Truth and error are interdependent; claims to truth can be made only in the light of previous error. In The Necessity of Errors, John Roberts explores how, up to Hegel, emphasis was placed on error as something that dissolves truth and needs to be eradicated. Drawing on the fragmented corpus of writing on error, from Locke to Luxemburg, Adorno to Vaneigem, and covering five key areas from philosophy to political praxis, this wide-ranging account explores how we learn from error, under what conditions, and with what means. Errors, Roberts finds, are productive, but not in any uniform sense or under all circumstances - a theory of errors needs a dialectics of error.