International Catholic University


Galileo: Science and Religion

Galileo and the Inquisition I

William E. Carroll

Wittgenstein once asked a friend, "Tell me, why do people always say it was natural for man to assume that the Sun went round the Earth rather than that the Earth was rotating?" His friend replied, "Well, obviously, because it just looks as though the Sun is going round the Earth." To which Wittgenstein responded, "Well, what would it have looked like if it had looked as though the Earth was rotating?"

On the occasion of the publication, in March 1987, of the Catholic Church's condemnation of in vitro fertilization, surrogate motherhood, and fetal experimentation, there appeared a cartoon in a Roman newspaper, in which two bishops are standing next to a telescope. In the distant night sky, in addition to Saturn and the Moon, there are dozens of test-tubes. One bishop turns to the other, who is in front of the telescope, and asks: "This time what should we do? Should we look or not?" The historical reference to Galileo was clear. In fact, at a press conference at the Vatican, Cardinal Ratzinger was asked whether he thought the Church's response to the new biology would not result in another "Galileo affair." The Cardinal smiled, perhaps realizing the persistent power -- at least in the popular imagination -- of the story of Galileo's encounter with the Inquisition more than three hundred and fifty years before. The Vatican office which Cardinal Ratzinger now heads, the Congregation for the Doctrine of the Faith, is the direct successor to the Holy Roman and Universal Inquisition.

In my initial lecture I sought to provide a global view of the "Galileo Affair," paying special attention to the persistence of the legend of Galileo's encounter with the Inquisition: a legend which sees Galileo as representing modern science's fighting to free itself from the clutches of blind faith, biblical literalism, and superstition. I argued that Galileo and the officials of the Inquisition shared common first principles about the nature of scientific truth and the complementarity between science and religion. In this lecture and the next two, I want to examine with you some of the particulars of this famous story in order to suggest an interpretation quite at variance with the popular legend.

Galileo was born in Pisa in 1564: the same year in which Michelangelo died and Shakespeare was born. It was 21 years after the publication of Copernicus' treatise on heliocentric astronomy; it was 47 years after the appearance of Luther's 95 theses and the beginning of the Reformation. In fact, the Protestant Reformation, the Catholic response -- especially the Council of Trent, whose final session ended in 1563 -- the destruction of the religious unity of Europe, and the ensuing wars of religion constitute the world in which Galileo will spend his entire life.

Galileo entered the University of Pisa in 1581 to prepare for a career in medicine, but his interests quickly turned to natural philosophy and mathematics. After teaching at Pisa for a few years, he left in 1592 for the University of Padua. It was at Padua, from 1592 to 1610, that he formulated the basic principles of his physics, especially his understanding of the laws of motion.

In 1609 he began to use the newly discovered telescope to observe the heavens, and in March 1610 he published The Starry Messenger in which he reported his discoveries that the Milky Way consists of innumerable stars, that the Moon has mountains, and that Jupiter has four satellites. Subsequently, he discovered the phases of Venus and spots on the surface of the Sun. He named the moons of Jupiter the "Medicean Stars" and was rewarded by Cosimo de' Medici, Grand Duke of Tuscany, with appointment as chief mathematician and philosopher at the Duke's court in Florence.

In order to understand the importance of Galileo's Starry Messenger, we need to place his observations in the context of developments in astronomy in the late sixteenth and early seventeenth centuries. Between 1572 and 1610 there were several new observations: the nova of 1572; Tycho Brahe's observations (and those of others, as well) of comets in 1577 and 1585; the super-nova of 1604; and Galileo's own observations in 1609/10. These observations persuaded several natural philosophers that some important features of the heavens described by Aristotle could no longer be accepted as accurate: in particular, the immutability of the heavens and the existence of solid, crystalline planetary spheres.

