The paper considers the applicability of the concept of public reason (PR), properly reinterpreted and recalibrated, to constitutional law. After the presentation of the general idea of PR as a legitimacy-conferring device, the paper discusses its proper scope (both in terms of its range and actors), and also attempts to rebut the main challenges to the idea of PR. It then undertakes a “translation” of the philosophical conception of PR into a constitutional doctrine of illicit legislative motivations, which taint the law with unconstitutionality, and then tests it by using a case study of US Supreme Court freedom of speech jurisprudence under the First Amendment.