86. In the First Part of his poem, we find Parmenides chiefly interested to prove that _it is_; but it is not quite obvious at first sight what it is precisely that _is_. He says simply, _What is, is_. To us this does not seem very clear, and that for two reasons. In the first place, we should never think of doubting it, and we cannot, therefore, understand why it should be asserted with such iteration and vigour. In the second place, we are accustomed to all sorts of distinctions between different kinds and degrees of reality, and we do not see which of these is meant. Such distinctions, however, were quite unknown in those days. “That which is,” with Parmenides, is primarily what, in popular language, we call matter or body; only it is not matter as distinguished from anything else. It is certainly regarded as spatially extended; for it is quite seriously spoken of as a sphere (fr. 8, 40). Moreover, Aristotle tells us that Parmenides believed in none but a sensible reality, which does not necessarily mean with him a reality that is actually perceived by the senses, but includes any which might be so perceived if the senses were more perfect than they are.[449] Parmenides does not say a word about “Being” anywhere.[450] The assertion that _it is_ amounts just to this, that the universe is a _plenum_; and that there is no such thing as empty space, either inside or outside the world. From this it follows that there can be no such thing as motion. Instead of endowing the One with an impulse to change, as Herakleitos had done, and thus making it capable of explaining the world, Parmenides dismissed change as an illusion. He showed once for all that if you take the One seriously you are bound to deny everything else. All previous solutions of the question, therefore, had missed the point. Anaximenes, who thought to save the unity of the primary substance by his theory of rarefaction and condensation, did not observe that, by assuming there was less of what is in one place than another, he virtually affirmed the existence of what is not (fr. 8, 42). The Pythagorean explanation implied that empty space or air existed outside the world, and that it entered into it to separate the units (§ 53). It, too, assumes the existence of what is not. Nor is the theory of Herakleitos any more satisfactory; for it is based upon the contradiction that fire both is and is not (fr. 6). Footnote 449: