 John Rawls first proposed a constructivist interpretation of Kant’s account of moral obligation and practical reason (Rawls 1980, 2000). On Rawls’ reading, Kant’s analysis of obligation commits him to a kind of constructivism, which is best understood in contrast to competing views of moral obligation (Rawls 1980, 1989, 2000). Kant holds that all previous ethical theories have failed to account for moral obligation because they have failed as theories of practical reason (Kant G 4: 441–444; C2 5: 35–41, 153, 157). They fail to explain how reason plays a role in our lives because they misunderstand its practical function and mischaracterize its relation with the ends of choice. Kant’s charge is directed against all previous moral doctrines, but his arguments specifically address sentimentalism and ‘dogmatic rationalism’. Sentimentalism, championed by Francis Hutcheson, David Hume, and Adam Smith, holds that ethical judgments stem from sentiments and regards reason as incapable of moving us to action on its own. According to the sentimentalist, the role of reason is solely instrumental. That is, reason merely finds the means to satisfy an agent’s ends, and it is not capable of indicating which ends are worth pursuing. This claim exposes sentimentalism as a “heteronomous” doctrine, which fails to establish the objectivity of moral obligations. This is because sentimentalism treats moral obligations as conditional upon our interests, and thus as having limited authority.