I am quoting pages 175 and 176 from Madness and Civilization Chapter VI: Doctors and Patients [Vintage Books Edition 1988 which I believe is the standard edition]. Foucault is writing about the classical period.
Its [i.e.madness] double nothingness is to be the visible form of that non-being which is evil, and to utter, in the void and in the sensation appearances of its delirium, the non-being of error. It is totally pure, since it is nothing if not the evanescent point of a subjectivity from which all presence of the truth has been removed; and totally impure, since this nothingness is the non-being of evil. The technique of cure, down to its physical symbols most highly charged with iconographic intensity - consolidation and return to movement on the one hand, purification and immersion on the other - is secretly organize around these two fundamental themes: the subject must be restored to his initial purity, and must be wrested from his pure subjectivity in order to be initiated into the world; the non-being that alienated from himself must be annihilated, and he must be restored to the plenitude of the exterior world, to the solid truth of being” (175, 176)
These passages are baffling to me. What does he mean by nothingness? non-being? the Non-being of error? The nothingness is the non-being of evil? And the idea that the the "non-being that alienated from himself must be annihilated?"