2. WHAT IS SCIENCE? (1.) _As opposed to sensation and the sensuous representation._—The Theatætus is devoted to the discussion of this question in opposition to the Protagorean sensualism. That all knowledge consists in perception, and that the two are one and the same thing, was the Protagorean proposition. From this it followed, as Protagoras himself had inferred, that things are, as they appear to me, that the perception or sensation is infallible. But since perception and sensation are infinitely diversified with different individuals, and even greatly vary in the same individual, it follows farther, that there are no objective determinations and predicates, that we can never affirm what a thing is in itself, that all conceptions, great, small, light, heavy, to increase, to diminish, &c., have only a relative significance, and consequently, also, the conceptions of species, as combinations of the changeful many, are wholly wanting in constancy and stability. In opposition to this Protagorean thesis, Plato urges the following objections and contradictions. _First._ The Protagorean doctrine leads to the most startling consequences. If being and appearance, knowledge and perception are one and the same thing, then is the irrational brute, which is capable of perception, as fully entitled to be called the measure of all things, as man, and if the representation is infallible, as the expression of my subjective character at a given time, then need there be no more instruction, no more scientific conclusion, no more strife, and no more refutation. _Second._ The Protagorean doctrine is a logical contradiction; for according to it Protagoras must yield the question to every one who disputes with him, since, as he himself affirms, no one is incorrect, but every one judges only according to truth; the pretended truth of Protagoras is therefore true for no man, not even for himself. _Third._ Protagoras destroys the knowledge of future events. That which I may regard as profitable may not therefore certainly prove itself as such in the result. To determine that which is really profitable implies a calculation of the future, but since the ability of men to form such a calculation is very diverse, it follows from this that not man as such, but only the wise man can be the measure of things. _Fourth._ The theory of Protagoras destroys perception. Perception, according to him, rests upon a distinction of the perceived object and the perceiving subject, and is the common product of the two. But in his view the objects are in such an uninterrupted flow, that they can neither become fixed in seeing nor in hearing. This condition of constant change renders all knowledge from sense, and hence (the identity of the two being assumed), all knowledge impossible. _Fifth._ Protagoras overlooks the apriori element in knowledge. It is seen in an analysis of the sense-perception itself, that all knowledge cannot be