French philosopher who, across six decades of work, has pursued a single improbable project: to rebuild, on the foundation of mathematical set theory, a systematic philosophy capable of defending universal truth and revolutionary politics against the ironic, anti-foundational mood of late-20th-century thought. Born in Rabat in 1937 to a mathematician-turned-socialist-mayor, he trained at the École normale supérieure under Louis Althusser and Jean Hyppolite, and was formed simultaneously by Sartrean commitment, Althusser's science of Marx, and Lacan's return to Freud. Radicalized by May 1968, he became a militant Maoist and a founder of the Union des communistes de France marxistes-léninistes — a small revolutionary group through which he developed, and never really disowned, a commitment to organized communist politics that survived the collapse of every institution that had carried it.
His philosophical masterwork Being and Event (1988) makes the stark claim that mathematics is ontology: set theory, in its post-Cantorian axiomatic form, is what we have to say about pure multiplicity, which is what there is. The interesting moments, however, are not being but the event — a rare, radically contingent rupture that is not inscribed in the situation it transforms, and that summons subjects who, by declaring fidelity to it, draw out its consequences as a new truth. History has, Badiou argues, exactly four kinds of truth procedure — politics, art, science, and love — and a life is worth living to the extent that one participates, in fidelity, in at least one of them. The 2006 sequel Logics of Worlds supplies a phenomenology of "appearing" to match the ontology of being.
Badiou is among the last unapologetic defenders of the communist hypothesis: the wager that an egalitarian politics beyond private property and the managerial state is a genuine and recurring human possibility, interrupted and restarted across Spartacus, the French Revolution, the Paris Commune, October 1917, the Chinese Cultural Revolution, and (in his reading) May '68. This has made him enormously influential — Žižek and a generation of anglophone radicals read him closely — and a target for critics who find his fidelity to Maoism and to certain of his polemical interventions (notably on Israel, Palestine, and anti-Semitism in France) apologetic or worse. He is, in any case, impossible to read indifferently: the last philosopher of that French generation still writing as if philosophy were for something.