According to Pietro Alighieri (1300-1364), the Poet’s son and commentator of the Commedia, the Noble Castle that Dante encounters in Limbo is the symbolic image of human knowledge. Its seven walls and seven gates—Pietro argued—are an allegorical reference to the seven parts of philosophy: physics, metaphysics, ethics, politics, economics, mathematics, and dialectics. Others have seen it as representing the arts of the Trivium and Quadrivium. Indeed, the Noble Castle houses the “great-hearted souls,” those ancient spirits who are excluded from salvation because, as pagans, they were not baptized. In addition to the great classical philosophers—Aristotle, Socrates, and Plato—they include eminent scientists of the ancient and medieval world: Thales of Miletus, Dioscorides, Euclid, Ptolemy, Hippocrates, Galen, Avicenna, and Averroes.