The founder of modern Utilitarianism and one of the most relentless reformers the English-speaking world has produced. Bentham's opening move is disarmingly simple: "nature has placed mankind under the governance of two sovereign masters, pain and pleasure." From that foundation he tried to derive an entire science of legislation — actions, laws, and institutions to be judged solely by their tendency to increase or decrease net happiness, "the greatest happiness of the greatest number."
This single-criterion radicalism made Bentham a scourge of tradition, mystification, and legal Latin alike. He called natural rights "nonsense upon stilts." He wrote meticulous draft legal codes, campaigned for the decriminalization of homosexuality (unpublished in his lifetime, for obvious reasons), designed the Panopticon prison, and trained a generation of "Philosophical Radicals" — including James Mill, whose son John Stuart Mill would humanize and ultimately partly repudiate the doctrine.
Bentham is easy to caricature: the cold calculator, the patron saint of surveillance, the man who left his embalmed body on display at University College London (where it still sits). But the reformist legislation of 19th-century Britain — on prisons, poor laws, the franchise, education, legal procedure — is unthinkable without him.