Thomas Hobbes is best known for his political philosophy, especially his work Leviathan (1651), which presents a comprehensive theory of social contract and government. He argued that without a strong central authority, humans live in a state of nature where life is war of all against all. To escape this, individuals consent to surrender their rights to a sovereign—a 'Leviathan'—who enforces laws and ensures peace. His mechanistic view of human nature, grounded in materialism, also influenced early modern psychology and ethics.

Hobbes's state of nature is a hypothetical condition before the establishment of society, where there is no government or laws. He believed that in this state, human beings are driven by appetites and aversions, leading to a constant competition for resources and security. Because there is no moral order, everyone has a right to everything, resulting in a 'war of all against all.' Life is solitary, poor, nasty, brutish, and short. This grim picture justifies the necessity of a social contract and a powerful sovereign to impose order.

For Hobbes, the social contract is a mutual agreement among individuals to relinquish their natural rights and submit to a common authority, the sovereign, in exchange for protection and peace. This contract is not between the people and the ruler but among the people themselves, who authorize the sovereign to act on their behalf. Once established, the sovereign's power is absolute and cannot be revoked, because any division of authority would lead back to chaos. This contract is the foundation of political legitimacy and social order.

Hobbes advocates for absolute sovereignty because he believes that any limitation on the sovereign's power would reintroduce the state of nature, where conflicting authorities lead to civil war. He argues that the sovereign must possess ultimate authority over laws, religion, and policy to maintain peace. While he acknowledged that absolute rulers could be tyrannical, he thought that even bad government is preferable to the anarchy of the state of nature. His defense of absolutism is pragmatic, rooted in his fear of social disorder rather than a belief in divine right.

Hobbes's political theory is built on a specific conception of human nature as fundamentally self-interested and competitive. He believed humans are driven by desires for power, which he defines as the present means to secure future goods. This pursuit of power inevitably leads to conflict, as resources are scarce. However, Hobbes also recognized reason as a tool for self-preservation, guiding individuals to seek peace. This rational calculation underpins the social contract, showing that even in a state of nature, reason can lead to the establishment of civil society.

Hobbes's most famous work is Leviathan (1651), a comprehensive treatise on political philosophy, religion, and human nature. His earlier Elements of Law (1640) outlined his ideas on human psychology and politics. De Cive (1642) was his first systematic work on political philosophy, focusing on the relation between the individual and the state. He also wrote De Corpore (1655) and De Homine (1658), which expounded his materialist philosophy. His translation of Thucydides' History of the Peloponnesian War also influenced his views on power and conflict.

Hobbes's social contract theory profoundly influenced later political philosophers, including John Locke, Jean-Jacques Rousseau, and Immanuel Kant. His emphasis on consent, rights, and the purpose of government laid the groundwork for liberalism, though his absolutist conclusions were later challenged. His secular and scientific approach to politics broke from medieval theology, making him a pioneer of modern political science. In international relations, his realist view of state behavior as competitive and conflict-prone remains highly influential in realist theories.

While both Hobbes and John Locke use the social contract, they differ radically. Hobbes posits a state of nature that is a war of all against all, leading him to advocate for absolute sovereignty as necessary for peace. Locke, however, sees the state of nature as governed by natural law, where people have rights to life, liberty, and property. For Locke, the social contract is limited: if the sovereign violates natural rights, the people can resist and replace them. Locke's contract is conditional, while Hobbes's is permanent and unilateral.