The chief object of Plato in tracing the origin of society is to show the point at which regular government superseded the patriarchical authority, and the separate customs of different families were systematized by legislators, and took the form of laws consented to by them all. According to Plato, the only sound principle on which any government could be based was a mixture or balance of power. The balance of power saved Sparta, when the two other Heraclid states fell into disorder. Here is probably the first trace of a political idea, which has exercised a vast influence both in ancient and modern times. And yet we might fairly ask, a little parodying the language of Plato--O legislator, is unanimity only 'the struggle for existence'; or is the balance of powers in a state better than the harmony of them? In the fourth book we approach the realities of politics, and Plato begins to ascend to the height of his great argument. The reign of Cronos has passed away, and various forms of government have succeeded, which are all based on self-interest and self-preservation. Right and wrong, instead of being measured by the will of God, are created by the law of the state. The strongest assertions are made of the purely spiritual nature of religion--'Without holiness no man is accepted of God'; and of the duty of filial obedience,--'Honour thy parents.' The legislator must teach these precepts as well as command them. He is to be the educator as well as the lawgiver of future ages, and his laws are themselves to form a part of the education of the state. Unlike the poet, he must be definite and rational; he cannot be allowed to say one thing at one time, and another thing at another--he must know what he is about. And yet legislation has a poetical or rhetorical element, and must find words which will wing their way to the hearts of men. Laws must be promulgated before they are put in execution, and mankind must be reasoned with before they are punished. The legislator, when he promulgates a particular law, will courteously entreat those who are willing to hear his voice. Upon the rebellious only does the heavy blow descend. A sermon and a law in one, blending the secular punishment with the religious sanction, appeared to Plato a new idea which might have a great result in reforming the world. The experiment had never been tried of reasoning with mankind; the laws of others had never had any preambles, and Plato seems to have great pleasure in contemplating his discovery.