The ethical system of Plato appears to be a strict deduction from his physical. As man in his view is a compound of matter, vivified by a portion of the world-soul, which the divine reason takes and unites with a portion of itself, so his virtues correspond to this threefold composition.(380) For man has an immortal portion in his soul, the reason, the godlike, in him, but the divine reason, in joining a portion of the world-soul with matter, invests it with two mortal parts, one the courageous, or manly, the other, sensuous desire, or the female element, having their seat in the body's activity. To these answer respectively the virtues of prudence, of courage, and of temperance, while justice comes in afterwards as a right ordering of the three, or as prudence applied to practice. The seat of all irregular desires, of all evil, in fact, is to Plato in this union of the soul with matter. As this matter is primordial, evil in its origin does not indeed spring from God, but it is beyond his power: it springs from that state of things which existed before the action of God on chaos:(381) it must stand over against the good: and of necessity encompasses this mortal nature and the place of its habitation: and to man it lies not in the perverted use of free-will, but in his original composition, wherein his body is its seat. But in this triple composition of man Plato does not seem to have clearly apprehended a human personality at all: he has not even attempted to explain(382) in what the unity of the soul consists besides these its three portions, two of which, being tied to the body, drop off at death.
It is in the practice of Plato as a teacher that we can most fitly consider the conception which the Greek philosophers in general had concerning the method of studying and imparting philosophy altogether. It was about the fortieth year of his life, and twelve years after the death of his master Socrates, that Plato, having already travelled widely, settled at Athens.(383) Here he purchased a fixed residence at the Academia, which became from that time a philosophical school for study, conversation, oral lectures, and friendly meetings. Here he drew around him an inner circle of scholars to whom he addressed his unwritten doctrines,(384) especially his doctrine of Ideas, the key to his whole system, according as they were able, after preparation, to receive them: and here besides he gave lectures which might be attended not only by that inner circle of choice disciples but by studious persons in general. This residence of Plato served for three hundred years, from 387 before Christ until the siege of Athens by Sylla in 87, as the centre of Plato's philosophy viewed as a teaching power. Now in this Plato had before him the great example of Pythagoras, in the first age of Greek philosophy. Concerning the doctrines of that philosopher we know little with certainty,(385) but all are agreed as to his manner of teaching them. His attempt was to establish a community which should carry in its bosom, propagate, and perpetuate a doctrine in morals, politics, religion, and philosophy. His whole procedure was by oral teaching, for he left not a word written. It was in fact a religious order of life which he first practised in his own person, and then endeavoured to communicate to others. Into this order trial for everyone preceded reception.(386) His scholars were for a long period required to practise silent obedience and unconditional submission to the authority of the doctrine delivered to them. Severe daily examination was imposed upon all. The publishing of his doctrine, especially his speculation as to the nature of God, was strictly forbidden. The upright life, the learning which then could only be attained by personal inquiry, the persuasiveness of Pythagoras, were together so effective that he succeeded in establishing such a community both in Crotona and in other cities of Southern Italy. It was persecuted and suffered continual disasters, but still this Pythagorean community, bearing on its founder's doctrines and manner of life, existed for several generations after his death, during which many of the most distinguished Greeks belonged to it. Such was the poet Æschylus, whose mind was formed on Pythagorean principles. In Plato's time the Pythagorean Archytas was at the head of the state of Tarentum: and Plato himself was largely imbued with Pythagorean tenets.(387)