We have now to consider what were the speculative motives that led Aristotle to overlook these contradictions, and to find rest in a theory even less satisfactory than the earlier systems which he is always attacking with relentless animosity. The first motive, we believe, was the train of reasoning, already laid before the reader, by which universal essences, the objects of knowledge, gradually came to be identified with particular objects, the sole existing realities. For the arguments against such an identification, as put forward by our philosopher himself, still remained unanswered. The individuals comprising a species were still too transient for certainty and too numerous for comprehension. But when for the antithesis between Form and Matter was substituted the antithesis between Actuality and Possibility, two modes of evasion presented themselves. The first was to distinguish between actual knowledge and potential knowledge. The former corresponded to existing particulars, the latter to general ideas.[253] This, however, besides breaking up the unity of knowledge, was inconsistent with the whole tenor of Aristotle’s previous teaching. What can be more actual than demonstration, and how can there be any demonstration of transient particulars? The other mode of reconciliation was perhaps suggested by the need of an external cause to raise Possibility into Actuality. Such a cause might be conceived with all the advantages and without the drawbacks of a Platonic Idea. It would be at once the moving agent and the model of perfection; it could reconcile the general and the particular by the simple fact of being eternal in time, comprehensive in space, and unique in kind. Aristotle found such a cause, or rather a whole series of such causes, in the celestial spheres. In his system, these bear just the same relation to terrestrial phenomena that Plato’s Ideas bear to the world of sense. They are, in fact, the Ideas made sensible and superficial, placed alongside of, instead of beneath or behind, the transient particulars which they irradiate and sustain.