The thesis seeks to examine the apparent tensions between fundamental ideas found in the literature of fifteenth century English legal theory and practice. The theorists studied are Fortescue and Pecock (whose ideas are set against those of their English and continental predecessors and contemporaries) and the practical materials examined are the Year Books and the Statutes. The work attempts to advance the view that legal thought in fifteenth-century England, both theoretical and practical, is characterised by two essentially opposing concepts of law. According to one outlook, described as the heteronomist thesis, in legal theory human law relies for its authority upon <i>natural law</i> and <i>justice</i>: it is not the human will which is used to shape law, it is these entities, divine in origin, which operate as external influences in the shaping of law. In the Year Books and in legislation the heteronomist thesis expresses itself as a mode or practice of disposing of cases and problems. Persistently during the period, legislators, judges and counsel rely upon <i>conscience</i> and <i>divine law</i>, and to a lesser extent <i>natural law</i> (distinctly moral ideas), and <i>reason</i> and <i>mischief</i> (more mundane ideas of good sense and wrong), to shape and legitimise their respective decisions. According to the other outlook, the autonomist thesis, it is simply the human will that shapes law, rather than abstractions like morality. In legal practice, and to a lesser extent in legal theory, popular <i>consent</i> and <i>usage</i> are conceived to supply the authority which immediately underlies law: it is from these that law and legal decisions originate. This prepares the way for the beginnings of a positivist view of law, by stressing the autonomy of law, its independence from the influences of abstract right and wrong. Legal theory supposes that humanly-created rules are still laws even when they depart from the requirements of abstract right and wrong. In the practical sphere, legislative enactments which are against <i>conscience</i> or which produce <i>mischief</i> are treated as entities which have to be expressly repealed, they are not automatically void. Similarly, judges commonly reject arguments based explicitly upon the claims of <i>reason</i> or <i>conscience</i>: the courts are shy of departing from the demands of law, from the <i>rigor iuris</i>; to depart from established practice would result in <i>inconvenience</i>, inconsistency, and this the courts would exclude even if it meant that a particular individual suffered a <i>mischief</i>.