Berkeley's Direct References to Plato's Theaetetus

These are the explicit, direct references to Plato's Theaetetus that are found in his Siris. They make an interesting selection. Besides the Theaetetus, Berkeley explicitly refers to Timaeus, Phaedrus, Epinomis, Republic, the Platonic Epistles, Phaedo, and Alcibiades Major.

253. We know a thing when we can understand it: and we understand it, when we can interpret or tell what it signifies. Strictly the sense knows nothing. We perceive indeed sounds by hearing, and characters by sight: but we are not therefore said to understand them. After the same manner, the phaenomena of nature are alike visible to all: but all have not alike learned the connexion of natural things, or understand what they signify, or know how to vaticinate by them. There is no question, saith Socrates in Theaeteto, concerning that which is agreeable to each person; but concerning what will in time to come be agreeable, of which all men are not equally judges, He who foreknoweth what will be in every kind, is the wisest. According to Socrates, you and the cook may judge of a dish on the table equally well; but while the dish is making, the cook can better foretell what will ensue from this or that manner of composing it. Nor is this manner of reasoning confined only to morals or politics; but extends also natural science.

304. There is according to Plato properly no knowledge, but only opinion concerning things sensible and perishable, not because they are naturally abstruse and involved in darkness, but because their nature and existence is uncertain, ever fleeting and changing; or rather, because they do not in strict truth exist at all, being always generating or in fieri, that is, in a perpetual flux, without any thing stable or permanent in them to constitute an object of real science. The Pythagoreans and Platonics distinguish between το γιγνομενον and το ον, that which is ever generated and that which exists. Sensible things and corporeal forms are perpetually producing and perishing, appearing and disappearing, never resting in one state, but always in motion and change; and therefore in effect, not one being but a succession of beings: while το ον is understood to be somewhat of an abstract or spiritual nature, and the proper object of intellectual knowledge. Therefore as there can be no knowledge of things flowing and instable, the opinion of Protagoras and Theaetetus, that sense was science, is absurd. And indeed nothing is more evident than that the apparent sizes and shapes, for instance, of things are in a constant flux, ever differing as they are view'd at different distances, or with glasses more or less accurate. As for those absolute magnitudes and figures, which certain Cartesians and other moderns suppose to be in things, that must seem a vain supposition, to whoever considers, it is supported by no argument of reason, and no experiment of sense.

305. As understanding perceiveth not, that is, doth not hear or see or feel, so sense knoweth not: And although the mind may use both sense and phancy, as means whereby to arrive at knowledge yet sense or soul, so far forth as sensitive, knoweth nothing. For, as it is rightly observed in the Theaetetus of Plato, science consists not in the passive perceptions, but in the reasoning upon them, τω περι εκεινων συλλογισμω.

311. As to an absolute actual existence of sensible or corporeal things, it doth not seem to have been admitted either by Plato or Aristotle. In the Theaetetus we are told, that if anyone saith a thing is or is made, he must withal say, for what, or in respect of what, it is or is made; for that any thing should exist in itself or absolutely, is absurd. Agreeably to which doctrine it is also farther affirmed by Plato, that it is impossible a thing should be sweet, and sweet to no body....

316. And as the Platonic philosophy supposed intellectual notions to be originally inexistent or innate in the soul, so likewise it supposed sensible qualities to exist (though not originally) in the soul, and there only. Socrates saith to Theaetetus, You must not think the white colour that you see is in any thing without your eyes, or in your eyes, or in any place at all....

348. Socrates, in the Theætetus of Plato, speaketh of two parties of philosophers, the ρεοντες and οι του ολου στασιωται, the flowing philosophers who held all things to be in a perpetual flux, always generating and never existing; and those others who maintained the universe to be fixed and immoveable. The difference seems to have been this, that Heraclitus, Protagoras, Empedocles, and in general those of the former sect, considered things sensible and natural; whereas Parmenides and his party considered το παν, not as the sensible but as the intelligible world, abstracted from all sensible things.

367. As for the perfect intuition of divine things, that he supposeth to be the lot of pure souls, beholding by a pure light, initiated, happy, free and unstained from those bodies, wherein we are now imprisoned like oysters. But in this mortal state, we must be satisfy'd to make the best of those glympses within our reach. It is Plato's remark in his Theaetetus, that while we sit still we are never the wiser, but going into the river and moving up and down, is the way to discover its depths and shallows. If we exercise and bestir ourselves, we may even here discover something.