In 1588, when Galileo was a 24-year-old unknown, a medical school dropout, he was invited to deliver a couple of lectures on Dante’s “Divine Comedy.” Many in Galileo’s audience would have been shocked, even dismayed, to see this young upstart take the stage and start poking holes in what they believed about the poet’s meticulously constructed fantasy world. Ever since its 1314 publication, scholars had toiled to map the physical features of Dante’s Inferno—the blasted valleys and caverns, the roiling rivers of fire. What Galileo said, put simply, is that many commonly accepted dimensions did not stand up to mathematical scrutiny.