The reasoning by which Galileo and his contemporaries defended the Copernican model of the solar system (the “heliocentric” model – that is, with the Earth orbiting the Sun rather than the Sun orbiting the Earth) can puzzle us even today. Here are a few of the questions that we could ask about the reasoning given by Copernicus, Galileo, and their contemporaries. Did Copernicus’s arguments support the heliocentric model strongly enough to justify believing it true? Or was it unjustified until Galileo amassed telescopic evidence for it? Or was it unjustified until even later – when Newtonian physics was developed? Or did it remain unjustified until even later – when various mechanical and optical discoveries were made in the nineteenth century? Was the Catholic Church justified at the time of Galileo in regarding Copernicus’s theory as just one among many fairly successful techniques for predicting the night sky’s appearance? Did Galileo bring his sentence (at his famous – and notorious – trial) on himself? Could Galileo argue persuasively for his telescope’s reliability? Could Galileo use mere “thought-experiments” (as opposed to actual experiments) to defend Copernicanism? In this course, we will grapple with these and related questions in order to arrive at a more nuanced understanding of the logic by which scientific theories in general are tested and, ultimately, justified. We will also try to use this historical episode to understand better how political, social, and cultural factors can influence the reception of a scientific theory – even today! We will learn some of the means by which the biases and presuppositions introduced by these factors were overcome (eventually) in the Copernican Revolution, and we will apply some of these lessons to current science. At various points during our discussions, each student will submit in written form his or her own best reconstructions of some of the arguments that were given for or against the Copernican model. In other words, each student will offer his or her best advice regarding how a given scientist might have argued for or against Copernicanism, anticipating possible objections and responses. Students will occasionally form groups to examine and to critique one another’s proposals, with each group finally presenting its best thoughts orally to the rest of the class for further discussion. Students will, in effect, be putting Galileo on trial once again – not for heresy or for disobeying authority, but for having convincing or for having insufficient evidence for his Copernicanism. In all of these ways, students will learn how to appreciate sympathetically the competing astronomical theories from the perspective of the 16th and 17th centuries, when the truth was in some doubt. Along the way, students will wrestle with some of the puzzles and apparent paradoxes arising even from today’s best philosophical accounts of the logic of theory testing in science. No previous background in science will be assumed. Students will not need to purchase any books.