John Locke
The English physician and philosopher who argued that government exists only by consent and may be overthrown when it fails, supplying the vocabulary of every liberal revolution since.
John Locke trained as a physician, spent much of his adult life as an adviser and occasional conspirator in the household of a leading Whig politician, and fled to the Netherlands in 1683 when his patron's faction was implicated in a plot to kill the king. He published his major political work anonymously in 1689, after the Glorious Revolution had made its argument safe to make.
That timing matters. Locke is often read as a serene philosopher establishing timeless principles. He was a partisan in a specific and dangerous fight about whether an English king could rule without Parliament, and his ideas were weapons before they were doctrine.
The argument
The Two Treatises of Government dismantles the theory of divine-right monarchy and then builds a replacement. The replacement runs roughly like this.
People exist, prior to any government, in a state of nature: not Hobbes's war of all against all, but a condition of rough equality governed by reason, in which everyone holds natural rights to life, liberty, and property. Property, for Locke, arises from labor. What you have mixed your work with becomes yours before any state exists to say so, which means property rights precede government rather than being created by it.
The state of nature is workable but insecure, because there is no neutral judge. So people consent to establish a government whose entire purpose is to protect those pre-existing rights. It follows that government authority is limited to that purpose, that it is held in trust, and, decisively, that a ruler who attacks the rights he was appointed to defend has broken the trust and may be removed. Locke calls this the appeal to heaven, which in practice means rebellion.
He also argued, in the Letter Concerning Toleration, that the state has no business coercing religious belief, since sincere faith cannot be produced by force. In the Essay Concerning Human Understanding he argued that the mind begins without innate ideas and is furnished entirely by experience, which had a political payoff: if minds are made rather than born, then hierarchy is not natural and education matters enormously.
The influence and the debt
Read the American Declaration of Independence and you are reading Locke, sometimes almost verbatim: rights that are unalienable, governments deriving their just powers from the consent of the governed, and the right of the people to alter or abolish a government that becomes destructive of those ends. Jefferson changed property to the pursuit of happiness, which is the one interesting edit.
His fingerprints are equally on the French Declaration of the Rights of Man, on the Enlightenment generally, and on the assumption, so widespread now that it is invisible, that a government must justify itself to those it governs rather than the other way around.
The contradiction
Locke's biography contains something his admirers have to face. He held shares in the Royal African Company, which trafficked enslaved Africans, and he helped draft the Fundamental Constitutions of Carolina, which granted colonists absolute power over their slaves. His theory of property, in which unimproved land may be claimed by whoever cultivates it, was also directly useful for dispossessing indigenous Americans.
The usual defenses, that he opposed slavery in the abstract, or that his justifications were narrow, do not really dissolve the problem. The more honest reading is that the founding text of universal rights was written by a man who understood the universe of rights-holders more narrowly than his own words implied, and that the subsequent history of liberalism is largely the story of other people forcing the words to mean what they say.
The echo
That is Locke's real legacy and it is a strange one. He supplied a set of premises so powerful that they kept escaping the limits their author placed on them. Abolitionists, suffragists, anticolonial movements, and civil rights campaigners all found that the fastest route to their goal was to take the founding language literally and ask why it did not apply to them. An argument that outruns its author is the most useful kind there is.