The Church and Galileo

I

In 1633, at the end of one of the most famous trials in history, the Inquisition found Galileo "vehemently suspected of heresy"¹ for holding and defending the thesis that the earth revolves around the sun and for thinking "that one may hold and defend as probable an opinion after it has been declared and defined contrary to the Holy Scripture."² Vehement suspicion of heresy was a technical term meaning much more than it may sound to modern ears; in fact, it was a specific category of religious crime intermediate in seriousness between formal heresy and mild suspicion of heresy. The content of Galileo's "suspected heresy"was twofold. The first was an astronomical or cosmological claim about physical reality, which Galileo had supported and defended in his Dialogue on the Two Chief World Systems, Ptolemaic and Copernican (1632).The second was a methodological principle or rule about how to proceed in the search for physical truth or the acquisition of natural knowledge; it may be rephrased as the principle that Scripture is not an authority and may be disregarded as irrelevant in astronomy and natural philosophy; Galileo had practiced this principle in this book and had justified it explicitly in privately circulated essays in 1613–1616.

A number of penalties accompanied this verdict. First, Galileo had to recite immediately an "abjuration" of the "above mentioned errors and heresies."³ Second, the Dialogue was banned. Third, he was condemned to house arrest to the end of his life (1642). Finally, he had to recite the seven penitential psalms once a week for three years.

II

This condemnation was the climax of a series of events⁴ that started in 1543, when Nicolaus Copernicus published an epoch-making book, On the Revolutions of the Heavenly Spheres, in which he advanced a new argument in favor of the idea that the earth revolves around the sun. The Copernican theory immediately came under attack for reasons stemming from astronomical observation, Aristotelian physics, traditional epistemology, and scriptural interpretation. These objections were advanced by astronomers, mathematicians, and natural philosophers, as well as theologians and churchmen, and by Protestants as well as Catholics. Thus, Copernicanism attracted few followers. Galileo, in the first twenty years of his career (1589–1609), was not one of them. His stance toward Copernicanism then was one of indirect pursuit, an attitude that is not only weaker than acceptance but also weaker than direct pursuit: his research focused on physics rather than astronomy; he was critical of Aristotelian physics and favorably inclined toward an Archimedean approach; he had intuited that Copernicanism was more consistent with the new physics he was developing than was the geostatic theory; but at that time, he felt that the arguments against Copernicanism were stronger than those in favor of it.

However, in 1609, by means of the newly invented telescope, he made several startling discoveries, which he published in The Sidereal Messenger (Venice, 1610): that the moon's surface is rough, full of mountains and valleys; that innumerable other stars exist besides those visible with the naked eye; that the Milky Way and the nebulas are dense collections of large numbers of individual stars; and that the planet Jupiter has four moons revolving around it at different distances and with different periods. Soon thereafter, he also discovered the phases of Venus and sunspots, and he published the Sunspot Letters (Rome, 1613). The new telescopic evidence removed most of the observational-astronomical objections against the earth's motion and added new evidence in its favor. Galileo now believed not only that the geokinetic theory had greater explanatory coherence than the geostatic theory (as Copernicus had shown) and that it was physically and mechanically more adequate (as Galileo's new physics suggested) but also that it was empirically and observationally more accurate in astronomy...