In researching whether there was anything more substantial in Foucault's attention to index cards, I came across this passage in a book:
Foucault had [sic] often been accused by critics of being cavalier in his research. The French historian Jacques Léonard asks in relation to Discipline and Punish for example: When a philosopher engages with historians, they wonder ... whether he is a sufficiently erudite scholar to dare to talk this way: does he have enough index cards, are they comprehensive, well-catalogued? Are his files as thick as our own?The author obviously thinks they are. I am less sure, but I would like to know more about whether Foucault used index cards, and if so, how?