Thoughts, Words, and Promises: <i>Fides</i> and the Legal Significance of the Inner Sphere

Abstract This paper traces how the Roman notion of fides evolved into the concept of good faith, which is central to contract law theory. It explores how fides was initially understood in Roman society as a divine presence within the inner sphere of individuals, symbolizing honesty and loyalty in social relationships. To illustrate how fides transitioned into the legal domain through natural law and procedural law, the paper examines three key developments over the longue durée of European private law: the emergence of the bonus vir (good and reasonable man) standard, the evolution of consensual contracts, and the doctrine of aequitas naturalis (natural equity). The legal evolution of good faith from a moral-religious phenomenon to a legal principle highlights how loyalty to promises and honesty, which belong to the inner sphere, and their relationship to their outward manifestations were addressed in law. It also demonstrates how the values tied to fides became integral to European legal traditions thereby shaping modern contract law.