Abstract: A rationale is developed for the numerus clausus - the principle that the number of property forms is limited by the law. Current theories, based on third party information costs, imply that as the cost of providing notice of novel forms decreases, the numerus clausus principle should weaken. Here, the numerus clausus is seen not as a limitation on the available number of forms (Smith), nor as a limitation on the divisibility of rights (Hansmann & Kraakman), but as a strong legal preference for maintaining rules of exclusion in the face of private pressures to move to fine grained modes of governance. Exclusionary rules are superior to governance rules because they compensate for the cognitive and motivational limitations of the parties to transactions. These limitations do not vary with the cost of notice, because they arise from the ‘human element’ inherent in the transacting process. Exclusionary rules allow for the operation of differential human capital, free from the need for restitution or permission that characterizes governance rules, thereby enabling a Hayekian search process. This search process (sometimes) requires that the control of a resource be spread across more than one decision-making unit over the lifetime of the resource. This implies that satisficing, in which any one decision-making unit finds a course of action that is ‘good enough’ rather than ‘best’, should be a goal of property law, and not just a clarifying assumption of rational choice theory.