Also, as I noted in my first lecture, Galileo did not think that his telescopic discoveries provided a proof for the view that the earth rotated on its axis and revolved about the Sun. He did think that they provided arguments for the plausibility of Copernican astronomy. His discovery of the phases of Venus required only that Venus must revolve about the Sun. Even the discovery of spots on the Sun, and the fact that these spots moved across the Sun's surface, only provided evidence that the Sun was not an immutable body. None of Galileo's telescopic discoveries required the abandonment of a modified geocentric system; much less did they affirm the truth of a heliocentric one. Furthermore, Galileo understood the difference between providing plausible arguments for a position and demonstrating that it is true. Although Galileo's telescopic observations were not sufficient to demonstrate the truth of Copernican astronomy, they did serve to call into question the received geocentric cosmology, which was a melange of views having their source in Ptolemy and Aristotle. They were also a powerful incentive for Galileo to discover a demonstration for the motion of the earth. In The Starry Messenger, Galileo claimed that his most important discovery were the four moons of Jupiter. This discovery, according to Galileo,

. . . [provides] an excellent and splendid argument for taking away the scruples of those who, while tolerating with equanimity the revolution of the planets around the Sun in the Copernican system, are so disturbed by the attendance of one Moon around the Earth while the two together complete the annual orb around the Sun that they conclude that this constitution of the universe must be overthrown as impossible. For here we have not only one planet revolving around another while both run through a great circle around the Sun: but our vision offers us four stars wandering around Jupiter like the Moon around the Earth while all together with Jupiter traverse a great circle around the Sun in the space of twelve years. [Sidereus Nuncius . . . , pp. 84-5]

Copernican astronomy required two centers of heavenly motion: the Moon's revolving around the Earth, and the Earth and the other planets' revolving around the Sun. Yet, such a universe, with more than one center of motion, seemed inconceivable. Since it was now clear that four moons revolved around Jupiter, and Jupiter itself moved around another center, an important objection to Copernican astronomy disappeared.

How could any thinking person not accept the new heavens, the novità celesti, to which Galileo had drawn attention? Stillman Drake, one of the famous scholars of Galileo in our own age, categorizes the opposition to Galileo in this way:

The arguments brought forth against [Galileo's] new discoveries were so silly that it is hard for the modern mind to take them seriously. . . . The chief argument was that the phenomena he had described were merely illusions created by his telescope and had no real existence in the skies. . . . One of his opponents who admitted that the surface of the moon looked quite rugged, maintained that it was actually quite smooth and spherical as Aristotle had said, reconciling the two ideas by saying that the moon was covered with a smooth and transparent material through which [the rugged surface] could be discerned. . . . One after another, all attempts to cleanse the heavens of new celestial bodies came to grief. Philosophers had come up against a set of facts which their theories were unable to explain. The more persistent and determined adversaries of Galileo had to give up arguing and resort to threats. [Drake, pp. 73-4]

Were the objections to Galileo's claims all so silly, as Drake calls them? Galileo argued that his new optical device revealed things in the heavens as they really were, even though they were invisible to the naked eye. Galileo provided no theoretical arguments, in the science of optics, to demonstrate the reliability of the instrument he had used. In this respect, it is useful to cite the remarks of the philosopher and historian of science, Hans Blumenberg, who warns us against an all-too-easy dismissal of Galileo's opponents: "The fool's role that Galileo's opponents have long played in the historiography of natural science has rendered them harmless for us and obscured their significance as indicators of the difficulties in our relation to reality that are always present and become especially acute in historical situations where radical change is under way. The failure of their obstructed faculty of vision is only a correlate of the exaggerated expectations that Galileo himself had invested in his optical discoveries." [Blumenberg, p. 662]

In an intriguing recent article, Roger Ariew notes that Galileo's arguments for there being mountains on the Moon presupposes that the Moon reflects light from the Sun. But, to assume that the moon reflects light is to assume that the surface of the Moon is rough and uneven: to assume, that is, that the Moon is like the Earth. In this regard it is interesting to note that Medieval natural philosophers rejected the view that the Moon reflected light because a smooth and polished surface (which they were convinced the Moon was) would function like a mirror and reflect light rays in a way other than from a rough surface (i.e., the whole surface would not reflect light equally). Thus, they thought that the Moon receives light from the Sun, becomes luminescent, and then light emanates from it. Averroes is a principal source of this view:

