This article suggests a new approach to understanding the self-formation of subjectivity in the work of Michel Foucault that emphasizes the influence of his mentor, the philosopher and historian of science Georges Canguilhem. I argue that Foucault adapts Canguilhem’s biological–epistemological notion of ‘error’ in order to achieve two things: to provide a notion of subjective self-formation compatible with the claims of his ‘archaeology of knowledge’ and ‘genealogy of power’, and to provide an alternative to the phenomenological theory of the subject. (...) The notion of ‘error’ accomplishes these goals because it posits that experience is the result of knowledge, not vice versa. To illustrate the notion of ‘erroneous’ subjectivity, I turn to two volumes edited by Foucault: I, Pierre Rivière, having slaughtered my mother, my sister and my brother… and Herculine Barbin; Being the Recently Discovered Memoirs of a Nineteenth- Century French Hermaphrodite. Both juxtapose ‘subjective’ accounts of ‘abnormal’ individuals with ‘objective’ documents written by judges, psychiatrists and medical professionals in order to show how ‘erroneous subjectivities’ may transgress epistemological limits and form new concepts. (shrink)