John Locke's theory of the social contract, articulated in his "Two Treatises of Government" (1689), is a cornerstone of liberal political thought. It posits that government's legitimacy derives from the consent of the governed and is fundamentally aimed at protecting individual rights.

Locke begins with a 'state of nature,' a pre-political condition where individuals are free and equal, governed by the 'law of nature' (reason). In this state, individuals possess inherent natural rights: life, liberty, and property. Unlike Thomas Hobbes's state of nature, Locke's is not necessarily a state of war, but it is inconvenient due to the absence of an impartial judge to resolve disputes and enforce natural law.

To overcome these inconveniences and better secure their natural rights, individuals voluntarily enter into a social contract. They consent to form a civil society and establish a government. However, this consent is conditional. The primary purpose of government is to protect the natural rights of its citizens, especially their property. The government's power is therefore limited and derived from the people.

Crucially, Locke argued that if the government acts contrary to the trust placed in it by the people, by violating their natural rights or exceeding its legitimate authority, the people retain the right to resist, alter, or even overthrow that government. This concept of the 'right to revolution' was profoundly influential, notably inspiring the American and French Revolutions.

Locke's social contract theory laid the groundwork for modern liberal democracy, emphasizing limited government, the rule of law, individual rights, and the principle of popular sovereignty, where political authority ultimately rests with the people.