The invariance of the laws of nature was one of the most popular themes of nineteenth century philosophy. For it was not till last century that general acceptance was accorded to the doctrine of the "Uniformity of Law," adumbrated in ancient times by Epicurus and Lucretius. It is now a cardinal axiom of science that the same cause in the same conditions is always followed by the same effect. There exists in nature no indeterminate element; all things are governed by fixed laws, and the discovery of these laws is the main business of science. It is necessary to guard against reading into this statement an erroneous idea of the content of a "law of nature." Such a law is of course not an enactment of any sort; and it is not even to be thought of as an actual explanation of the how and why of the phenomena with which it has to do. It really is nothing but an expression of our belief in the pronouncement of the preceding paragraph, that like conditions do produce like results. It is a prediction based on past experience, and is of value merely in that past experience leads us to credit its accuracy. The composite essay beginning on page 19 discusses this question of the reality of natural laws, and should be consulted in connection with the present contribution.--Editor. This great philosophic principle was derived of course from the study of natural science; i.e., from observations and experiments conducted upon the earth. Their comprehensiveness is therefore limited by the fact that the observer is always in a state of rest, or nearly so, as compared with the earth. All observers upon the earth are moving through space at the same velocity; and it was possible to argue that the uniformity of law might only hold good, when experiments were conducted at this velocity. An observer moving at very different velocity might discover that the laws of nature under these new conditions were somewhat different.