation of scientifictheories capable of indicating the causal basis of the laws. And the perpetuity principle functions so as to form a bridge between the theories and the laws. (shrink)