What does it mean to say that one “ought” to undergo an emotion? In The Imperative of Responsibility, Hans Jonas provocatively asserts that twentieth-century citizens “ought” to fear for the well-being of future generations. I argue that Jonas's demand is not straightforwardly reducible to claims about the fittingness, expedience, or aretaic desirability of fear, and I present an interpretation of its content and coherence using Aristotle's moral psychology of fear in the Rhetoric, Politics, and Nicomachean Ethics as a framework. Aristotle's account of fear as an anticipatory, imaginative stance that alters perception and judgment helps to clarify that Jonas's demand concerns acts of affect-laden, imaginative reflection through which one might revise one's affective sensibilities with regard to future persons. I conclude by considering several objections to Jonas's first-order argument, and indicating several clarifications and caveats that are important for formulating strong normative assertions about political emotions more generally.