How does ethical rationalism use reason rather than custom or feeling to judge right and wrong?

Ethical rationalism posits that moral principles are objective truths discoverable through reason, rather than being derived from subjective feelings or social customs.12

Proponents of this view, such as the Cambridge Platonists and later intuitionists like David Ross, argue that fundamental moral rules are as self-evident as the truths of logic or mathematics. In this framework, reason allows individuals to recognize prima facie duties—such as the obligation to keep promises or make reparations—as intrinsic to the nature of human action. When multiple duties conflict, rational intuition is used to weigh their comparative strengths.1

Alternatively, in utilitarian rationalism, reason functions in two ways: it uses inductive reasoning to infer the consequences of an action, and it relies on a priori, self-evident judgment to determine that certain outcomes are intrinsically better than others. By grounding morality in these rational processes, ethical rationalists maintain that moral standards are objective and independent of the varying attitudes or desires of individuals.12