Here, as elsewhere, we shall employ the genetic method of investigation. Aristotle’s writings do not, indeed, present that gradual development of ideas which makes the Platonic Dialogues so interesting. Still they exhibit traces of such a development, and the most important among them seems to have been compiled from notes taken by the philosopher before his conclusions were definitely reasoned out, or worked up into a consistent whole. It is this fragmentary collection which, from having been placed by some unknown editor after the _Physics_, has received a name still associated with every kind of speculation that cannot be tested by a direct or indirect appeal to the evidence of external sense. Whether there exist any realities beyond what are revealed to us by this evidence, and what sensible evidence itself may be worth, were problems already actively canvassed in Aristotle’s time. His _Metaphysics_ at once takes us into the thick of the debate. The first question of that age was, What are the causes and principles of things? On one side stood the materialists—the old Ionian physicists and their living representatives. They said that all things came from water or air or fire, or from a mixture of the four elements, or from the interaction of opposites, such as wet and dry, hot and cold. Aristotle, following in the track of his master, Plato, blames them for ignoring the incorporeal substances, by which he does not mean what would now be understood—feelings or states of consciousness, or even the spiritual substratum of consciousness—but rather the general qualities or assemblages of qualities which remain constant amid the fluctuations of sensible phenomena; considered, let us observe, not as subjective thoughts, but as objective realities. Another deficiency in the older physical theories is that they either ignore the efficient cause of motion altogether (like Thales), or assign causes not adequate to the purpose (like Empedocles); or when they hit on the true cause do not make the right use of it (like Anaxagoras). Lastly, they have omitted to study the final cause of a thing—the good for which it exists.