The Theaetetus discusses more fully the theory of knowledge. It opens with a comparison of the Socratic method to midwifery; it delivers the mind of the thoughts with which it is in travail. The first tentative definition of knowledge is that it is sensation. This is in agreement with the Protagorean doctrine that man is the measure of all things. Yet sensation implies change, whereas we cannot help thinking that objects retain their identity; if knowledge is sensation a pig has as good a claim to be called the measure of all things as a man. Again, Protagoras has no right to teach others if each man's sensations are a law unto him. Nor is the Heracleitean doctrine much better which taught that all things are in a state of flux. If nothing retains the same quality for two consecutive moments it is impossible to have predication, and knowledge must be hopeless. In fact, sensation is not man's function as a reasoning being, but rather comparison. Neither is knowledge true opinion, for this at once demands the demarcation of false opinion or error; the latter is negative, and will be understood only when positive knowledge is determined. Perhaps knowledge is true opinion plus reason; but it is difficult to decide what is gained by adding "with reason", words which may mean either true opinion or knowledge itself, thus involving either tautology or a begging of the question. The dialogue at least has shown what knowledge is not.
Locke, Berkeley and Hume, the eighteenth century sensation philosophers, were similarly refuted by Kant. The mind by its mere ability to compare two things proves that it can have two concepts at least before it at the same time, and can retain them for a longer period than a mere passing sensation implies. Yet the problem of knowledge still remains as difficult as Plato knew it to be.
"Is the Sophist the same as the Statesman and the Philosopher?" Such is the question raised in the Sophist. Six definitions are suggested, all unsatisfactory. The fixed characteristic of the Sophist is his seeming to know everything without doing so; this definition leads straight to the concept of false opinion, a thing whose object both is and is not. "That which is not" provokes an inquiry into what is, Being. Dualism, Monism, Materialism and Idealism are all discussed, the conclusion being that the Sophist is a counterfeit of the Philosopher, a wilful impostor who makes people contradict themselves by quibbling.
The Politicus carries on the discussion. In this dialogue we may see the dying glories of Plato's genius. In his search for the true pastor or king he separates the divine from the human leader; the true king alone has scientific knowledge superior to law and written enactments which men use when they fail to discover the real monarch. This scientific knowledge of fixed and definite principles can come only from Education. A most remarkable myth follows, which is practically the Greek version of the Fall. The state of innocence is described as preceding a decline into barbarism; a restoration can be effected only by a divine interposition and by the growth of a study of art or by the influence of society. The arts themselves are the children of a supernatural revelation.
The Timaeus and the long treatise the Laws criticise the theories of the Republic. The former is full of world-speculations of a most difficult kind, the latter admits the weakness of the Ideal State, making concessions to inevitable human failings.
Though written in an early period, the Apology may form a fitting end to these dialogues. Socrates was condemned on the charge of corrupting the Athenian youth and for impiety. To most Athenians he must have been not only not different from the Sophists he was never weary of exposing, but the greatest Sophist of them all. He was unfortunate in his friends, among whom were Critias the infamous tyrant and Alcibiades who sold the great secret. The older men must have regarded with suspicion his influence over the youth in a city which seemed to be losing all its national virtues; many of them were personally aggrieved by his annoying habit of exposing their ignorance. He was given a chance of escape by acknowledging his fault and consenting to pay a small fine. Instead, he proposed for himself the greatest honour his city could give any of her benefactors, public maintenance in the town-hall.
His defence contains many superb passages and is a masterpiece of gentle irony and subtle exposure of error. Its conclusion is masterly.
"At point of death men often prophesy. My prophecy to you, my
slayers, is that when I am gone you will have to face a far more
serious penalty than mine. You have killed me because you wish
to avoid giving an account of your lives. After me will come more
accusers of you and more severe. You cannot stop criticism except
by reforming yourselves. If death is a sleep, then to me it is
gain; if in the next world a man is delivered from unjust judges
and there meets with true judges, the journey is worth while.
There will be found all the heroes of old, slain by wicked
sentences; them I shall meet and compare my agonies with theirs.
Best of all, I shall be able to carry out my search into true and
false knowledge and shall find out the wise and the unwise. No
evil can happen to a virtuous man in life or in death. If my sons
when they grow up care about riches more than virtue, rebuke them
for thinking they are something when they are naught. My time has
come; we must separate. I go to death, you to life; which of the
two is better only God knows."
Two lessons of supreme importance are to be learned from Plato. In the first place he insists on credentials from the accepted teachers of a nation. On examination most of them, like Gorgias, would be found incapable of defining the subjects for the teaching of which they receive money. The sole hope of a country is Education, for it alone can deliver from ignorance, a slavery worse than death; the uneducated person is the dupe of his own passions or prejudices and is the plaything of the horde of impostors who beg for his vote at elections or stampede him into strikes.
Again, the possibility of knowledge depends upon accurate definition and the scientific comparisons of instances. These involve long and fatiguing thought and very often the reward is scanty enough; no conclusion is possible sometimes except that it is clear what a thing cannot be. The human intelligence has learned a most valuable lesson when it has recognised its own impotence at the outset of an inquiry and its own limitations at the end thereof. Knowledge, Good, Justice, Immortality are conceptions so mighty that our tiny minds have no compasses to set upon them. Better far a distrust in ourselves than the somewhat impudent and undoubtedly insistent claim to certitude advanced by the materialistic apostles of modern non-humanitarianism. When questioned about the ultimates all human knowledge must admit that it hangs upon the slender thread of a theory or postulate. The student of philosophy is more honest than others; he has the candour to confess the assumptions he makes before he tries to think at all.