On August 10, 1632, five leaders of the Society of Jesus convened in a somber Roman palazzo to pass judgment on a simple idea: that a continuous line is composed of distinct and limitlessly tiny parts. The doctrine would become the foundation of calculus, but on that fateful day the judges ruled that it was forbidden. With the stroke of a pen they set off a war for the soul of the modern world. Amir Alexander's Infinitesimal is the story of the struggle that pitted Europe's entrenched powers against voices for tolerance and change. It takes us from the bloody religious strife of the sixteenth century to the battlefields of the English civil war and the fierce confrontations between leading thinkers like Galileo and Hobbes. We see how a small mathematical disagreement became a contest over the nature of the heavens and the earth: Was the world entirely known and ruled by a divinely sanctioned rationality and hierarchy? Or was it a vast and mysterious place, ripe for exploration? The legitimacy of popes and kings, as well as our modern beliefs in human liberty and progressive science, hung in the balance; the answer hinged on the infinitesimal.