 Kant’s discussions of theoretical reason are not obviously connected to his account of practical reason. His accounts of truth, scientific method and the limited insights of theoretical reason are complex, as is his view of practical reason and morality. No one doubts that knowledge and scientific enquiry, no less than action, are subject to demands of rationality. However, if Kant’s account of reason is based—as O’Neill above all has argued—in avoiding principles of enquiry and of action that others cannot also adopt, it would be possible to see the underlying unity of these demands. We would understand, for example, why Kant so strenuously resists claims to transcendent insight. To give authority to such claims—those of revelation and religious authority, for example—would be irrational insofar as they rest on principles of belief that cannot be adopted by all.