(Canto 9, lines 62-63)
Dante does not explain which "doctrine" he means. Several interpretations have been proposed, according to some of which he intends Medusa to be a symbol for heresy, or for sexual lust, set to distract and delude people. As the Furies are traditionally goddesses of vengeance, they have also been proposed to symbolize the "regret", which tries to make Dante abandon his resolve. Be that as it may, even if the "doctrine" here might allude to something concrete but now obscure, it is a general theme of the poem that man must overcome obstacles like Medusa and the Furies on his way to rebirth and atonement for sins; in some measure, the "reason" symbolized by Virgil will help him not to "look in the wrong direction", but to succeed completely, heavenly grace in the form of the angel about to arrive is also necessary.