Dr. Chris Decaen Lecture: Galileo & The Church


Galileo Galilei, Scriptural Exegete, and the Church of Rome, Advocate of Science

By Dr. Chris Decaen

Note: The following text is the transcript of a lecture Dr. Decaen presented at Thomas Aquinas College on August 27, 2010.


Without a doubt, the so-called "Galileo Affair" is the most famous instance of the alleged opposition between faith and reason, and particularly between the Catholic Church and natural science. Galileo's corpse was barely cold when Enlightenment critics of the Church began exhuming Galileo, the martyr to science, as sufficient evidence for demonstrating Christianity's hostility to science. Nor have the critics been silent since; even now when Rome asserts that the faith implies limits to how we practice medicine, perform scientific research, or interpret certain scientific theories, the opposition is ready to identify each of these instances as yet another "Galileo affair."

Rome herself, in recent years, has been willing to admit a certain justice to this criticism, insofar as she has taken occasion to express regret about her role in what happened to Galileo. This was most manifest in Pope John Paul II's 1992 address to the Pontifical Academy of Sciences, which the media at the time dubbed the Catholic Church's "apology" for condemning Galileo. Unsurprisingly, however, John Paul's words were more nuanced than the journalists' reporting; on the one hand, he expressed hope that "theologians, scholars, and historians … will study the Galileo case more deeply and, in frank recognition of wrongs from whatever side they come, [will] dispel the mistrust that still opposes, in many minds, a fruitful concord between science and faith." On the other hand, he followed up these words by saying that "One might perhaps be surprised that … I am returning to the Galileo case. Has not this case long been shelved and have not the errors committed been recognized? That is certainly true." Nevertheless, the Pope went on to say that we must always remember the Galileo affair, lest history repeat itself: However, the underlying problems of this case concern both the nature of science and the message of faith. It is therefore not to be excluded that one day we shall find ourselves in a similar situation, one which will require both sides to have an informed awareness of the field and of the limits of their own competencies.

In short, the Pope agrees with the Church's critics who believe the principles underlying the Galileo affair are perennial, and that therefore the threat of another confrontation between faith and reason on this level is a real danger, for the Church and for the world.

In this talk I do not want to speculate about whether or how another "Galileo affair" could happen, nor about all the intricacies of what happened in the original Galileo affair - for it was indeed a many-faceted chain of events, involving not only science and theology but even perhaps a dose of personal vendetta, or at least what today we might call "personality conflicts." Further, a great deal of sensationalistic myth has crept into the popular account of what happened to Galileo, such as that he was tortured as a heretic, or left to rot in a Roman dungeon for the rest of his life, or burnt at the stake, all because the Pope couldn't stomach the idea that he was not at the center of the universe. All of these would make for good theatre, but none of them is true. My task this evening, however, is neither the debunking of myths nor (the more difficult task) the synthesizing of the complexities of the Galileo affair, although both are worthy tasks.

My aim, rather, is in a way simpler. I hope tonight to try, using the outlines of the Galileo affair as a background, to explain an often forgotten, but central, aspect of what led to Galileo's tragic clash with Rome, and to use this to consider the proper relationship between faith and reason, particularly as manifested in the relationship between Scriptural exegesis and the demonstrative power of natural science. To state my thesis explicitly (and to make the intentionally provocative title of my lecture more intelligible), I hope to show that the Galileo affair was largely a result of not only legitimate but fundamental insights on both sides: namely, on the one hand, the Catholic Church's desire to guard the dignity of the human intellect by insisting on keeping the bar high when deciding whether or not a scientific theory is certain, and, on the other hand, Galileo's caution about the nuance with which we should be read Holy Writ, holding fast to the principle that it asserts nothing false, while also recognizing that it's not always asserting what a superficial reading of it might suggest. In short, Rome was upholding the Aristotelian demand for logical rigor; while Galileo, the Augustinian openness about the literal meaning of Scripture. It was at the intersection of these two concerns that the conflict arose.


Any summary of the Galileo affair has to back up, briefly, to Copernicus. (Although nothing I have to say about Galileo bears immediately on what the juniors will be reading in the science tutorial this year, unfortunately in what follows I may spoil a few surprises for the sophomores in mathematics. I apologize to your tutors in advance. Perhaps I will be vague or superficial enough that I won't do too much damage.) When, in 1543, Nicholaus Copernicus published his revolutionary work of astronomy proposing that the sun, not the earth, is at the center of the universe, he dedicated it to Pope Paul III. It was almost 100 years before Galileo's trial in Rome, and the Church then did not react to the new hypothesis, much less did it react with hostility. Indeed, Copernicus had been encouraged by several members of the Roman curia to publish his ideas. Rather than endorse or condemn the novel idea of a sun-centered, or "heliocentric," cosmos, Rome appeared to have adopted a "wait and see" attitude.

Free to spread, then, the heliocentric approach slowly began to percolate through the scientific community for the next several decades, though the majority view of educated and uneducated alike remained that the earth is at the center of the cosmos. Although it is unclear exactly when Galileo himself became a "Copernican," certainly his experience, in the first decade of the 17th century, of turning his telescope on the sun, moon, and planets did much to solidify his opinion. For immediately thereafter he popularized heliocentrism by, in 1610, publishing two little books (The Starry Messenger and The Letters on Sun Spots),that included his drawings of his telescope-enhanced observations of: spots on the sun, mountains on the moon, moon-like phases for Venus, and moon-like satellites orbiting Jupiter. Galileo's basic argument for heliocentrism in these little books was straight forward: The earth cannot be the center of every motion in the sky, because Jupiter itself has moons, and the phases of Venus make it quite clear that Venus revolves around the Sun.

