Errol Lord’s The Importance of Being Rational is a systematic defense of “Reasons Responsiveness” (RR), the thesis that being rational consists in responding correctly to the objective normative reasons you possess—where these are understood as the facts you are in a position to both know and use as reasons for what they favor. This is a fascinating idea of great philosophical interest. Among other important upshots, it would (1) imply the falsity of the popular idea that rationality is a matter of coherence; (2) vindicate the intuitive—though recently controversial—thought that rationality is normative; and (3) provide an important element in, and so a reason to favor, two wider philosophical programs: “Reasons Fundamentalism,” which aims to understand all normative phenomena (among them, rationality) in terms of reasons, and “Knowledge-First Epistemology,” which aims to understand prominent epistemic notions (among them, justification) in terms of knowledge.