These same Idealist philosophers also maintained--That Forms, or the intelligible world, were eternally the same and unchangeable. Plato here affirms that this ideas or opinion is not true: he contends that the intelligible world includes both change and unchangeableness, motion and rest, difference and sameness, life, mind, intelligence, &c. He argues that the intelligible world, whether assumed as consisting of one Form or of many Forms, could not be regarded either as wholly changeable or wholly unchangeable: it must comprise both constituents alike. If all were changeable, or if all were unchangeable, there could be no Object of knowledge; and, by consequence, no knowledge.[108] But the fact that there _is_ knowledge (cognition, conception), is the fundamental fact from which we must reason; and any conclusion which contradicts this must be untrue. Therefore the intelligible world is not all homogeneous, but contains different and even opposite Forms--change and unchangeableness--motion and rest--different and same.[109] [Footnote 108: Plato, Sophist. p. 249 B. [Greek: xumbai/nei d' ou)=n a)kinê/tôn te o)/ntôn nou=n mêdeni\ peri\ mêdeno\s ei)=nai mêdamou=.]] [Footnote 109: Plato, Sophist. p. 249 C.] [Side-note: Plato's reasoning against the Materialists.] Let us now look at Plato's argument, and his definition of existence, as they bear upon the doctrine of the opposing Materialist philosophers, whom he states to have held that bodies alone existed, and that the Incorporeal did not exist:--in other words that all real existence was concrete and particular: that the abstract (universals, forms, attributes) had no real existence, certainly no separate existence. As I before remarked, it is not quite clear what or how much these philosophers denied. But as far as we can gather from Plato's language, what they denied was, the existence of attributes _apart from_ a substance. They did not deny the existence of just and wise men, but the existence of justice and wisdom, apart from men real or supposable. [Side-note: Difference between Concrete and Abstract, not then made conspicuous. Large meaning here given by Plato to Ens--comprehending not only objects of Perception, but objects of Conception besides.]