This essay, coauthored by the editor and a member of the editorial board of Common Knowledge, introduces the fifth installment of the journal's symposium “Fuzzy Studies,” which is about the “consequence of blur.” Beginning with a review of Enlightenment ideas about ideas — especially Descartes's argument that a mind “unclouded and attentive” can be “wholly freed from doubt” (Rules III, 5) — this essay then turns to assess the validity of counter-Enlightenment arguments, mostly Russian but also anglophone and French, against the association of clarity and certainty. A line of descent is drawn from the speaker of Descartes's Meditations on First Philosophy to Dostoevsky's underground man, and the latter is shown to differ from the former mainly in that the underground man sympathizes consciously with Descartes's bête noire, the “evil deceiver” (Descartes's speaker sympathizes perhaps unconsciously). Enlightenment, in other words, is not as it appears to be, for rationalists too insist on their caprice. Hyperrationality, it is argued, can be a form of madness, a mania over clarity, distinctions, rules, principles, and unquestionable truths. Examples from Russian and anglophone literature are given of how easily the distance from idée claire to idée fixe is to traverse. The end product of un-self-doubting rationality tends to be delay or stoppage of the intellect, which this essay terms noostasis (and proposes is the opposite of ekstasis).