Three centuries later, Foucault argued that Descartes hadn't just excluded the glass man from his rationality — he had employed him. “The man who imagines he is made of glass is not mad”, Foucault writes, “for any sleeper can have this image in a dream; but he is mad if, believing he is made of glass, he thereby concludes that he is fragile, that he is in danger of breaking, that he must touch no object which might be too resistant, that he must in fact remain motionless, and so on.” To declare the glass man’s thinking invalid was to establish, for the first time, what valid thinking looked like: “Madness is expelled, rejected, denounced in its very impossibility from the very interiority of thought itself.” Jacques Derrida would counter that Descartes had actually done the opposite — by admitting that the dreamer is indistinguishable from the madman, Descartes had made the madman’s condition that of rational life: “reason is madder than madness.” Both Foucault and Descartes agreed, implicitly, that the glass man Descartes had tried to discredit had become the hinge on which their debate turned. But neither Cervantes nor Descartes (nor Foucault and Derrida, for that matter) wrote in detail about the historical occasion of the delusion itself. Why glass? Why this material, this century, these sufferers?