The record of men and of movements, History teaches us the growth and development of ideas. Our civilization is the final expression of the two great master-thoughts of the race. Seeking an explanation of the pressing phenomena of life, man has peopled the world with spiritual beings to whom he has assigned benign or malign influences, to be invoked or propitiated. To the great ‘uncharted region’ (Gilbert Murray) with its mysteries, his religions offer a guide; and through ‘a belief in spiritual beings’ (Tylor’s definition of religion) he has built an altar of righteousness in his heart. The birth of the other dominant idea, long delayed, is comparatively recent. ‘The discovery of things as they really are’ (Plato) by a study of nature was the great gift of the Greeks. Knowledge, _scientia_, knowledge of things we see, patiently acquired by searching out the secrets of nature, is the basis of our material civilization. The true and lawful goal of the sciences, seen dimly and so expressed by Bacon, is the acquisition of new powers by new discoveries--that goal has been reached. Niagara has been harnessed, and man’s dominion has extended from earth and sea to the air. The progress of physics and of chemistry has revolutionized man’s ways and works, while the new biology has changed his mental outlook. The greater part of this progress has taken place within the memory of those living, and the mass of scientific work has accumulated at such a rate that specialism has become inevitable. While this has the obvious advantage resulting from a division of labour, there is the penalty of a narrowed horizon, and groups of men work side by side whose language is unintelligible to each other. Here is where the historian comes in, with two definite objects, teaching the method by which the knowledge has been gained, the evolution of the subject, and correlating the innumerable subdivisions in a philosophy at once, in Plato’s words, a science in itself as well as of other sciences. For example, the student of physics may know Crookes’s tubes and their relation to Röntgen, but he cannot have a true conception of the atomic theory without a knowledge of Democritus; and the exponent of Madame Curie and of Sir J. J. Thomson will find his happiest illustrations from the writings of Lucretius. It is unfortunate that the progress of science makes useless the very works that made progress possible; and the student is too apt to think that because useless now they have never been of value.