Integrating organization theory, organizational economics, and organizational law considerations, it is argued that the ‘nature of the firm’ can be more completely understood if it is considered a complete society-establishing contract, including constitutional pacts on procedures for the selection of actions, rather than a nexus of incomplete transactional contracts complemented by authority, power, or relational norms. The explanation is more general since firm-establishing contracts are a sub-set of those society-establishing contracts that are capable of regulating any venture in condition of high uncertainty and potential conflict, and because the constitutional regime adopted (authority-based, democratic, or other) becomes a specification of particular types of firms rather than part of the explanation of the firm. Evidence from published studies, as well as from newly gathered data on firm-founding contracts and other partnership establishing contracts (500 record database on large multi-party projects), document that actual contracts under uncertainty do fit the hypothesized pattern.