The founding modern theorist of political order. Writing during the English Civil War, Hobbes asked the most basic question in political philosophy — why should I obey? — and answered it with a thought experiment that still structures the field: imagine humanity in a "state of nature" with no common power. Life there, he famously wrote, would be "solitary, poor, nasty, brutish, and short." The only rational escape is a mutual covenant to transfer our natural liberty to a sovereign strong enough to enforce peace. This is the ancestor of every The Social Contract argument that followed.
Hobbes's sovereign was near-absolute — not because he loved tyranny but because he feared civil war more. Divide the sovereign, he argued, and you're back in the state of nature. John Locke and Jean-Jacques Rousseau would each rework the contract to reach very different conclusions, but they all argued on Hobbes's turf.
He was also a rigorous materialist: minds are matter in motion, values are appetites, "good" is what we desire. This naturalistic ethics horrified contemporaries and made him, for centuries, the philosopher respectable people defined themselves against — while quietly borrowing from.