ONE:XI.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 Aristotle��s 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.