Plato in denying the Protagorean formula, constitutes himself the measure for all. Counter proposition to the formula _ib._ Import of the Protagorean formula is best seen when we state explicitly the counter-proposition 150 Unpopularity of the Protagorean formula--Most believers insist upon making themselves a measure for others, as well as for themselves. Appeal to Abstractions 150 Aristotle failed in his attempts to refute the Protagorean formula--Every reader of Aristotle will claim the right of examining for himself Aristotle's canons of truth 152 Plato's examination of the other doctrine--That knowledge is Sensible Perception. He adverts to sensible facts which are different with different Percipients 153 Such is not the case with all the facts of sense. The conditions of unanimity are best found among select facts of sense--weighing, measuring, &c. 154 Arguments of Sokrates in examining this question. Divergence between one man and another arises, not merely from different sensual impressibility, but from mental and associative difference 155 Argument--That sensible Perception does not include memory--Probability that those who held the doctrine meant to include memory 157 Argument from the analogy of seeing and not seeing at the same time _ib._ Sokrates maintains that we do not see _with_ our eyes, but that the mind sees _through_ the eyes: that the mind often conceives and judges by itself without the aid of any bodily organ 159 Indication of several judgments which the mind makes by itself--It perceives Existence, Difference, &c. 160 Sokrates maintains that knowledge is to be found, not in the Sensible Perceptions themselves, but in the comparisons add computations of the mind respecting them 161 Examination of this view--Distinction from the views of modern philosophers 162 Different views given by Plato in other dialogues 163 Plato's discussion of this question here exhibits a remarkable advance in analytical psychology. The mind rises from Sensation, first to Opinion, then to Cognition 164 Plato did not recognise Verification from experience, or from facts of sense, as either necessary or possible 168 Second definition given by Theætêtus--That Cognition consists in right or true opinion _ib._ Objection by Sokrates--This definition assumes that there are false opinions. But how can false opinions be possible? How can we conceive Non-Ens: or confound together two distinct realities? _ib._ Waxen memorial tablet in the mind, on which past impressions are engraved. False opinion consists in wrongly identifying present sensations with past impressions 169 Sokrates refutes this assumption. Dilemma. Either false opinion is impossible, or else a man may know what he does not know 170 He draws distinction between possessing knowledge, and having it actually in hand. Simile of the pigeon-cage with caught pigeons turned into it and flying about _ib._