To appreciate the labours of Plotinus, we must, first of all, compare his whole philosophic method with that of his predecessors. Now, Zeller himself has shown quite clearly that in reach of thought, in power of synthesis, in accuracy of reasoning, not one of these can be compared to the founder of Neo-Platonism for a single moment.[507] We may go still further and declare with confidence that no philosopher of equal speculative genius had appeared in Hellas since Chrysippus, or, very possibly, since Aristotle. The only ground for disputing his claims to take rank with the great masters of Hellenic thought seems to be that his system culminates on the objective side in something which lies beyond existence, and on the subjective side in a mystical ecstasy which is the negation of reason. We have shown, however, that if the One is represented as transcending reality, so also is the Idea of Good which corresponds to it in Plato’s scheme; and that the One is reached if not grasped by a process of reasoning which, although unsound, still offers itself as reasoning alone, and moves in complete independence of any revelation or intuition such as those to which the genuine systems of mysticism so freely resort. It cannot be too often repeated that the One in no way conflicts with the world of real existence, but, on the contrary, creates and completes it. Now, within that world, with which alone reason is properly concerned, Plotinus never betrays any want of confidence in its power to discover truth; nor, contrary to what Zeller assumes, does he seem to have been in the least affected by the efforts of the later Sceptics to invalidate its pretensions in this respect.[508] Their criticism was, in fact, chiefly directed against Stoicism, and did not touch the spiritualistic position at all. That there can be no certain knowledge afforded by sensation, or, speaking more generally, by the action of an outward object on an inward subject, Plotinus himself fully admits or rather contends.[509] But while distrusting the ability of external perception, taken alone, to establish the existence of an external object by which it is caused, he expressly claims such a power for reason or understanding.[510] For him, as for Aristotle, and probably for Plato also, the mind is one with its real object; in every act of cognition the idea becomes conscious of itself. We do not say that Scepticism is powerless against such a theory as this, but, in point of fact, it was a theory which the ancient Sceptics had not attacked, and their arguments no more led Plotinus to despair of reason, than the similar arguments of Protagoras and Gorgias had led Plato and Aristotle to despair of it six centuries before. If Sextus and his school contributed anything to the great philosophical revolution of the succeeding age, it was by so weakening the materialistic systems as to render them less capable of opposing the spiritualistic revival when it came.