In the first centuries of the University's existence, the teachers were paid with money collected directly by the students (collectio). This was given as an oblatio, an offering, as knowledge was considered to be a gift of God which could not be bought or sold. Documents still exist which state how much a teacher should receive for a single lesson or for a whole course. However, the students did not always contribute to the collectio, so the commune had to intervene in order to guarantee the continuity of studies. Therefore around the mid-fourteenth century it was the commune which paid the most famous professors a salary. This is how the triangle which constituted the medieval phase of the University was formed, a triangle of professors, students and the commune. In addition the professors were divided into collegia (one of Civil Law and the other of Canon Law). This is when the distinction between full professors and other teachers was first made. Initially there was a distinction drawn between doctores legentes and legentes non doctores, but at the end of the thirteenth century the statutes of the commune provided for the distinction between full professors and provisional lecturers. The latter were students who had already passed a certain number of examinations and held a debate in public and were then given the responsibility of teaching a subject on a temporary basis.