ONE:Descartes had already accomplished a great simplification of the speculative problem by summing up all existence under the two heads of extension and thought. It remained to account for these, and to reduce them to a single idea. As we have seen, they were derived from Greek philosophy, and the bond which was to unite them must be sought for in the same direction. It will be remembered that the systems of Plato and Aristotle were bounded at either extremity by a determinate and by an indeterminate principle. With the one, existence ranged between the Idea of Good at the upper end of the scale and empty space at the lower; with the other, between absolute Thought and First Matter. It was by combining the two definite terms, space and thought, that Descartes had constructed his system; and after subtracting these the two indefinite terms remained. In one respect they were even more opposed to each other than were the terms with which they had been respectively associated. The Idea403 of Good represented unity, identity, and constancy, as against plurality, difference, and change; while Aristotles Matter was, by its very definition, multiform, fluctuating, and indeterminate. Nevertheless, there were equally important analogies traceable between them. No very clear account could be given of either, and both were customarily described by negatives. If Matter fell short of complete existence, the Good transcended all existence. If the one was a universal capacity for assuming Forms, the other was the source whence all Forms proceeded. When the distinctive characteristics of an individual were thought away, the question might well be mooted into which principle it would return. The ambiguous use of the word Power contributed still further to their identification, for it was not less applicable to the receptive than to the productive faculty. Now we have just seen into what importance the idea of Power suddenly sprang at the Renaissance: with Bruno it was the only abiding reality of Nature; with Hobbes it was the only object of human desire.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 so344 weakening the materialistic systems as to render them less capable of opposing the spiritualistic revival when it came.