The Protagorean doctrine--Man is the measure of all things--is simply the presentation in complete view of a common fact--uncovering an aspect of it which the received phraseology hides. Truth and Falsehood have reference to some believing subject--and the words have no meaning except in that relation. Protagoras brings to view this subjective side of the same complex fact, of which Truth and Falsehood denote the objective side. He refuses to admit the object absolute--the pretended _thing in itself_--Truth without a believer. His doctrine maintains the indefeasible and necessary involution of the percipient mind in every perception--of the concipient mind in every conception--of the cognizant mind in every cognition. Farther, Protagoras acknowledges many distinct believing or knowing Subjects: and affirms that every object known must be relative to (or in his language, _measured by_) the knowing Subject: that every _cognitum_ must have its _cognoscens_, and every _cognoscibile_ its _cognitionis capax_: that the words have no meaning unless this be supposed: that these two names designate two opposite poles or aspects of the indivisible fact of cognition--actual or potential--not two factors, which are in themselves separate or separable, and which come together to make a compound product. A man cannot in any case get clear of or discard his own mind as a Subject. Self is necessarily omnipresent; concerned in every moment of consciousness, and equally concerned in all, though more distinctly attended to in some than in others.[18] The Subject, self, or Ego, is that which all our moments of consciousness have in common and alike: Object is that in which they do or may differ--although some object or other there always must be. The position laid down by Descartes--_Cogito, ergo sum_--might have been stated with equal truth--_Cogito, ergo est (cogitatum aliquid)_: _sum cogitans--est cogitatum_--are two opposite aspects of the same indivisible mental fact--_cogitatio_. In some cases, doubtless, the objective aspect may absorb our attention, eclipsing the subjective: in other cases, the subjective attracts exclusive notice: but in all cases and in every act of consciousness, both are involved as co-existent and correlative. That alone exists, to every man, which stands, or is believed by him to be capable of standing, in some mode of his consciousness as an Object correlative with himself as a Subject. If he believes in its existence, his own believing mind is part and parcel of such fact of belief, not less than the object believed in: if he disbelieves it, his own disbelieving mind is the like. Consciousness in all varieties has for its two poles Subject and Object: there cannot be one of these poles without the opposite pole--north without south--any more than there can be concave without convex (to use a comparison familiar with Aristotle), or front without back: which are not two things originally different and coming into conjunction, but two