It is always difficult, in treating of any branch of knowledge, to put before the beginner a correct preliminary notion of the nature and scope of the study to which he is to be introduced, but the difficulty is exceptionally great in the case of the body of investigations traditionally known as Metaphysics. The questions which the science seeks to answer are, indeed, in principle of the simplest and most familiar kind, but it is their very simplicity and familiarity which constitute the chief difficulty of the subject. We are naturally slow to admit that there is anything we do not understand in terms and ideas which we are constantly using, not only in the special sciences, but in our non-systematised everyday thought and language about the course of the world. Hence, when the metaphysician begins to ask troublesome questions about the meaning and validity of these common and familiar notions, ordinary practical men, and even intelligent students of the special sciences, are apt to complain that he is wasting his time by raising idle and uncalled-for difficulties about the self-evident. Consequently the writer on Metaphysics is almost inevitably compelled to begin by rebutting the natural and current prejudice which regards his science as non-existent and its problems as illusory. The full vindication of metaphysical inquiry from this charge of futility can only be furnished by such a systematic examination of the actual problems of the study as will be attempted, in outline, in the succeeding chapters of this work. All that can be done in an Introduction is to present such a general description of the kind of questions to be subsequently discussed, and their relation to the more special problems of the various sciences, as may incline the reader to give an impartial hearing to what is to follow.