It has been demonstrated that if the moon acquires the power of lighting up from the sun, it is not by reflection. . . . If it illuminates, it is by becoming a luminous body itself. The sun renders it luminescent first and then the light emanates from it in the same way that it emanates from the other stars; that is, an infinite multitude of rays are issued from each point of the moon. If its power of illumination issued from reflection, it would illuminate some determined places on earth depending upon its circumstances; reflection is produced only for some determined angles. . . . Since the various parts of the celestial body are distinguished with respect to whether they are translucent or not, or luminescent, it is not impossible that the various parts of the moon receive the light of the sun differently. [cited in Ariew, p. 219]

There is a long discussion in the first section of Galileo's Dialogue Concerning the Two Chief World Systems in defense of the claim, based on telescopic observations, that the surface of the Moon is rough and uneven. The interlocutors compare the reflection of light from a wall with a rough surface with that from a flat mirror, and observe that the former illuminates the entirety of the surface opposite it, whereas the reflection from the flat mirror only illuminates a small portion where its bright reflection fell. Next they examine the reflection cast by a spherical mirror and decide that only a minute area of its surface would appear illuminated to the observer. "The rest would remain . . . unilluminated and therefore invisible. . . . [T]he whole Moon would be invisible [if it were a perfectly smooth spherical surface reflecting light] since that particle which gave the reflection would be lost by reason of its smallness and great distance." [Galileo, Dialogue . . ., p. 74] In The Assayer, published a decade earlier, in response to the question as to why the Moon is not smooth, Galileo writes: "it and all the other planets are inherently dark and shine by light from the sun. Hence they must have rough surfaces (fu necessario che fussero di superficie scabrosa), for if they were smooth as mirrors (liscia e tersa come uno specchio) no reflection would reach us from them and they would be quite invisible (tutto invisibili) to us." [Drake (ed.), Discoveries and Opinions of Galileo, p. 263.]

As Ariew observes, "the conclusion one ought to draw is that, since the medieval theory of the moon is not that light is reflected off the surface of the moon, but that the moon receives sunlight in proportion to its density, Galileo's observations of mountains on the moon, which assumes that sunlight is reflected off the moon, cannot succeed in destroying the medieval lunar theory; it can only be an independent account of the moon and lunar light based on radically different premises. Galileo concludes that the moon is like the earth by claiming to see spots on the lunar surface as the shadows that mountains would cause, if the light of the moon were reflected off the surface of the moon. But since the light of the moon diffuses throughout, since one does not see a simple image of the sun reflected off the moon, to assume that the light of the moon is received by reflection would be to assume that the surface of the moon is rough -- to assume the moon is like the earth. From the perspective of medieval lunar theory, Galileo's reasoning is oddly circular." [Ariew, p. 223]

The public position which Galileo occupied in Florence from 1610 involved him in controversy. As the best-known advocate for Copernican astronomy, he was a lightning rod for criticism. Philosophers, for example, were concerned with the apparent violation of the principles of Aristotelian physics involved in the notion that the Earth moved or that celestial bodies were in any way like the Earth. Criticism also came from some theologians who were troubled about the relationship between Copernican astronomy and the Bible.

In early 1615, well after the debate had begun, a Carmelite priest in Naples, Paolo Foscarini, published an essay in which he claimed that the Bible could be interpreted in such a way as to be consistent with Copernican astronomy. Foscarini, calling upon exegetical principles of well-known Catholic theologians such as Melchior Cano, observed that "when Sacred Scripture attributes something to God or to any creature which would be improper and incommensurate, then it should be interpreted and explained . . . either metaphorically. . . or according to our mode of consideration, apprehension, conception, understanding, [and] knowing . . . [as] the Holy Spirit frequently and deliberately adopts the vulgar and common way of speaking." A classic example of this mode of speaking in Scripture are statements about God's stretching out His hand, walking in the garden, or showing emotions. All such statements must be taken metaphorically or as accommodating our limited mode of understanding God. If the claim that the Earth moves were true -- a claim about one of God's creatures -- "it would be easy," Foscarini writes, "to reconcile it with those passages of Sacred Scripture which are contrary to it . . . by saying that in those places Scripture speaks according to our mode of understanding, and according to appearances, and in respect to us. For thus it is that these bodies appear to be related to us and are described by the common and vulgar mode of thinking; namely, the Earth seems to stand still and to be immobile, and the sun seems to revolve around it." Foscarini concludes that "in matters which pertain to the natural sciences and which are discovered and are open to investigation by human reason, Sacred Scripture ought not to be interpreted otherwise than according to what human reason itself establishes from natural experience and according to what is clear from innumerable data. . . . [If the heliocentric system is true] we ought not to affirm emphatically that the sacred writings favor the Ptolemaic or the Aristotelian opinion, and thus create a crisis for the inviolable and most August sacred writings themselves. Rather we ought to interpret those writings in such a way as to make clear to all that their truth is in no way contrary to the arguments and experiences of the human sciences." [cited in Blackwell, pp. 226, 232, and 259]

