We may, it seems to me, say that of all kinds and varieties of knowledge that only is entitled to the name “science” which can be described as Knowledge of Causes, or Knowledge of the Order of Nature. It is this knowledge to which the great founder of European science—Aristotle the Greek—pointed as _true_ knowledge: τότε ἐπιστάμεθα ὅταν τὴν αἰτίαν εἴδωμεν. Science is that knowledge which enables us to demonstrate so far as our limited faculties permit, that the appearances which we recognise in the world around us are dependent in definite ways on certain properties of matter: science is that knowledge which enables, or tends to enable us, to assign its true place in the series of events constituting the universe, to any and every thing which we can perceive. The method by which scientific knowledge is gained—knowledge of the causes of nature—is precisely the same as that by which knowledge of causes in every-day life is gained. Something—an appearance—has to be accounted for: the question in both cases is “Through what cause, in relation to what antecedent is this appearance brought about?” In scientific inquiry, as in every-day life, a hypothesis—a provisional answer or guess—is the reply; and the truth of that guess or hypothesis is then _tested_. This testing is an essential part of the process. “If my guess be true, then so-and-so as to which I can decide by inspection or experiment, must be true also,” is the form which the argument takes, and the inquiry is thus brought to a point where _observation_ can decide the truth of the hypothesis or first guess. In every-day life we have often to be content without fully testing the truth of our guesses, and hurry into action based on such unverified suppositions. Science, on the other hand, can always wait, and demands again and again the testing and verification of guesses before they are admitted as established truths fit to be used in the testing of new guesses and the building up of scientific doctrine.