(Canto 4, line 106)
The "noble castle" in Limbo to separate the souls of certain great persons from the rest is an invention of Dante, according to some an expression of his "proto-humanism". There has been a certain amount of debate concerning the exact meaning of the castle. Some think the castle represents science, with the seven walls or seven portals standing for the "seven arts" (often called "the seven liberal arts"): that is, the "Trivium" of grammar, rhetoric and logic and the "Quadrivium" of arithmetic, algebra, astronomy and music. Other commentators separate the walls from the portals, with the walls representing the seven virtues, that is the cardinal virtues of prudence, justice, restraint and courage together with the theological virtues of faith, hope and love, while the portals stand for the seven arts. Still others believe the castle stands for philosophy, while the walls (and its portals) represent its seven branches, that is, physics, metaphysics, ethics, politics, economics, mathematics and dialectics. Reggio thinks that the castle stands for the noble character of man and points to the passage in the Convivio that says "wherever there is virtue there is nobility", whereby it would be acceptable to view the walls of the castle as representing the seven virtues. Concerning the "rivulet" "defending" the castle, it is often believed it stands for some kind of barrier - that is to say, in order to achieve the superior inner character of man (enter the castle), it is necessary to overcome the lowly thoughts and bad habits which people have.