In observing the Sun through the telescope, Galileo had seen numerous spots on its surface. Elsewhere, these same spots had been noted by Christoph Scheiner, a Jesuit mathematician residing at the time in Bavaria. Mark Welser, an eminent citizen of Augusta who was acquainted with both Galileo and Scheiner, asked their opinions on the new discovery. Their conflicting ideas on the nature of sunspots reflected the basic split between the inflexibly philo-Aristotelian position of Scheiner, unable to forego the concept of an incorruptible heaven, and Galileo’s increasing acceptance of the new Copernican cosmology. The three letters sent by Galileo to Welser were published by the Lycean Academy under the title Istoria e dimostrazioni intorno alle macchie solari e loro accidenti [History and demonstrations concerning sunspots and their properties]. In these letters Galileo did not stop at close examination of the phenomenon of sunspots, but took it as a springboard for attacking the non-critical conformism of his adversaries, and ended by proclaiming the natural truth of the heliocentric position and the Earth’s motion.