Foscarini sent his essay to Cardinal Roberto Bellarmino, the learned Jesuit and important officer of the Inquisition in Rome. Bellarmino, already an old man, had spent his professional career refuting the views of Protestant theologians. Late in the 16th century he had been named Professor of Controversial Theology at the new Jesuit university in Rome, and he was skilled in the intricacies of biblical interpretation as well as in challenges to the authority of the Church.

Cardinal Bellarmino's response to Foscarini, a copy of which the Cardinal sent to Galileo, is one of the most important documents for our analysis. In April 1615, the Cardinal writes:

First . . . it appears to me that [you] and Signore Galileo are proceeding prudently by limiting yourselves to speaking hypothetically and not absolutely [ex suppositione e non assolutamente], as I have always believed Copernicus did [come io ho sempre creduto che habbia parlato Copernico]. For to say that, by assuming [che supposto] the earth moves and the sun stands still, one saves all the appearances [si salvano tutte le apparenze] better than by postulating [porre] eccentrics and epicycles is to speak well [benissimo detto]. This has no danger in it, and it suffices for mathematicians. But to wish to affirm that the sun is really fixed in the center of the heavens [che realmente il sole sta nel centro del mundo] and merely turns upon itself without traveling from east to west, and that the earth . . . revolves very swiftly around the sun, is a very dangerous thing [cosa molta pericolosa], likely not only to irritate all the scholastic theologians and philosophers, but also to harm our Holy Faith by rendering Holy Scripture false [di nuocere alla Santa Fede con rendere false le Sante Scritture]. . . .

Notice the distinction Cardinal Bellarmino draws between speaking "suppositionally" and speaking "absolutely." To speak suppositionally (hypothetically), in the sense the Cardinal means, is "to save the appearances," and in astronomy "to save the appearances" is to provide a consistent mathematical description of the observed phenomena. Hence, Bellarmino refers to the eccentrics and epicycles of Ptolemaic astronomy, which are mathematical constructs to describe observed movements in the heavens. To speak "absolutely" would be to specify what the movements in the heavens really are. This is a standard distinction employed by medieval scientists and philosophers. Aquinas, for example, observes that Ptolemaic astronomy provides only a model for the observed phenomena and that one could very well have a mathematical model in which the earth moves.

Bellarmino is wrong in thinking that Copernicus was only interested in saving the phenomena. Perhaps he is only offering pastoral advice to Galileo and Foscarini, suggesting to them a safe way to advance their arguments.

Cardinal Bellarmino next raises a theological objection:

Second. I say that, as you know, the Council [of Trent] would prohibit expounding the Scriptures contrary to the common agreement [ il commune consenso] of the Holy Fathers; and if Your Reverence would read not only all their works but the commentaries of modern writers on Genesis, Psalms, Ecclesiastes, and Joshua, you would find that all agree in expounding literally [ad literam] that the sun is in the heavens and travels swiftly around the earth, while the earth is far from the heavens and remains motionless in the center of the world [sta nel centro del mondo, immobile]. Now consider, with your sense of prudence [con la sua prudenza], whether the Church could support [possa sopportare] giving Scripture a meaning contrary to the Holy Fathers and to all the Greek and Latin expositors. Nor may one reply that this is a not a matter of faith, because if it is not a matter of faith with regard to the subject matter [ex parte obiecti], it is with regard to the one who has spoken [ex parte dicentis]. Thus that man would be just as much a heretic who denied that Abraham had two sons and Jacob twelve, as one who denied the virgin birth of Christ, for both are declared by the Holy Ghost through the mouths of the prophets and apostles.

