Summary: Franciscan brothers of the XIII and XIV centuries were confronted with the idea of poverty and the practice of a poor life. To the purpose of defending their conception of Franciscan poverty as a mirror of evangelical poverty, they produced a strong theoretical contribution to the evolution of canonical law, a new paradigm of law opposed, as Berman stresses, to classical Roman law. They developed the new idea of claim, subjective right, adding it to the old idea of objective right. In Continental countries, the categories of medieval Roman law were stretched, as Bartolus testifies, to permit the practice of Franciscan ideals in the social life of friars. But in the United Kingdom their action was in relation with the tradition of contracts of feoffment to uses: this kind of contract, not present in Continental tradition, was a sound legal instrument for living the Franciscan ideal. So, Franciscan friars in the United Kingdom gave a strong impetus to the development of feoffment to uses into the trust, a legal institution that has marked the history of capitalism. By searching the Heaven on this earth, Franciscan brothers gave to our civilisation a strong contribution as regards jurisprudence and economics.