This chapter lays the foundational philosophical, anthropological and psychological assumptions for an evolutionary account of law. It begins with a ‘thin’ notion of human nature, inspired by Hume and Smith, highlighting scarcity, self-interest (including concern for close ones) and limited generosity, which lead to the emergence of societal remedies like division of labour and mutual aid to overcome individual limitations. It further integrates Hayek’s concepts of bounded rationality and dispersed knowledge, emphasising that human decision-making occurs with incomplete information, shaped by social devices like prices and abstract legal principles. The chapter then navigates two core methodological debates. Firstly, it proposes a moderate methodological individualism to overcome the strict dichotomy between methodological individualism and holism, arguing that while social phenomena originate from individual actions, they are not simply a sum of individual parts and can develop emergent properties, thereby avoiding the fallacy of composition. Secondly, it examines the Homo Economicus (self-interested utility maximiser) and Homo Sociologicus (rule-follower) models of agent motivation. The evolutionary view reconciles these by acknowledging individuals as self-interested but also capable of forming relationships, acting with sympathy, and unconsciously creating and following rules that emerge from their interactions, adapting to changing desires and goals. This nuanced perspective highlights that social and legal orders are unintended consequences of complex individual interrelationships.