For medieval thinkers/writers, auctoritas was a very important issue/concept. In my studies on this mevieval concept I have come to think that there are three different meanings of auctorias as it is used by medievals. The origin of the first meaning can be found in Roman law: usus auctoritas (right of possession), auctoritas judiciorum (the authority of the judges), etc. This is a strict technical meaning, which I call the socio-juridical meaning of auctoritas. In this first sens, auctoritas is generally opposed to potestas. Cicero has left us plenty of examples for the first meaning. Innocentius III, in his Epistola ad Bulgaros (1204), speaks of a distinction between the authority of the pope and the power of the prince: "... ut ostendatur quanta sit differentia inter auctoritatem pontificis et principis potestatem" (PL 215, col. 284).
The second meaning has developed from the Greek education model, i.e., the reading of various auctores (in fact, quotations from various authors) gathered in the so-called florilegia for a specific purpose (see the use of Homer for learning the Greek grammar by the Greek ancient youth). This ancient education model have made history in the Middle Ages because it was the basic model of studying of rhetoric, eloquence, and so on.