All of these observations, however, Galileo knew, could be quite well accommodated to a modified geocentric hypothesis, such as had already been proposed more than a decade earlier by the Danish astronomer Tycho Brahe. (For you sophomores, I'll save the details for your math tutorial later this year.) Galileo's telescopic observations, then, in no way showed that Copernicus was right in putting the earth in motion rather than the sun. Rather, they more suggested that there was a serious mistake in the Aristotelian cosmology that divided the imperfect and corruptible matter here below from the perfect and incorruptible matter thought to compose the heavens, which cosmology was in fact tightly connected to the geocentric hypothesis. Neither a spotted sun nor a bumpy moon is perfect or incorruptible, and lunar mountains are probably susceptible to the same laws of erosion as the mountains around here. Still, this was a long way from proving that the earth revolved on its own axis or that it orbited the sun. Thus, while these discoveries made Galileo an instant celebrity throughout Europe, the majority of the academic establishment remained unconvinced by heliocentrism.

This was the situation, then, when Galileo, in 1615, left his home in Florence for an extended stay in Rome to encourage the Roman curia, and hopefully even the Pope himself, to endorse the Copernican hypothesis. For recall that the Protestant revolt had been in motion for almost exactly 100 years at this point, and Galileo's correspondent and advocate, Johannes Kepler, a Lutheran, had been recently censured and then finally excommunicated by the Lutheran clergy for his heliocentrism. The Magisterium of the Catholic Church had been silent about the new hypothesis for the better part of a century now, and Galileo was concerned that the Church not make the same mistake the Lutherans had by condemning the theory, and those associated with it (like himself). He did not meet with success. Ironically, in part as a result of the rumblings in the city caused by Galileo's recent evangelization for Copernicanism, Pope Paul V had asked the Congregation of the Holy Inquisition (or, more briefly, the Holy Office) to call together a committee of theologians to assess the orthodoxy of the heliocentric hypothesis; their conclusion was negative. In 1616 the Holy Office approved their conclusion, issuing a decree that in itself was one of the greatest scandals in Church history, namely, that heliocentrism is "foolish and absurd in philosophy" and "contrary to Holy Scripture." I will return shortly to this period to elaborate the reasons why Galileo failed to convince the Holy Office, but for now I will simply run through the basic historical facts as they unfolded here on.

At the end of Galileo's sojourn in Rome, two more disasters befell Galileo. First, Copernicus's work was put on the Index of Forbidden Books as being in need of "correction." The second fell when Galileo was summoned by (now saint) Robert Cardinal Bellarmine, Pope Paul V's chief theological advisor, whom the Pope had asked to convey privately to Galileo the decision of the Holy Office, and to order Galileo "to relinquish altogether the said opinion that the Sun is the center of the world and at rest and that the Earth moves; nor henceforth to hold, teach, or defend it in any way verbally or in writing." No doubt crushed, but apparently acquiescing in the order to remain silent, Galileo returned home to Florence.

Galileo did not, however, table his Copernican thoughts altogether. Over the next decade he continued in private to try to strengthen the argument for heliocentrism - and in no small part because he had received praise and encouragement from the newly elected Pope Urban VIII, an old friend with whom in years past he had discussed heliocentrism, and who therefore, Galileo seemed to hope, might be willing to reopen the question. In 1632 Galileo's work found its completion in the publication of a 500-page fictitious conversation between three friends, entitled A Dialogue Concerning the Two Chief World Systems. Apparently Galileo thought that in this book he was still fulfilling the letter of the injunction he had received from Cardinal Bellarmine about teaching heliocentrism as the truth. For, he probably thought, not only does the very vehicle of a dialogue ensure that the author never speaks in his own voice, but also the character in it named "Salviati" (obviously filling the role of teacher in the dialogue) several times protests that he is not trying to show that heliocentrism is true but only to give the strongest arguments on both sides of the question. But the man protests too much, as the most superficial reader can see that Salviati, the learned scientist defending heliocentrism with erudition and panache, is Galileo's mouthpiece, especially given that the second character, "Sagredo," is portrayed as open-minded and intelligent and he is quickly won over by Salviati's arguments in favor of heliocentrism, while the third, "Simplicio" (whose name in Italian as in English calls to mind "simpleton"), is painfully and even laughably stubborn in defending the geocentric view. So, in spite of the explicitly stated intent, the Dialogue is clearly an extended and systematic refutation of geocentrism and a defense of heliocentrism.

It should have been no surprise, then, when a year later, in 1633, Galileo found himself summoned to Rome to be tried by the Holy Office for having violated the terms of the 1616 injunction. After an initial resistance, by the end of the trial Galileo admitted that he was defending heliocentrism in the Dialogue, and then recanted his belief in heliocentrism. The Dialogue was then put on the Index of Forbidden Books, and, for penance, Pope Urban sentenced Galileo to house-arrest at his country home just outside Florence for what turned out to be the last nine years of his life.

