The scattered condition of the scholars in Paris often made lodging difficult. Some students rented rooms from townspeople, who often exacted high rates while the students demanded lower. This tension between scholars and citizens would have developed into a sort of civil war if Robert de Courçon had not found the remedy of taxation. It was upheld in the Bull of Gregory IX of 1231, but with an important modification: its exercise was to be shared with the citizens. The aim was to offer the students a shelter where they would fear neither annoyance from the owners nor the dangers of the world. Thus were founded the colleges (colligere, to assemble); meaning not centers of instruction, but simple student boarding-houses. Each had a special goal, being established for students of the same nationality or the same science. Often, masters lived in each college and oversaw its activities.