To the Eleatic system thus originating with Xenophanes is to be attributed the dialectic phase henceforward so prominently exhibited by Greek philosophy. It abandoned, for the most part, the pursuits which had occupied the Ionians--the investigation of visible nature, the phenomena of material things, and the laws presiding over them; conceiving such to be merely deceptive, and attaching itself to what seemed to be the only true knowledge--an investigation of Being and of God. By the Eleats, since all change appeared to be an impossibility, the phenomena of succession presented by the world were regarded as a pure illusion, and they asserted that Time, and Motion, and Space are phantasms of the imagination, or vain deceptions of the senses. They therefore separated reason from opinion, attributing to the former conceptions of absolute truth, and to the latter imperfections arising from the fictions of sense. It was on this principle that Parmenides divided his work on "Nature" into two books, the first on Reason, the second on Opinion. Starting from the nature of Being, the uncreated and unchangeable, he denied altogether the idea of succession in time, and also the relations of space, and pronounced change and motion, of whatever kind they may be, mere illusions of opinion. His pantheism appears in the declaration that the All is thought and intelligence; and this, indeed, constitutes the essential feature of his doctrine, for, by thus placing thought and being in parallelism with each other, and interconnecting them by the conception that it is for the sake of being that thought exists, he showed that they must necessarily be conceived of as one.