During those nine years Galileo was not still inert. In 1638, he composed a second dialogue, with a title and structure that bore a resemblance to its predecessor: Discourses and Mathematical Demonstrations on Two New Sciences - the work that the juniors have begun reading in the science tutorial. Though yet another conversation between Salviati, Sagredo, and Simplicio, this work no longer pursued the heliocentric hypothesis (except in perhaps the most subtle of ways, but that's another lecture). And that silence was a good thing, as the Two New Sciences was arguably his greatest work, the one with the most lasting scientific merit, pointing the natural philosophers of his day in a direction that Isaac Newton and others would follow out to a much deeper understanding of nature as a whole, including but not limited to the dispute about whether the earth or the sun is the center of the cosmos.


Let that suffice as an outline of the string of events I am calling the "Galileo affair." In order to see more clearly what I think were the principal matters under dispute that led to Galileo's being silenced and eventually to his trial and house arrest, I want to circle back to the 1616 condemnation of heliocentrism and a crucial part of what led up to it: namely, the matter of whether Scripture itself teaches that the earth is at rest, and whether this means that science and faith are in conflict. Or to put it another way, does science prove heliocentrism, and if so, does this mean Scripture is in error?

In 1615, a year before the prohibition of Copernicus's work that set all of this in motion, Galileo and Cardinal Bellarmine had an indirect exchange of letters pertaining to heliocentrism and Scripture. Galileo was responding to a series of doubts that some of his colleagues were expressing about heliocentrism, pointing to passages in Scripture that appear to conflict with it, the most celebrated of which being when Joshua commands the sun to stop moving so as to extend the daylight hours needed for conquering the Amorites. Galileo's first response to these concerns, in the form of a letter to a friend, which he then copied and sent on to Cardinal Bellarmine, outlined his views on the relationship between Church authority and natural science. About a month later Cardinal Bellarmine responded with a letter of his own that partially agreed with Galileo's views, though it expressed caution. After reading this response, Galileo began composing a point-by-point response to Bellarmine, although he then set this aside in favor of a lengthy essay that articulated and defended in far greater detail his view of the relation between Scripture and science, especially as regards heliocentrism. This essay he circulated in the form of an open letter to Lady Christina of Loraine the Grand Duchess of Tuscany, an old friend who had nevertheless recently publicly asserted that heliocentrism was heretical because of its conflict with Joshua, chapter 10. These letters, especially the last, make it clear what the sticking point was about heliocentrism.

They also make it clear that the dispute was not about who should submit to whom, the Church to science or science to the Church. For neither Bellarmine nor Galileo entertains the possibility that science could truly prove something that is in fact contrary to the Faith; Galileo was not a fideist any more than orthodox theologians were. In short, neither Galileo nor the Church in the 17th century saw faith and reason as at war, such that the one must prevail over the other. As Galileo puts it several times in the "Letter to the Grand Duchess," "two truths can never contradict each other." As a result, although he frequently notes that the aims of Scripture and of science are distinct, and so it would seem that they would rarely have things to say about the same subjects, Galileo rejects the idea that one should have to make a choice between them, or worse, that one should just learn to live with contradictory thoughts. Such a view would ignore the fact that God is the source of revelation and of nature, and He cannot oppose His very Self, a principle on which Galileo, following a long tradition of Fathers and Doctors of the Church, insists.

Indeed, Galileo's conviction that faith and science are essentially compatible is perhaps the central premise of his understanding of their relationship. For thereby he asserts that any conflict between them is only apparent, and that, therefore, certain principles must be followed in the discernment of wherein lies the illusion. The chief among these depends upon properly assessing the weight of the scientist's position. For, as Galileo insists repeatedly in the "Letter to the Grand Duchess," not all propositions the scientist adheres to are equally certain. Calling to mind the Aristotelian distinction commonplace among philosophers and theologians of his day between scientifically demonstrated knowledge and persuasive but undemonstrated opinions, Galileo says that the Catholic should react to any apparent conflict according to the quality of the scientist's argument. For he reiterates in the "Letter to the Grand Duchess" that

even in those [physical] propositions which are not matters of faith, this authority [i.e., that of Scripture] ought to be preferred over that of all human writings that are supported only by bare assertions or probable arguments, and not set forth in a demonstrative way. This I hold to be necessary and proper to the same extent that divine wisdom surpasses all human judgment and conjecture.

Galileo's principle here is fairly cut and dry: What natural science has proved in the strictest sense is simply true, and therefore every Scripture passages that appears to say otherwise, while not false, is being misunderstood, and the theologian's task then becomes a closer scrutiny of the passages in question in order to find the other possible meaning(s). Whatever natural science has not proved in the strict sense, however, even if persuasive arguments have been offered in its defense, if it nevertheless appears to oppose Scripture, should be set aside by the believer as not merely improbable but as false.

