John Locke

"The end of law is not to abolish or restrain, but to preserve and enlarge freedom. For in all the states of created beings capable of law, where there is no law, there is no freedom."

John Locke (1632 - 1704) was an English moral and political philosopher who advocated empiricism, skepticism, liberalism, and religious toleration.
 
In An Essay on Human Understanding (1689) he argued that all thought originates in the senses (empiricism) - which is to say, through induction on the basis of observation. The objective reality they signify is beyond our ability to know, and what knowledge we do possess is always provisional. Metaphysics forms no part of philosophy. This position put him at odds with the Rationalism of Descartes and Spinoza, who argued that knowledge arises through reason - which is to say, through deduction from axioms. For Descartes and Spinoza, the senses produce illusions, while reason produces certain knowledge. The first method is characteristic of what was then called natural philosophy, and is today called science. The second method is characteristic of mathematics.
 
According to Locke the mind begins as a blank slate. There are no innate ideas or moral principles, nor is there any such thing as human nature. We are what our environment makes us. Free will is an illusion - man is free when he has the power to do what he wants, and under compulsion when he does not. It is thus the man, not the will, which is free or compelled. The mind, being simply the operation of reason on the senses, can only be subject to different sense impressions, or more or less reasonable. For better or worse, man always does what he thinks reasonable.
 
Nevertheless, we are justified in holding people accountable for their actions. Human beings are endowed with reason, and can there for distinguish between actions which lead to pleasure (that are good) and those that lead to pain (that are evil.) Criminality is the result of faulty reason, and the purpose of punishment is its rehabilitation. Accordingly, the severity of punishment correlates with the power of reason - premeditated crimes are worse than impassioned, impassioned than reckless, reckless than unknowing. A person who has no reason - a child, an idiot, or a lunatic, for instance - is not liable to criminal punishment. (The older theory apportioned punishment to the relative social status of the involved parties - a commoner who insulted a lord did so at the peril of their life, but a lord who murdered a commoner merely risked a fine.)
In Two Treatises on Government (1690) Locke rejected

Absolutism and advocated limited government. Man originally lived in a state of nature, he said. But, becoming dissatisfied, they formed a Social Contract to preserve their Natural Rights - life, liberty, and property. Who ever remains within the community past the age of reason (about 14, at the time) implicitly enters into the contract, and is bound by it. Among the most important of these liberties is that of reason, which the state has no right to violate. It can compel action, but not belief - hence religious persecution is a violation of man's Natural Rights.
 
The other party to the contract is the state, which derives it's legitimacy from the protection of the Natural Rights of the community. The state is justified in using its power to preserve the Natural Rights of the people, but if it attacks those rights is at war with the community. The contract, being breached, is dissolved - the people may remove it and create a new government in its place.
John Locke decisively influenced the political and legal theory of the Enlightenment, as well as the philosophy and practice of science, and the development of the skeptical tradition in epistemology. He was widely read by the framers of the American constitution and the leaders of the French Revolution, who hoped to create governments that would serve rather than rule the people.

Stanford Encyclopedia of Philosophy: John Locke
http://plato.stanford.edu/entries/locke/

Part of a series on the Enlightenment (III of XVII)
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