The whole procedure of Protagoras conveys the impression that his inversion was of a purely dialectical nature. For the style and manner in which he formulates his new point of view leaves to humanity for all its mental life nothing but mere opinion. His dictum as to man being the measure of all things takes no account of a natural order of things. To this perhaps may be attributed the fact that his philosophy, however arresting it may have been in his own day and time, set forth personally by this gifted mind, has yet proved itself to be but little permanent. After the see-saw between idealism and materialism had proceeded for some two thousand years more, the new inversion of the higher order set in with a mighty whirlwind, the most powerful, the most systematically-delivered attack upon the notion of _substans_ that Western philosophy had ever experienced--the philosophy of Hume. Hume’s philosophy, briefly stated, consists in the investigation of what exhibits itself to sense-perception considered as based on a possible content of _substans_--in unravelling it to the last thread and pointing out to his contemporaries with irrefutable clearness and acuteness, “See there, you people! a constant in itself is nowhere to be found!” Hume is frequently alluded to as a sceptic. I consider, on the contrary, that his philosophy is the purest criticism precisely where in philosophy criticism may be practised at all--namely, upon the concept of _substans_, whether in material or immaterial form. Every criticism of _substans_ culminates naturally in criticism of the notion of an _I_. For Hume, the _I_, the self, became a bundle, a collection of separate mental representations “that follow one another with inconceivable rapidity and are in a state of perpetual flow, continual motion.” But a criticism of the notion of _substans_ is incomplete without a criticism of the concept of cause; for the intuition that all that exists must have an adequate cause is likewise a necessity of thought. Now, where there is a constant in itself, a _substans_ in things, causality is an actual _following after one another_ of cause and effect, this “constant in itself” being also “cause in itself” of that which happens, the latter therefore, as “effect in itself,” representing a simple following upon that cause in itself, in such sort that between the two there exists a necessary--I might almost say--a rigid dependence; whereupon the question, “How is a relation between the two possible?” becomes a problem that defies solution. Hence it follows that one is bound to hold the problem of causality as a correlate of _substans_. If the latter falls, the former falls along with it.