He states the distinction, and the two courses of action, again later in the "Letter," this time with examples:

[A]mong physical propositions there are some with regard to which all human science and reason cannot supply more than a plausible opinion and a probable conjecture in place of a sure and demonstrated knowledge - for example, whether the stars are ensouled. Then there are other propositions of which we have (or may confidently expect) positive assurances through experiments, long observation, and rigorous demonstration - for example, whether or not the earth and the heavens move, and whether or not the heavens are spherical. As to the first sort of propositions, I have no doubt that where human reasoning cannot reach-and where consequently we can have no science but only opinion and faith - it is necessary in piety to comply absolutely with the strict sense of Scripture. But as to the other kind, I should think, as said before, that first we are to make certain of the fact,which will reveal to us the true senses of the Bible, and these will most certainly be found to agree with the proved fact (even though at first the words sounded otherwise), for two truths can never contradict each other. I take this to be an orthodox and indisputable doctrine, and I find it specifically in St. Augustine… [I]f what they [i.e., astronomers] say is proved by unquestionable arguments,this holy Father does not say that the astronomers are to be ordered to dissolve their proofs and declare their own conclusions to be false. Rather, he says it must be demonstrated that what is meant in the Bible … is not contrary to their proofs.

Notice two things in this passage: first, his choice of illustrations of matters of conjecture vs. matters of demonstration, and second, his introduction of precedent for his view - St. Augustine.

As to the first, Galileo takes as an example of something that can be defended but not decisively proved by natural science the notion that the heavenly bodies are moved by immaterial beings that are actually souls for those bodies. Not to go into this view in detail, I'll just say that it is as old as Aristotle, although Aristotle himself was somewhat ambiguous about what it means to call such beings "souls." Especially in the Middle Ages, but even into Galileo's time, it remained an enduring matter of dispute among philosophers and theologians whether these beings (which were by then understood to be angels) were united to the stars merely as movers, agents pushing the stars through the heavens, or in fact as souls vivifying those stars. Galileo saw both possibilities, reasonably enough, as merely conjectures, and the arguments on neither side as being demonstrative.

His examples of scientific views that had truly and unambiguously been proven are equally telling. He gives two, but I'll mention the second one first because it is more straightforward. Astronomers had long before shown that the geometry of the motions of the heavenly bodies trace out points on a sphere with the North Star at the pole. Hence, here we have a clear example of something that has been shown with demonstrations based on careful observations.

Galileo's second example of something demonstrated by natural science is directly relevant to the dispute about heliocentrism: namely, whether the earth or the heavens move. (In point of fact, he says "earth andthe heavens," but I take him to have misspoken, since no one - at that time, any way - held thatboththe earth moves around the sun and that the sun moves around the earth.) Galileo is asserting here that the dispute about heliocentrism vs. geocentrism is a matter of demonstration, something that either has already been settled by demonstrative argument and/or experiment, or we, as he puts it, "may confidently expect," will soon be settled this way. And in such a case, Galileo says, Scripture should be rethought wherever it might appear to contradict.

The second point to notice in that passage, again, is that Galileo claims his approach to Scripture is that of St. Augustine, whom he quotes more than a dozen times, frequently at great length, in the "Letter to the Grand Duchess." He also cites St. Thomas Aquinas in defense along the same lines. And although St. Augustine and St. Thomas never speak quite as explicitly as Galileo here, it seems to me that his interpretation of them is reasonable. For example, Galileo quotes the following passage from St. Augustine's Literal Commentary on Genesis,where he touches on the relation of science and Scriptural exegesis several times:

It is to be held as an unquestionable truth that whatever the sages of this world have demonstrated concerning physical matters is in no way contrary to our Bibles; hence whatever the sages teach in their books that is contrary to the holy Scriptures may be concluded without any hesitation to be quite false. And, according to our ability, let us make this evident, and let us keep the faith of our Lord, in whom are hidden all the treasures of wisdom, so that we neither become seduced by the verbiage of false philosophy, nor frightened by the superstition of counterfeit religion.

Likewise, though Galileo does not quote it, St. Thomas approvingly summarizes these points from St. Augustine by saying:

As Augustine teaches, in questions of this sort two things must be observed: First, that the truth of Scripture be held firmly, and second, that because Divine Scripture can be expounded in many ways, no one may adhere so precisely to any exposition that, if this be established with certain reason to be false, he presume to assert it to be the sense of Scripture. Otherwise Scripture may be derided by the unfaithful due to this, and the path of belief might thereby be withheld from them.

Here we see St. Augustine and Galileo of one mind in distinguishing things science has legitimately proved from things about which science has only opinion, and that any apparent conflict in the former case should involve a more reflective reading of Scripture, but any conflict in the latter case should end in a rejection of the scientist's mere opinion.

Nor by citing St. Augustine and St. Thomas is Galileo proposing an unheard of view in his own day. Rather, it seems that Cardinal Bellarmine holds essentially the same view in the aforementioned letter passed on to Galileo shortly before Galileo wrote the "Letter to the Grand Duchess." Therein Cardinal Bellarmine says that

if there were a true demonstration that the Sun was in the center of the universe and the Earth the third sphere [i.e., the third planet from the center of the cosmos], and that the Sun did not go around the Earth but the Earth went around the Sun, then it would be necessary to use careful consideration in explaining the Scriptures that seemed contrary, and we should rather have to say that we do not understand them than to say that something is false which had been proven.

