The most startling of these sayings is that which affirms that good and evil are the same (fr. 57). This does not mean in the least, however, that good is evil or that evil is good, but simply that they are the two inseparable halves of one and the same thing. A thing can become good only in so far as it is already evil, and evil only in so far as it is already good, and everything depends on the contrast. The illustration given in fr. 58 shows this clearly. Torture, one would say, was an evil, and yet it is made a good by the presence of another evil, namely, disease; as is shown by the fact that surgeons expect a fee for inflicting it upon their patients. Justice, on the other hand, which is a good, would be altogether unknown were it not for the existence of injustice, which is an evil (fr. 60). And that is why it is not good for men to get everything they wish (fr. 104). Just as the cessation of strife in the world would mean its destruction, so the disappearance of hunger, disease, and weariness would mean the disappearance of satisfaction, health, and rest. This leads to a theory of relativity which prepares the way for the doctrine of Protagoras, that “Man is the measure of all things.”[418] Sea-water is good for fish and bad for men (fr. 52), and so with many other things. At the same time, Herakleitos is not a believer in absolute relativity. The process of the world is not merely a circle, but an “upward and downward path.” At the upper end, where the two paths meet, we have the pure fire, in which, as there is no separation, there is no relativity. We are told expressly that, while to man some things are evil and some things are good, all things are good to God (fr. 61). Now by God there is no doubt that Herakleitos meant Fire. He also calls it the “one wise,” and perhaps said that it “knows all things.” There can hardly be any question that what he meant to say was that in it the opposition and relativity which are universal in the world disappear. It is doubtless to this that frs. 96, 97, and 98 refer. Footnote 418: Plato’s exposition of the relativity of knowledge in the _Theaetetus_ (152 d sqq.) can hardly go back to Herakleitos himself, but is meant to show how Herakleiteanism might naturally give rise to such a doctrine. If the soul is a stream and things are a stream, then of course knowledge is relative. Very possibly the later Herakleiteans had worked out the theory in this direction, but in the days of Herakleitos himself the problem of knowledge had not yet arisen. [Sidenote: The Wise.]