The Cardinal's reference to the decree of the fourth session (1546) of the Council of Trent is particularly important. In addition to making clear what books constituted the canon of Scripture, the Council decreed that with respect to "matters of faith and morals" no one is permitted to interpret the Bible contrary to "that sense which Holy Mother Church, to whom it belongs to judge their true sense and meaning, has held and does hold." Nor may one interpret Scripture contrary to the "unanimous agreement" of the Church Fathers. Bellarmino extends the sense of "faith and morals" to include historical and scientific claims found in the Bible, since to deny the truth of what the Bible says on any matter calls into question the affirmation that the entire Bible is God's revealed word.

Despite the Cardinal's claim that the Church's understanding of the Bible was involved in the dispute, he is willing to examine the arguments of the new astronomy.

Third . . ., if there were a true demonstration [ci fusse vera dimostrazione] that the sun is in the center of the universe [nel centro del mondo] . . . and that the sun does not circle the earth but the earth circles the sun, then one would have to proceed with great care in explaining the Scriptures that appear contrary [che paiono contrarie], and say rather that we do not understand them than that what is demonstrated is false. But I will not believe that there is such a demonstration until it is shown to me [Ma non crederó che ci sia tal dimostrazione, fin che non mi sia mostrata]. Nor is it the same to demonstrate that by supposing the sun to be at the center and the earth in the heaven one can save the appearances, and to demonstrate that in truth [che in verità] the sun is at the center and the earth in heaven; for I believe the first demonstration may be available, but I have very grave doubts [grandissimo dubbio] about the second, and in the case of doubt one must not abandon [non si de[v]e lasciare] the Holy Scripture as interpreted by the Holy Fathers. . . .

This final paragraph in Bellarmino's response to Foscarini is very important. Note, that he again draws a distinction between saving the appearances and demonstrating the truth of a position. Note further that, despite his very grave doubts, he admits the possibility of a demonstration for the motion of the earth, although he is aware of no such demonstration. In the absence of such a demonstration, prudence, at least, requires that the traditional interpretation of those passages of the Bible which claim that the earth is motionless, be maintained.

Galileo shared Cardinal Bellarmino's understanding of the difference between an astronomy which "saves the appearances" and an astronomy which demonstrates what is truly so. In a note to a friend in 1615, Galileo observed: "Two kinds of suppositions have been made . . . by astronomers: some are primary and with regard to the absolute truth in nature; others are secondary, and these are posited imaginatively to render an account of the appearances in the movements of the stars . . . ." These latter suppositions, designed to save the appearances, are, according to Galileo, "chimerical and fictive . . . false in nature, and introduced only for the sake of astronomical computation." Galileo described his task as the discovery of the "true constitution of the universe," an understanding which is "unique, true, real, and which cannot be other than it is." [Galileo, Opere, Vol. 5, 102]