Cardinal Bellarmine, however, ends this letter on a pessimistic note - perhaps even a gentle challenge to Galileo:

But I do not think that there is any such demonstration, since none has been shown to me. To demonstrate that the appearances are saved by assuming the Sun at the center and the Earth in the heavens is not the same thing as to demonstrate that in fact the Sun is in the center and the Earth in the heavens. I believe that the first demonstration may exist, but I have very grave doubts about the second; and in the case of doubt one may not abandon the Holy Scriptures as expounded by the holy Fathers.

Galileo, St. Augustine, and Cardinal Bellarmine seem to agree, then, that there are only two possible scenarios: a demonstrated proposition that requires a reinterpretation of Scripture, or a merely probable proposition that should be rejected in favor of the ordinary understanding of Scripture.

For the remainder of this lecture, then, I want to discuss both the principles of, and Galileo's particular situation with respect to, these two options, for they pose at least two questions right away. First, granting that when a rigorously demonstrated physical view appears to conflict with Scripture we should reinterpret Scripture, does that not put the Church in a situation where the meaning of a passage, and even of articles of the Creed, is always tentative and even arbitrary? Second, was Galileo really in this situation of having a demonstration for heliocentrism? Could he go beyond probable arguments and strictly speaking prove heliocentrism?


Taking up the first, let us assume for now that Galileo, or any scientist, has a necessary demonstration for some physical proposition, and that it appears to conflict with Scripture. How, then, do we go about interpreting the problematic passages? Is there any principled approach to such exegesis based in Church Tradition? Galileo himself in this matter points us again to St. Augustine for guidance. For in his commentary on the literal meaning of Genesis, St. Augustine emphasizes that the literal, or intended, meaning of a passage is not always the surface reading of it. Whence he says, in a passage that Galileo quotes,

In points that are obscure, or far from clear, if we should read anything in the Bible that may allow of several constructions consistently with the faith to be taught, let us not commit ourselves to any one of these with such precipitous obstinacy that when, perhaps, the truth is more diligently searched into, this may fall to the ground, and we with it. Then we would indeed be seen to have contended not for the sense of divine Scripture, but for our own ideas . . .

Notice that St. Augustine is not speaking exclusively of passages that appear to conflict with established science; whenever the matter is obscure, we should not insist on our own interpretation if many are compatible with the faith.

Certainly the clearest cases of where Scripture takes up obscure matters are those concerning God Himself, and therefore here most obviously one should be cautious about insisting on a particular meaning, especially when one finds such apparently inappropriate language used to describe Him. For He is frequently said to have hands and feet, and even to get angry and to repent, none of which is consistent with sound philosophy or even with other passages in Scripture that assert that God is wholly immaterial and unchangeable. Likewise, St. Augustine points out, Scripture frequently employs metaphors, idioms, perhaps even irony, or figures of speech that are simply common ways of speaking and whose surface, or you might say "word for word," meaning is not the one intended by the Sacred authors. For example, the literal meaning of the frequent Scriptural metaphor that speaks of matters or men being "in the hand of the LORD," is not that God has a physical member with five digits, or even if He does, that these things are in fact in his hand; rather, the expression means that the LORD governs and has care over this matter. Nor is this figure of speech distinctive of our ways of describing immaterial beings, for we even now speak of matters as being "in the president's hands," and no one understands this to imply that he carries such things in his digits, or even to imply that he has physical hands, since the expression would be equally applicable if the president in fact had no hands. Rather, the sense is (still) simply that the office of the president gives him power of deciding this matter.

To take another example, Psalm 95 says, "The LORD is God, the mighty God, the great king over all the gods"; one could imagine someone reading this psalm as implicitly asserting polytheism, and some modern Scripture scholars do just that, saying Judaism hadn't evolved explicitly past its polytheistic roots at the time this psalm was composed. But one need not say that that is the literal meaning, the author's intended meaning, in this psalm. The Psalmist could be using the word "gods" loosely, as an accommodation to common ways of speaking so as to include lesser supernatural beings; or he could have been speaking ironically, as if to say "Yahweh is the king of all the other so-called 'gods' of the gentiles," a figure of speech that arguably turns up in some of the prophets. And so on. In short, the literal sense is not always straightforward, and therefore, as St. Augustine says, one must be extremely cautious in asserting he has definitively found that sense, even if no apparent conflict with natural science has arisen.

St. Thomas Aquinas develops St. Augustine's view, saying that Scripture often speaks according to the limited understanding of the listener, not that of the Sacred Author. Whence, following St. Augustine, St. Thomas several times explains that some of the details of the Genesis creation story are Moses' accommodations to the servile minds (and therefore speech) of the newly liberated Hebrew slaves. For example, in discussing the firmament's division of the waters on the Second Day of creation, he says:

I respond saying that, in superficially considering Genesis according to the letter, someone could conceive such an image [of the division of the waters] according to a position of certain ancient philosophers. [He may have Thales in mind, who said all things are water or come to be from water.] … But because such a position is discovered to be false through true arguments, it cannot be said that this is the understanding of the Scripture [passage]. Rather, it should be considered that Moses spoke to an uneducated [rudi] people, stooping to their ignorance [quorum condescendens imbecillitati], [and] proposed to them only those things that appear to the senses most manifestly. All men, moreover, be they ever so uneducated, apprehend by the senses that earth and water are bodies; air, however, is not perceived by all to be a body, insofar as some philosophers said air is nothing, naming what is full of air as a 'vacuum.' And therefore Moses makes express mention of water and earth, but he does not expressly name air lest he propound to uneducated men something unfamiliar to them.

