The concept of bona fides is an indigenously ancient Roman invention. Roman bona fides – the notion of private law – was linked to Roman fides widely present in the public sphere, which permeated its culture and influenced the Roman state to become one of the most resilient mega-organizations in the world. Fides embraced trust and trustworthiness, i.e. keeping one’s word also in non-equal relationships, and even among enemies. Bona fides in the realm of private law went much further also preventing against the deceit and abuse, and motivating to respect tacit obligations as referred to the basic goods to be realized in a given polity. Not surprisingly, in the ius commune it was bona fides that was proclaimed as the basis of commercial transactions (lex mercatoria), and in modern times fides and bona fides was invoked as the foundation of international law called by the rebranded Roman name ius gentium. Not only modern doctrine, but also the European legal tradition allows to argue for two statements. Bona fides is not just a technical concept of private law, but is the foundation of the legal order in general. To understand the purpose and use of bona fides in the public law sphere, centuries of private law reflection on Roman bona fides should be a constant source of inspiration. After all, the public and private spheres are linked by human person and the moral framework as reflected in the constitutional order.