A fiduciary is a person who undertakes to act for the benefit of another person. He owes the duty of loyalty to beneficiary, is required to disgorge any unauthorized gain from his position, and has the burden of disproving his disloyalty in litigation. This article presents a theory that unifies a wide variety of human relationships as fiduciary relationships and explains how the law regulating these relationships works as a coherent legal system. Its theoretical foundation is the legal impossibility of self-contract, its analytical framework is a variant of trust game, and its justificatory principle is that of both corrective justice and Pareto optimality.