Great as was the progress made by Greek philosophy in the fourth century before Christ, quite as great were the difficulties with which it had perpetually to contend; quite as difficult the problems on the solution of which it had to labour. Aristotle had already pointed out the weak points in the system of Plato, which rendered it impossible for him to accept that system as satisfactory. From the platform of later knowledge still further objections might be raised to it. Even in Aristotle’s own system inconsistencies on some of the most important points might be found, concealed under a certain indefiniteness of expression, but fatal if once brought to light to the soundness of the whole. For with all his ingenuity, Aristotle never succeeded in harmoniously blending all the elements out of which his system was composed. Thus the divergencies of his immediate followers from the original Aristotelian teaching may be accounted for. Nor were these defects of a kind that could be easily disposed of. The deeper the enquiry is carried, the clearer it becomes that they were defects embedded in the foundations of the systems both of Plato and Aristotle, and underlying the whole previous range of philosophic thought. Omitting details and minor points, they may all be ultimately referred to two: either to an imperfect knowledge and experience of the world, or to the overhaste of idealistic philosophy to draw conclusions. To the former defect may be attributed the mistakes in natural science into which Plato and Aristotle fell, and the limited character of their view of history; to the latter, the Platonic theory of ideas with all that it involves—the antithesis of ideas and appearances, of reason and the senses, of knowledge and ignorance, of the present world and the world to come—and likewise the corresponding points in the system of Aristotle; such, for instance (to name some of the principal ones only), as the relation of the particular and the general, of form and matter, of God and the world, of the theory of final causes and natural explanations, of the rational and the irrational parts of the soul, of speculative theory and practice.