Every situation has what Badiou calls evental sites: elements that are on the edge of the void, included in the situation but not presented, belonging without being counted. An event arises from such a site: it is a self-referential multiple that includes itself as an element — structurally inconsistent, unpresentable, belonging to itself. Cantor's discovery of infinite sets beyond the merely countable, the French Revolution's rupture of the monarchical order, Schoenberg's invention of the twelve-tone row, the encounter of two people who fall in love — each of these is an event in Badiou's sense: a happening that the existing counting procedures of its situation cannot assimilate or domesticate.
Because an event is structurally undecidable — it cannot be verified by the existing knowledge of the situation — the first response to it is always a wager: naming the event, asserting that something happened, committing to investigating what consequences follow from taking the event to be real. This commitment is fidelity (fidélité). To be faithful to an event is to transform elements of the situation according to their relationship to the event — to ask of each element, in each situation you find it: does this connect to what happened, or not? The long work of fidelity constitutes what Badiou calls a generic procedure, a truth process that unfolds over time in the world.
The subject, for Badiou, is not a pre-given agent or a Cartesian cogito. It is what emerges from and within fidelity: a localised finite fragment of a truth procedure, constituted by the work of tracing the consequences of an event. The mathematician who pursues the implications of a new concept, the political militant who sustains the work of an emancipatory rupture, the lovers who build their love in the aftermath of the encounter — each is a subject in Badiou's sense. The subject does not exist before the event; it is created by the response to it. This is what Badiou means by the statement that man is "a being of event" rather than a stable natural kind.
The event–fidelity–subject triad is the central structure of Being and Event (1988). It is developed through a series of case studies — Cantor's discovery of infinity, the French Revolution, the poetry of Mallarmé, the life of Pascal — each analysed as an example of a different truth procedure. The theory is significantly revised and extended in Logics of Worlds (2006).
