Theodorus, a famous geometrician of Cyrene and a follower of Protagoras, is represented to have met Socrates at Athens, and to have been asked by him whether among his pupils there were any who promised to become eminent. Theodorus particularizes one above all the rest, who, while he is speaking, is seen approaching. His name is Theætetus. Socrates, having heard him so highly spoken of by Theodorus, at once opens upon the subject which he wishes to discuss, and asks What science is. Theætetus, in answer, enumerates several particular sciences, but is soon led to understand that the question is not, how many sciences there are, but what science itself is; and by an instance in point shews that he does so. Still he doubts his own ability to answer the question proposed, but is at length induced to make the attempt by Socrates pleasantly describing himself as inheriting his own mother’s skill in midwifery, by which he is able to bring to the birth and deliver the mental conceptions of those whose souls are pregnant with ideas[134]. ----- Footnote 134: § 1-22. ----- Theætetus, then, first of all says that science is nothing else than perception. This, Socrates observes, is the opinion of Protagoras, differently expressed; for he said, that man is the measure of all things, in other words that all things are such as they appear to each person. In order to examine the truth of this doctrine Socrates begins by stating it more fully. Protagoras asserts that nothing exists of itself, nor can any thing be designated by any quality, for what we call great will, in reference to something else, be also small, and what we call heavy, light, and so on, so that nothing ever exists but is always becoming. Consequently all things spring from motion, and the relation that they bear to each other. Thus, with respect to colour, it does not actually exist, it is neither in the object seen nor in the eye itself, but results from the application of the eye to the object, and so is the intermediate production of both. Again if you compare six with four they appear to be half as many again, but if with twelve, only the half, whence it appears that the same number is at one time great, at another small, which would not be the case if numbers had a fixed and determined magnitude. The principle then on which all things depend is this, That the universe is nothing but motion, of which there are two species, the one active, the other passive, by the union of which that which is perceivable and perception itself consist. Thus when the eye and a corresponding object, meeting together, produce whiteness and its connate perception, the eye sees, and becomes not vision, but a seeing eye, and the object itself becomes not whiteness but white: so that nothing is essentially one, but is always being produced by something else, and therefore the word “being” must be entirely done away with. But here it may be objected that the perceptions produced in persons who