After thus criticising the formula — Homo Mensura — Plato proceeds to canvass the other doctrine, which he ascribes to Protagoras along with others, and which he puts into the mouth of Theætêtus — “That knowledge is sensible perception”. He connects that doctrine with the above-mentioned formula, by illustrations which exhibit great divergence between one percipient Subject and another. He gives us, as examples of sensible perception, the case of the wind, cold to one man, not cold to another: that of the wine, sweet to a man in health, bitter if he be sickly.[81] Perhaps Protagoras may have dwelt upon cases like these, as best calculated to illustrate the relativity of all affirmations: for though the judgments are in reality both equally relative, whether two judges pronounce alike, or whether they pronounce differently, under the same conditions — yet where they judge differently, each stands forth in his own individuality, and the relativity of the judgment is less likely to be disputed.
[81] Plato, Theætêt. pp. 152 A, 159 C.
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.
But though some facts of sense are thus equivocal, generating dissension rather than unanimity among different individuals — such is by no means true of the facts of sense taken generally.[82] On the contrary, it is only these facts — the world of reality, experience, and particulars — which afford a groundwork and assurance of unanimity in human belief, under all varieties of teaching or locality. Counting, measuring, weighing, are facts of sense simple and fundamental, and comparisons of those facts: capable of being so exhibited that no two persons shall either see them differently or mistrust them. Of two persons exposed to the same wind, one may feel cold, and the other not: but both of them will see the barometer or thermometer alike.[83] Πάντα μέτρῳ καὶ ἀριθμῷ καὶ σταθμῷ — would be the perfection of science, if it could be obtained. Plato himself recognises, in more than one place, the irresistible efficacy of weight and measure in producing unanimity; and in forestalling those disputes which are sure to arise where weight and measure cannot be applied.[84] It is therefore among select facts of sense, carefully observed and properly compared, that the groundwork of unanimity is to be sought, so far as any rational and universal groundwork for it is attainable. In other words, it is here that we must seek for the basis of knowledge or cognition.
[82] Aristotle (Metaphysic. Γ. p. 1010, a. 25 seq.) in arguing against Herakleitus and his followers, who dwelt upon τὰ αἰσθητὰ as ever fluctuating and undefinable, urges against them that this is not true of all αἰσθητά, but only of those in the sublunary region of the Kosmos. But this region is (he says) only an imperceptibly small part of the entire Kosmos; the objects in the vast superlunary or celestial region of the Kosmos were far more numerous, and were also eternal and unchangeable, in constant and uniform circular rotation. Accordingly, if you predicate one or other about αἰσθητὰ generally, you ought to predicate constancy and unchangeability, not flux and variation, since the former predicates are true of much the larger proportion of αἰσθητά. See the Scholia on the above passage of Aristotle’s Metaphysica, and also upon Book A. 991, a. 9.
[83] Mr. Campbell, in his Preface to the Theætêtus (p. lxxxiii.), while comparing the points in the dialogue with modern metaphysical views, observes. “Modern Experimental Science is equally distrustful of individual impressions of sense, but has found means of measuring the motions by which they are caused, through the effect of the same motions upon other things besides our senses. When the same wind is blowing one of us feels warm and another cold (Theætêt. p. 152), but the mercury of the thermometer tells the same tale to all. And though the individual consciousness remains the sole judge of the exact impression momentarily received by each person, yet we are certain that the sensation of heat and cold, like the expansion and contraction of the mercury, is in every case dependent on a universal law.”
It might seem from Mr. Campbell’s language (I do not imagine that he means it so) as if Modern Experimental Science had arrived at something more trustworthy than “individual impressions of sense”. But the expansion or contraction of the mercury are just as much facts of sense as the feeling of heat or cold; only they are facts of sense determinate and uniform to all, whereas the feeling of heat or cold is indeterminate and liable to differ with different persons. The certainty about “universal law governing the sensations of heat and cold,” was not at all felt in the days of Plato.
[84] Thus in the Philêbus (pp. 55-56) Plato declares that numbering, measuring, and weighing, are the characteristic marks of all the various processes which deserve the name of Arts; and that among the different Arts those of the carpenter, builder, &c., are superior to those of the physician, pilot, husbandman, military commander, musical composer, &c., because the two first-named employ more measurement and a greater number of measuring instruments, the rule, line, plummet, compass, &c.
“When we talk about iron or silver” (says Sokrates in the Platonic Phædrus, p. 263 A-B) “we are all of one mind, but when we talk about the Just and the Good we are all at variance with each other, and each man is at variance with himself”. Compare an analogous passage, Alkibiad. I. p. 109.
Here Plato himself recognises the verifications of sense as the main guarantee for accuracy: and the compared facts of sense, when select and simplified, as ensuring the nearest approach to unanimity among believers.