So arguably the two chief doctors of the Church agree that it is reasonable to expect that in some places Scripture will speak loosely, according to the abilities, opinions, and disposition of the original audience. This does not mean Scripture sometimes says things that are false; it does mean, however, that the surface meaning sometimes does not capture the whole truth of the matter spoken about. Although the passages are always as precise as they need to be, given the circumstances and what the author intends to assert, they are not always as precise as is possible.

In the case of passages that appear to conflict with heliocentrism, moreover, it seems possible that the surface meaning may not be the intended meaning. Indeed, perhaps we can set aside apparently conflicting passages from the Psalms or the Wisdom literature since, because of their explicitly poetical nature, one expects to find metaphors and circumlocutions there. Historical narratives, however, pose a more serious challenge, and this is no doubt part of the reason why the Grand Duchess and others thought the book of Joshua was not as easy to explain away. Consider the words of Joshua, chapter 10:

Then spoke Joshua to the LORD in the day when the LORD gave the Amorites over to the men of Israel; and he said in the sight of Israel, "Sun, stand thou still at Gibeon, and thou Moon in the valley of Aijalon." And the Sun stood still, and the Moon stayed, until the nation took vengeance on their enemies. Is this not written in the Book of Jashar? The Sun stayed in the midst of heaven, and did not hasten to go down for about a whole day. There has been no day like it before or since, when the LORD hearkened to the voice of a man; for the LORD fought for Israel. (Joshua 10:12-14)

The surface meaning here certainly is more immediately congruous with a stationary earth and a moving sun. But this might be giving the text too rigid of a reading. Surely Moses' successor as leader of the Israelites could reasonably be expected to take a page out of the books of his master, who also wrote narratives and yet also wrote with great subtlety, accommodating his speech to his audience. Is there any compelling reason to assert that Joshua was trying to make any exact astronomical assertions, even implicitly, when he addressed the sun? Rather, what Joshua wants is for the day to stop progressing toward sunset. But since daytime is defined by the relative positions of the sun and the side of the earth facing it, only their relative motion must cease in order to lengthen the day. Whence, only that much need Joshua be, even implicitly, asserting, and that much would be compatible with heliocentrism. Now, of course I do not mean to say that Joshua was proposing heliocentrism itself, any more than he was proposing the theory of relativity. Rather, he was speaking in the common idiom of his day where the motion that causes the progression of the day is articulated in terms of the sun's apparent motion across the sky. It would seem that Joshua is repudiating heliocentrism no more than is the common man - even today - who speaks of the Sun "rising" in the east or "going down" in the west.

Regardless, then, whether Joshua himself knew the truth about whether the sun moves around the earth or vice-versa and note that St. Augustine, St. Thomas, and Galileo seem to agree that, because the Sacred Author is inspired, he always understands the whole truth of his words -nevertheless, context makes it possible, and even probable, that Joshua does not here intend to teach the Hebrews a particular point about astronomy. As St. Augustine puts it, in the Genesis commentary when he wonders about the true shape of the heavens,

It must be stated very briefly that our authors knew about the shape of the sky, whatever might be the truth of the matter. But the Spirit of God, who was speaking through them, did not wish to teach people about such things that would contribute nothing to their salvation.

Thus, astronomers should not find an insuperable obstacle when, for example, the Psalmist says that the LORD "stretched out the sky like a skin" (Ps 104:2), but Isaiah says the sky is a "suspended dome" (Is 40:22), and yet the astronomers themselves argue that it is a sphere. If it is likely, or at least possible, that the wisdom literature and the prophets, at least in these passages, are not intending to speak with scientific precision, then perhaps also neither is the historical narrative of the Book of Joshua. Likewise, then, Joshua himself probably should not be seen as here settling what will one day be a disputed question in astronomy when he is in fact merely commanding a miracle so that his army can conquer the Amorites at Gibeon.

To sum up, then: according to Galileo, to insist on this principle - that Scripture has many senses, and that even the literal sense is often difficult to determine with certainty because the Sacred Authors often proportion their words to their audience, and their principal goal is not to explain the natural but the supernatural - is both a sound principle of Scriptural exegesis, and one with a long pedigree, endorsed by the greatest doctors of the Church. Whence, universally only with fear and trembling should we hold that certain passages must be read so as to exclude all other interpretations; passages that appear to have physical implications that conflict with things known rigorously through natural reason only emphasize this need for caution.


