The most noticeable points in the 9th Book of the Republic are:—(1) the account of pleasure; (2) the number of the interval which divides the king from the tyrant; (3) the pattern which is in heaven.
1. Plato’s account of pleasure is remarkable for moderation, and in this respect contrasts with the later Platonists and the views which are attributed to them by Aristotle. He is not, like the Cynics, opposed to all pleasure, but rather desires that the several parts of the soul shall have their natural satisfaction; he even agrees with the Epicureans in describing pleasure as something more than the absence of pain. This is proved by the circumstance that there are pleasures which have no antecedent pains (as he also remarks in the “Philebus”), such as the pleasures of smell, and also the pleasures of hope and anticipation. In the previous book (558, 559) he had made the distinction between necessary and unnecessary pleasure, which is repeated by Aristotle, and he now observes that there are a further class of “wild beast” pleasures, corresponding to Aristotle’s θηριότης. He dwells upon the relative and unreal character of sensual pleasures and the illusion which arises out of the contrast of pleasure and pain, pointing out the superiority of the pleasures of reason, which are at rest, over the fleeting pleasures of sense and emotion. The preeminence of royal pleasure is shown by the fact that reason is able to form a judgment of the lower pleasures, while the two lower parts of the soul are incapable of judging the pleasures of reason. Thus, in his treatment of pleasure, as in many other subjects, the philosophy of Plato is “sawn up into quantities” by Aristotle; the analysis which was originally made by him became in the next generation the foundation of further technical distinctions. Both in Plato and Aristotle we note the illusion under which the ancients fell of regarding the transience of pleasure as a proof of its unreality, and of confounding the permanence of the intellectual pleasures with the unchangeableness of the knowledge from which they are derived. Neither do we like to admit that the pleasures of knowledge, though more elevating, are not more lasting than other pleasures, and are almost equally dependent on the accidents of our bodily state (compare Introduction to “Philebus”).
2. The number of the interval which separates the king from the tyrant, and royal from tyrannical pleasures, is 729, the cube of 9. Which Plato characteristically designates as a number concerned with human life, because nearly equivalent to the number of days and nights in the year. He is desirous of proclaiming that the interval between them is immeasurable, and invents a formula to give expression to his idea. Those who spoke of justice as a cube, of virtue as an art of measuring (“Protagoras” 357 A), saw no inappropriateness in conceiving the soul under the figure of a line, or the pleasure of the tyrant as separated from the pleasure of the king by the numerical interval of 729. And in modern times we sometimes use metaphorically what Plato employed as a philosophical formula. “It is not easy to estimate the loss of the tyrant, except perhaps in this way,” says Plato. So we might say, that although the life of a good man is not to be compared to that of a bad man, yet you may measure the difference between them by valuing one minute of the one at an hour of the other (“One day in thy courts is better than a thousand”), or you might say that “there is an infinite difference.” But this is not so much as saying, in homely phrase, “They are a thousand miles asunder.” And accordingly Plato finds the natural vehicle of his thoughts in a progression of numbers; this arithmetical formula he draws out with the utmost seriousness, and both here and in the number of generation seems to find an additional proof of the truth of his speculation in forming the number into a geometrical figure; just as persons in our own day are apt to fancy that a statement is verified when it has been only thrown into an abstract form. In speaking of the number 729 as proper to human life, he probably intended to intimate that one year of the tyrannical = 12 hours of the royal life.
The simple observation that the comparison of two similar solids is effected by the comparison of the cubes of their sides, is the mathematical groundwork of this fanciful expression. There is some difficulty in explaining the steps by which the number 729 is obtained; the oligarch is removed in the third degree from the royal and aristocratical, and the tyrant in the third degree from the oligarchical; but we have to arrange the terms as the sides of a square and to count the oligarch twice over, thus reckoning them not as = 5 but as = 9. The square of 9 is passed lightly over as only a step towards the cube.
3. Towards the close of the Republic, Plato seems to be more and more convinced of the ideal character of his own speculations. At the