Galileo the scientist shares with Aristotle and Aquinas, and with Cardinal Bellarmino, the view that science deals with the truth of things. It is important to remember that the Aristotelian notion of science that was current in the age of Galileo is different from what we generally consider science today. Scientific knowledge for Aristotle is knowledge of what is necessarily so, that is, cannot be otherwise, because it is based on the discovery of the causes that make things be what they are. Such sure, certain knowledge is quite different from the product of probable or conjectural reasoning: reasoning which lacks certitude because it falls short of identifying true and proper causes. Galileo, despite his disagreements with 17th century Aristotelians, never departed from Aristotle's ideal of science as sure, certain knowledge. Whether Galileo was arguing about the movement of the earth or about laws that govern the motion of falling bodies, his goal was to achieve true, scientific demonstrations. Cardinal Bellarmino exemplifies the same Aristotelian position: namely, that the natural scientist discovers the truths of nature. Thus, he demands that if Galileo, the scientist, wishes to speak "absolutely," he must provide a demonstration for the motion of the Earth: after all, that is what a good scientist does. Without a demonstration a scientist cannot conclude that, in fact, the Earth moves. Although Cardinal Bellarmino accepted the Aristotelian notion of science, he was more than ready to reject specific conclusions in Aristotelian cosmology. When he was a young professor at Louvain in the 1570s he embraced a biblical cosmology at odds with many of the details of Aristotle's description of the heavens. In particular, Bellarmino rejected Aristotle's view that the heavens were immutable and composed of special matter. In the 1570s and again in the second decade of the 17th century, Bellarmino admits that, were the confusion in astronomical theories resolved by scientific truth, "one would have to consider a way of interpreting the Scriptures which would put them in agreement with the ascertained truth, for it is certain that the true meaning of Scripture cannot be in contrast with any other truth, philosophical or astronomical." [Baldini and Coyne, p. 20]

The opposition within scientific circles in the early 17th century to claims that the Earth moved was generally based on the assumption that a geocentric astronomy was an essential part of a larger Aristotelian cosmology: the view, that is, that Aristotelian physics and metaphysics depended in some way on the affirmation that the Earth was immobile at the center of the universe. Thus, if one were to reject such a geocentric astronomy, then, so it seemed to many, the whole of Aristotelian science would have to be discarded. As a result of such an understanding of the interdependence of astronomy, cosmology, physics, and metaphysics, the acceptance of a moving Earth would involve a radical philosophical revolution. Hence, we might understand why many of Galileo's contemporaries were so troubled by his support for Copernican astronomy. Furthermore, although we now accept without question that the Earth moves, we need to guard against assuming that it is a simple matter to reach this conclusion and that, therefore, the scientific opponents of Galileo were either simple-minded or stubbornly blind to the truth.

An understanding of the theological dimensions of the encounter between Galileo and the Inquisition requires that we keep in mind this question concerning the scientific knowledge of the motion of the Earth. All sides in the controversy were committed to the Aristotelian ideal of scientific knowledge. Remember, Cardinal Bellarmino told Galileo that if there were a demonstration for the motion of the Earth, then the Bible would have to be interpreted accordingly. The Cardinal has simply reaffirmed traditional Catholic teaching that the truths of science and the truths of faith cannot contradict one another. Whether we turn to Augustine in the 4th century or Aquinas in the 13th, we can discover the common Catholic commitment to the harmony between reason and revelation. Furthermore, both Augustine and Aquinas warned against using the Bible as an encyclopedia of natural science. Galileo liked to quote the remarks of Cardinal Baronius: Scripture teaches you how to go to heaven, not how the heavens go.

In the next lecture, we will look at Galileo's response to Cardinal Bellarmino's letter to Foscarini and examine how what was prudential advice, that is, to avoid speaking as though Copernican astronomy were true, becomes in 1616 a disciplinary order of the Inquisition, according to which Galileo is required not to hold, teach, or defend the view that the Sun is in the center of the universe and that the Earth moves.

Texts Cited

Ariew, Roger. "Galileo's Lunar Observations in the Context of Medieval Lunar Theory," Studies in the History and Philosophy of Science 15, no. 3 (1984), pp. 212-227.

Baldini, Ugo and G. V. Coyne. The Louvain Lectures of Bellarmine and the Autograph Copy of his 1616 Declaration to Galileo. Vatican Observatory Publications, 1984.

Blackwell, Richard J. Galileo, Bellarmine, and the Bible. University of Notre Dame Press, 1991.

Blumenberg, Hans The Genesis of the Copernican World (trans. by Robert Wallace). Cambridge, Mass.: MIT Press, 1987.

Drake, Stillman (ed.) Discoveries and Opinions of Galileo. Garden City, New York: Doubleday, 1957.

Van Helden, Albert (trans./ed.). Sidereus Nuncius or The Sidereal Messenger. Chicago: The University of Chicago Press, 1989.

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