Heidegger’s subordination of reason to “care” in Being and Time has exposed him to the charge of irrationalism. Against this view, I argue that Being and Time offers a “normativity-first” account in which reason, as reason-giving (logon didonai), is an ineluctable demand constitutive of authentic selfhood. Examining Heidegger’s rejection of the neo-Kantian equation of reason with logic in his 1929 Kantbuch, I explain the threads that connect what Heidegger calls “pure sensible reason” to his extensive phenomenological account, in Being and Time, of the “everyday” and “authentic” modes of Dasein’s care-structure. As authenticity’s discursive mode, the “call of conscience” is Dasein’s portal into normative space. As the essay “On the Essence of Ground” makes plain, Dasein’s response to the call involves answerability for what it holds to be best in its practical life, hence reason-giving. Such an origin of reason contrasts with rationalism only in eschewing any principle of sufficient reason.