Commentary Par XI 2

Depending on whether we have read [Par X 138] in bono or in malo, that is, whether we have thought Dante meant to praise or blame Siger's 'syllogizing,' we decide that the noun form of that word is here used oppositionally or with the same intonation.  See C.Par.X.138 for reasons to prefer the first alternative; where Siger is admirable for his powers of reasoning, the normal run of men is not, merely using reason to advance their cupidinous designs.

Aversano (Aver.2000.2), p. 48, suggests that difettivi (flawed) is one of the key words of this canto, calling into question all prideful intellectual acts of humankind, which contrast with the humility embodied in Francis.