The Enlightenment was a period of nearly unbounded optimism and faith in the human race's ability to solve its own problems, including restructuring government and society along more reasonable lines. There were two main factors leading into this search for a rational approach to creating a better society. First of all, Deism, with its idea of a God detached from our affairs, gave us the ability and responsibility to solve our own problems. Second, this was a period of rapid social and economic changes, especially in England with its booming colonial empire and economy. London's population jumped from c.700,000 in 1715 to 2.7 million by 1815. Such rapid growth led to squalid living conditions, alcoholism (gin consumption increasing by a factor of 10 times), drug abuse, and crime. While Deism may have given us the power and responsibility to reform society, these conditions provided an urgent need for such reforms. The result was a flurry of new ideas in political science, economics, psychology, and social reform.
Enlightenment ideas on politics were rooted in John Locke's Two Treatises on Government (1694). Locke's basic idea was that government, rather than being at the whim of an absolute monarch with no checks on his power, existed merely as a trust to carry out the will of the people and protect their "lives, liberty, and property." If it failed in its duties or acted arbitrarily, the subjects had the right to form a new government, by revolution if necessary.
Locke's ideas largely summarized the achievements of the English Revolution of the 1600's. They had a tremendous impact on political thinkers in France chafing under the corrupt reigns of Louis XV and Louis XVI. Three of these men, Montesquieu, Voltaire, and Rousseau would profoundly influence French political thought and provide the theoretical justification for the French Revolution.
Montesquieu, sometimes seen as the father of political science, looked at various types of government and analyzed what made them work in his book, The Spirit of the Laws. Among the ideas he supposedly derived from England was the separation of powers in government, a vital part of our own constitution.
Voltaire, who first made his name by championing the cause of a Jew wrongly accused and executed for a crime, was probably the most famous of the Enlightenment philosophers. Voltaire wrote on a wide range of topics, but should be remembered here for advocating more civil and political liberties, at least for educated people who can understand the implications of their actions. Voltaire was less clear on what rights the illiterate masses should have.
Finally, there was Rousseau who said that people could only legitimately follow laws they themselves have made. Otherwise, they were the victims of someone else's tyranny. Therefore the ideal state is a small-scale democracy in which everyone participates. Together, the ideas of Locke, Montesquieu, Voltaire, and Rousseau provided the basic ideas we have today on personal rights and liberties and how a government can best be structured to guarantee those rights and liberties.
In economics, the most important figure was Adam Smith, whose The Wealth of Nations pushed for a wholly new attitude toward economics. Smith saw people as selfish and willing to work much harder and produce much more if they had the incentive to do so. He saw the mercantilism of the 1600's and 1700's, where the state tried to import gold and silver while exporting its goods, as stifling to an economy. Therefore, doing away with mercantilist monopolies and restrictions would provide more incentive to produce. There was no need to regulate the market since people's greed and the law of supply and demand would make the market self-regulating. Smith's free market policy, known as laissez faire ("hands off") was widely adopted in the 1800's as Britain, Europe, and the United States rapidly industrialized. It is still a vital part of our economic thinking today.
In psychology, there was Helvetius, who claimed our minds and personalities are blank slates at birth and that we are the products of our environment and the sum total of our past experiences. Combining Helvetius' "blank slate" theory with the prevailing optimism of the age was Jeremy Bentham. He felt we could teach people to act in rational ways by providing an ideal environment where they can learn the right sorts of behavior. Bentham's movement, Utilitarianism, became quite popular and pushed for a wide range of social reforms in such areas as prisons, law codes, and public health.