John Locke (1632–1704) strengthened the social contract theory, elaborating the idea that sovereignty reverts to the people whenever the king becomes a tyrant. He held that the natural state of individuals is a condition of perfect freedom to order their actions, not asking leave of any man.[XI-6] This state of liberty is not a state of license to individuals to destroy themselves or their neighbors.[XI-7] The state of liberty has the law of nature to govern it. Since all are equal, no one ought to harm another in his liberty or possessions. Locke affirmed that men are in a state of nature until by their own consent they join in a political society.[XI-8] In order to meet their needs effectively, they join in societies. One of these important needs is the preservation of property. Locke defended private property on the ground that it is a normal expression of individuality and necessary to individuality. Right and wrong are not determined by the ruler or the state; they existed before society developed. Here the Puritanism of Locke enters. He stressed moral values. He made the natural rights of individuals supreme; individuals may even overturn the government and still keep within their rights. Locke’s justification of revolution is his most startling doctrine. Imagine the heart-throb of the common people who heard Locke’s contention that the end of government is the good of mankind, that people should not submit to tyranny, that whoever uses his force without right and law puts himself in a state of war with those against whom he uses it, and that in such a state the people have a right to resist and defend themselves.[XI-9] Further, the people have a right to act as the supreme social force and to put legislation into new forms and into the hands of new executives. By these bold declarations Locke created a new public opinion, and aroused new moral power in the minds and hearts of the common people. By the middle of the eighteenth century the concept of individual freedom became crystallized in the doctrines of the natural rights of the individual, the contractual societary relationships between independent individuals, and the _laissez faire_ principle in governmental science. The physiocrats, who took up the ideas of natural liberty and economic freedom, exercised a tremendous influence in France during the three decades following 1750. Their leaders were Quesnay, de Gournay, Condorcet, and Turgot. They believed that there was a natural law ruling human lives, just as there is a natural law ruling the physical world. They chafed under social restraints. Under the natural law, every individual has natural rights, chief of which is the right to the free exercise of all his faculties so long as he does not infringe on the similar right of other individuals. Unlike John Locke and other English thinkers who accepted the idea of individual liberty, the physiocrats argued that this natural liberty could not be abridged by a social contract.