The. reference to Heraclitus' 'unity which agrees with itself by being at variance' is to his doctrine that the universe and things in it are maintained in existence by the simultaneous operation of contrary tensions. His favourite illustration of this truth is a bow, in which contrary tensions are exerted at the same time by the bow and its string. Eryximachus' criticism completely misrepresents the whole theory of Heraclitus, who constantly asserted not only that opposites are necessarily co-existent and involve one another, but even that they are one and the same. Since Plato shows elsewhere that he was perfectly well aware of Heraclitus' meaning, his object here must be to satirize Eryximachus by making him guilty of a gross misinterpretation.