quality:corruption, structural:reformatting
John Locke and Thomas Hobbes

Enlightenment philosophers are given credit for shaping liberal ideas. These ideas were first drawn together and systematized as a distinct ideology by the English philosopher John Locke, generally regarded as the father of modern liberalism. Thomas Hobbes attempted to determine the purpose and the justification of governing authority in post-civil war England. Employing the idea of a state of nature — a hypothetical war-like scenario prior to the state — he constructed the idea of a social contract that individuals enter into to guarantee their security and, in so doing, form the State, concluding that only an absolute sovereign would be fully able to sustain such security. Hobbes had developed the concept of the social contract, according to which individuals in the anarchic and brutal state of nature came together and voluntarily ceded some of their rights to an established state authority, which would create laws to regulate social interactions to mitigate or mediate conflicts and enforce justice. Whereas Hobbes advocated a strong monarchical commonwealth (the Leviathan), Locke developed the then-radical notion that government acquires consent from the governed, which has to be constantly present for the government to remain legitimate. While adopting Hobbes's idea of a state of nature and social contract, Locke nevertheless argued that when the monarch becomes a tyrant, it violates the social contract, which protects life, liberty and property as a natural right. He concluded that the people have a right to overthrow a tyrant. By placing the security of life, liberty and property as the supreme value of law and authority, Locke formulated the basis of liberalism based on social contract theory. To these early enlightenment thinkers, securing the essential amenities of life— liberty and private property —required forming a "sovereign" authority with universal jurisdiction.

His influential Two Treatises (1690), the foundational text of liberal ideology, outlined his major ideas. Once humans moved out of their natural state and formed societies, Locke argued, "that which begins and actually constitutes any political society is nothing but the consent of any number of freemen capable of a majority to unite and incorporate into such a society. And this is that, and that only, which did or could give beginning to any lawful government in the world". The stringent insistence that lawful government did not have a supernatural basis was a sharp break with the dominant theories of governance, which advocated the divine right of kings and echoed the earlier thought of Aristotle. Dr John Zvesper described this new thinking: "In the liberal understanding, there are no citizens within the regime who can claim to rule by natural or supernatural right, without the consent of the governed".

Locke had other intellectual opponents besides Hobbes. In the First Treatise, Locke aimed his arguments first and foremost at one of the doyens of 17th-century English conservative philosophy: Robert Filmer. Filmer's Patriarcha (1680) argued for the divine right of kings by appealing to biblical teaching, claiming that the authority granted to Adam by God gave successors of Adam in the male line of descent a right of dominion over all other humans and creatures in the world. However, Locke disagreed so thoroughly and obsessively with Filmer that the First Treatise is almost a sentence-by-sentence refutation of Patriarcha. Reinforcing his respect for consensus, Locke argued that "conjugal society is made up by a voluntary compact between men and women". Locke maintained that the grant of dominion in Genesis was not to men over women, as Filmer believed, but to humans over animals. Locke was not a feminist by modern standards, but the first major liberal thinker in history accomplished an equally major task on the road to making the world more pluralistic: integrating women into social theory.

Locke also originated the concept of the separation of church and state. Based on the social contract principle, Locke argued that the government lacked authority in the realm of individual conscience, as this was something rational people could not cede to the government for it or others to control. For Locke, this created a natural right to the liberty of conscience, which he argued must remain protected from any government authority. In his Letters Concerning Toleration, he also formulated a general defence for religious toleration. Three arguments are central:

Locke was also influenced by the liberal ideas of Presbyterian politician and poet John Milton, who was a staunch advocate of freedom in all its forms. Milton argued for disestablishment as the only effective way of achieving broad toleration. Rather than force a man's conscience, the government should recognise the persuasive force of the gospel. As assistant to Oliver Cromwell, Milton also drafted a constitution of the independents (Agreement of the People; 1647) that strongly stressed the equality of all humans as a consequence of democratic tendencies. In his Areopagitica, Milton provided one of the first arguments for the importance of freedom of speech—"the liberty to know, to utter, and to argue freely according to conscience, above all liberties". His central argument was that the individual could use reason to distinguish right from wrong. To exercise this right, everyone must have unlimited access to the ideas of his fellow men in " a free and open encounter ", which will allow good arguments to prevail.

In a natural state of affairs, liberals argued, humans were driven by the instincts of survival and self-preservation, and the only way to escape from such a dangerous existence was to form a common and supreme power capable of arbitrating between competing human desires. This power could be formed in the framework of a civil society that allows individuals to make a voluntary social contract with the sovereign authority, transferring their natural rights to that authority in return for the protection of life, liberty and property. These early liberals often disagreed about the most appropriate form of government, but all believed that liberty was natural and its restriction needed strong justification. Liberals generally believed in limited government, although several liberal philosophers decried government outright, with Thomas Paine writing, "government even in its best state is a necessary evil".