Summary: THE different branches of knowledge are often classified according to their subject matter. To each is assigned a definite region of experience or a definite group of ideas; the conclusions of those who study these with special application are supposed to be embodied in a series of propositions which convey all that is known on the subject to any one whose interest may be aroused in it. Distinctions drawn on this basis between the various experimental sciences, or between science and mathematics or history, are accepted without question. The physicist gives his material to the chemist to be analysed, asks the mathematician to solve his equations, and inquires of the historian whether his result was known to the ancients; he recognises that these matters lie within their province, not his, and yet he has perfect confidence in their answers. The exact boundaries may sometimes be difficult to draw, and where they occur, disputes may arise between those who approach them from opposite sides; but there are large areas within which all are content to accept the jurisdiction of the recognised occupants.