Granting, then, that Galileo's approach to deciding when and how to revisit the surface meaning of Scripture is both principled and orthodox, and that such an approach does not endanger the faith but supports it, still, are his views of what counts as a rigorous demonstration or proof equally principled and orthodox? Let us now express some doubts about our earlier assumption that Galileo is really in this situation of having a strict demonstration in hand, and therefore can insist that Scripture be reinterpreted to fit the heliocentric hypothesis. Galileo, as we saw earlier, knew full well that neither Copernicus's philosophical arguments for heliocentrism nor his own observations of spots on the sun and moons around Jupiter should have settled the matter. They were thought-provoking, powerful, and persuasive, but not demonstrative. Whence, in his draft response to Cardinal Bellarmine's aforementioned letter, Galileo had insisted that heliocentrism be held to the highest bar of demonstration:

Not to believe that a proof of the Earth's motion exists until one has been shown it is very prudent, nor do we demand that anyone believe such a thing without proof. Indeed, we seek, for the good of the holy Church, that everything the followers of this doctrine can set forth be examined with the greatest rigor, and that nothing be admitted unless it far outweighs the rival arguments. If these men [i.e., the defenders of heliocentrism] are only ninety percent right, then they are defeated…

Nor is Galileo merely being hopeful that someone someday will come up with just such a rigorous proof. Rather, in 1615 as he prepared to go to Rome, he thought he had one in hand. That proof was in the form of a today little-known and quite clever explanation he had been working out over the years of the hitherto mostly baffling phenomenon of the tides. In short, Galileo thought the only possible explanation of the tides was the Earth's motion; the tides were the "smoking gun" heliocentrism needed.

The theory is basically as follows. Galileo had observed that, when barges carrying fresh water from the mainland to Venice sped up, the water in the hold piled up at the stern (that's the back for you landlubbers), but when the barge slowed down, the water rose toward the bow (the front). He took this to mean that as a body containing water accelerates, this motion isn't immediately communicated to the water, so the water bunches up where it's forced to accumulate (because it is not moving fast enough), and the opposite happens when it decelerates.

Looking back to heliocentrism, then, Galileo inferred that if the earth orbits the sun while also rotating on its own axis, then the absolute motion of bodies on the earth would be accelerating during half the day and decelerating during the other half - accelerating when the direction of the rotational movement of one side of the earth is the same as that of the whole earth around the sun (since the two speeds would add to each other), and decelerating when the rotational motion is opposite that of the orbital motion around the sun (since the former would subtract from the latter). Rigid bodies, like land masses and barges, Galileo speculated, probably would not manifest effects of this acceleration and deceleration, but liquids would: The ocean should be acting like an accelerated liquid during roughly half the day (on arc CBL), and then like a decelerated liquid during the other half (LDC).

But, Eureka! This is exactly what we see in the tides: high tide, which is water "bunched up," and low tide, which is water troughing. Thus, Galileo thought, we have not only the explanation of the as-yet-never-explained tides and their cyclic nature, but also an effect-to-cause demonstration of heliocentrism. As he put it,

If the terrestrial globe were immovable, the ebb and flow of the oceans could not occur naturally; and … when we confer upon the globe the movements just assigned to it [by the heliocentric hypothesis], the seas are necessarily subjected to an ebb and flow agreeing in all respects with what is to be observed in them.

Without the earth both rotating on its axis and orbiting the sun, there would be no tides; therefore, on the geocentric hypothesis tides should never happen, but on the heliocentric hypothesis, they must. In early 1616, about nine months after Cardinal Bellarmine had insisted that a demonstration had to be forthcoming before Scriptural exegesis must accommodate heliocentrism, and six months after Galileo had eagerly agreed to this in the "Letter to the Grand Duchess, "Galileo began presenting this proof in Rome, both at public gatherings and in a booklet he had circulated. Cardinal Bellarmine was among one of the recipients of the written form, and he may have heard Galileo defend it orally. But less than two months later Galileo received the summons to see the Cardinal, and he was told that Copernicus's writings were being put on the Index, and that he himself should not teach heliocentrism. The proof, apparently, had been unconvincing. Indeed, when years later Galileo devoted the last part of his Dialogue on the Two Chief Worlds Systemsto explaining and defending this argument from the tides, the publication of this work again did not persuade Rome that the case for heliocentrism rose to the level of a demonstration. Why not?

Several reasons could be proposed, each with their own validity, again connected to the complexity of the "Galileo affair." I want to stress the most crucial one, though. Namely, Galileo's proof from the tides was not accepted as demonstrative because it wasn't in fact demonstrative. Moreover, it was false. In spite of its ingenious unification of two apparently unrelated phenomena - the tides and the hypothesized motion of the Earth - the theory was mistaken. Now, I don't want to be guilty of transposing common-place knowledge (or opinion, at any rate) onto men of Galileo's day. For "we all know now" (supposedly) that the tides are due to universal gravitation and the attractive power of the moon's mass. Hindsight is 20/20, but I don't think one needs the benefit of hindsight to see that there were severe defects with Galileo's theory - certainly sufficient, and even compelling, grounds to say that it fell far short of a demonstration.

Later this year you freshman will have the opportunity to study in great detail the conditions required for a strict demonstration laid out in Aristotle's Posterior Analytics, and I have neither the time tonight nor, probably, the ability to present them to you in the manner the subject deserves. But one does not need a detailed grasp of strict demonstration to make my argument here. For at minimum any effect-to-cause demonstration must be based on an adequate grasp of the effect to be explained, and in this respect Galileo's tide argument is lacking.

For perhaps the most obvious sign that the theory was not correct, much less demonstrated, was that it predicts only one tide-cycle per day: one high and one low tide in 24 hours. Long before Galileo's day it was well known to mariners and to anyone who might take the time to notice that there are two tide cycles, although apparently Galileo did not know this when he first worked out the tide argument before 1615. When his fellow Italians pointed out the double cycle to him, Galileo was undeterred, insisting that this must be due to the shape and depth of the Mediterranean sea floor, and added that in other parts of the world, under less complex circumstances, we will find only one tide cycle per day. He then let the matter rest without further inquiry for over a decade.

