Plato studied the philosophy of Heraclitus with special diligence; we find much of it quoted in his works, and he got his earlier philosophic education most indubitably from this source, so that Heraclitus may be called Plato’s teacher. Hippocrates, likewise, is a philosopher of Heraclitus’ school. What is preserved to us of Heraclitus’ philosophy at first seems very contradictory, but we find the Notion making its appearance, and a man of profound reflection revealed. Zeno began to abrogate the opposed predicates, and he shows the opposition in movement, an assertion of limitation and an abrogation of the same; Zeno expressed the infinite, but on its negative side only, in reference to its contradiction as being the untrue. In Heraclitus we see the perfection of knowledge so far as it has gone, a perfecting of the Idea into a totality, which is the beginning of Philosophy, since it expresses the essence of the Idea, the Notion of the infinite, the potentially and actively existent, as that which it is, i.e. as the unity of opposites. From Heraclitus dates the ever-remaining Idea which is the same in all philosophers to the present day, as it was the Idea of Plato and of Aristotle. 1. _The Logical Principle._ Concerning the universal principle, this bold mind, Aristotle tells us (Metaph. IV. 3 and 7), first uttered the great saying: “Being and non-being are the same; everything is and yet is not.” The truth only is as the unity of distinct opposites and, indeed, of the pure opposition of being and non-being; but with the Eleatics we have the abstract understanding that Being is alone the truth. We say, in place of using the expression of Heraclitus, that the Absolute is the unity of being and non-being. When we understand that proposition as that “Being is and yet is not,” this does not seem to make much sense, but only to imply complete negation and want of thought. But we have another sentence that gives the meaning of the principle better. For Heraclitus says: “Everything is in a state of flux; nothing subsists nor does it ever remain the same.” And Plato further says of Heraclitus: “He compares things to the current of a river: no one can go twice into the same stream,”[61] for it flows on and other water is disturbed. Aristotle tells us (Met. IV. 5) that his successors even said “it could not once be entered,” for it changed directly; what is, is not again. Aristotle (De Cœlo, III. 1) goes on to say that Heraclitus declares that “there is only one that remains, and from out of this all else is formed; all except this one is not enduring (_παγίως_).”