This book offers a conceptual analysis of reason, proposing a plausible theory to resolve persistent vagueness in the historical use of the concept. The proposed theory leads to the view that reason is fundamentally subjective and dependent on non-inferential matters of faith. Despite the inherent subjectivity, we can arrive at a consensus in reasoning exercises because we all participate in a universal minimum religion that grounds all reasoning. True atheism is therefore impossible. This analysis bears on debates about the relation between faith and reason as well as the relation between ethics based on reason and ethics based on sentiments. The book further shows that this new and original view of religiously constrained reason ought to equally inform our thinking about applied ethics, such as politics, law, and economics.