However, when in 1629, while he was nearing completion of the Dialogue on the Two Chief World Systems,which culminated in a lengthy explanation of the tide argument, he received a letter from the Tuscan ambassador to Spain, whom he had contacted to inquire about the tides on the Atlantic coast and elsewhere. The ambassador, having consulted extensively with mariners in Spain, assured Galileo that everywhere the tides followed a 12-hour cycle, two a day. Galileo's confidence in his argument was apparently unshaken, however, and he again convinced himself that the second tide cycle each day must also be due to the peculiarities of the ocean floor in the Atlantic and Pacific.

Several other problems plague this tide theory in itself, but I'll mention only one other, one concerning the true cause of the tide cycle. For Galileo's theory requires that high tides and low tides should be locked into the time of day, such that high tide occurs only at night, peaking around midnight, and low tide only during the day, peaking around noon. But no such connection between the time of day and the tide cycle was evident. Rather, any experienced mariner or careful observer in Galileo's day knew that the tide cycles bear some connection to the lunar cycle. Indeed, more than a dozen years before Galileo published the Dialogue,Kepler published a basically accurate theory of the tides associating them exclusively with the position of the Moon, not the Earth's dual motion. In the Dialogue, however, Salviati (Galileo's alter ego) dismisses Kepler's lunar tide theory:

But among the great men who have philosophized about this remarkable effect [i.e., the tides], I am more astonished at Kepler than at any other. Despite his open and acute mind, and though he has at his fingertips the motions attributed to the earth, he has nevertheless lent his ear and his assent to the moon's dominion over the waters, to occult properties, and to such puerilities.

So Galileo not only was aware of what we now consider the true account, but rejects it offhand, apparently because of a philosophical objection to the obscurity of the sort of causality Kepler was allowing for.

Such a single-minded commitment to his own position is perhaps particularly surprising, given that Galileo begins his presentation of the tide argument as follows:

In questions of natural science like this one at hand, a knowledge of the effects is what leads to an investigation and discovery of the causes. Without this, ours would be a blind journey, or one even more uncertain than that… Hence before all else it is necessary to have a knowledge of the effects whose causes we are seeking. … And though in other seas remote from us events may take place that do not occur in our Mediterranean, nevertheless the reason and cause which I shall produce will still be true, provided that it is verified and fully satisfied by the events which do take place in our sea; for ultimately one single, true, and primary cause must hold good for effects which are similar in kind.

In spite of this admission that we must be subject to the observable phenomena in inferring their causes, Galileo is willing to set aside several large and readily observable phenomena that are serious obstacles to his alleged demonstration.

These passages are, of course, not flattering, especially with the benefit of hindsight. But my goal here is not to make Galileo look the fool for the harshness of his assessment of Kepler's tide theory, and I certainly do not want to undermine you juniors taking seriously the Two New Sciences this semester. Rather, I am trying to manifest how committed Galileo was to his intuition both that heliocentrism is true and that this proves it, to the point of being blinded to entertaining significant objections to his proof. Perhaps the sheer ingeniousness of the theory makes is seductive in a way.

Perhaps another reason was Galileo's conviction that scrutinizing his own proof was not his job. For he had said before, in the "Letter to the Grand Duchess," that if a scientist is convinced he has a demonstration in hand, then it is up to others to show him that he is mistaken.

Again, to command that the very professors of astronomy themselves see to the refutation of their own observations and proofs as mere fallacies and sophisms is to enjoin something that lies beyond any possibility of accomplishment. For this would amount to commanding that they must not see what they see and must not understand what they know. … Now if truly demonstrated physical conclusions need not be subordinated to biblical passages, but the latter must rather be shown not to interfere with the former, then before a physical proposition is condemned it must be shown to be not rigorously demonstrated - and this is to be done not by those who hold the proposition to be true, but by those who judge it to be false.

But surely it is strange to say that one is not obliged to be self-critical when one thinks he has proof; intellectual honesty and even the desire for certainty argue the other way. At any rate, Galileo's audience was apparently trying, in some measure, to give his argument the cross-examination on which he was insisting, at least insofar the tide argument amounted to a demonstration. For several skeptical friends and acquaintances challenged Galileo precisely as regards the tide argument's conflict with observations.

In particular, to review, Galileo had given, in 1616 before the edict, copies of his tide argument to both Pope Paul and Cardinal Bellarmine, and yet they had been unconvinced. Likewise, in 1629 Pope Urban himself, an old friend of Galileo who had been familiar with the tide argument for years, repeatedly tried to dissuade him from including, or at least focusing on, it in his forthcoming Dialogue;Galileo acquiesced only insofar as he didn't entitle the Dialogue on the Two Chief World Systems as a Dialogue on the Tides, which had been his original plan. Although there is no surviving record of exactly what Pope Urban's reservations were - other than his repeated assertion that Galileo wasn't offering a demonstration (so heliocentrism should be considered only a hypothesis) - one wonders whether his objections had something to do with some of the more obvious problems I've mentioned. At least this much is clear: Neither Cardinal Bellarmine and Pope Paul in 1616, nor Pope Urban in 1633, thought the tide argument rose to the level of demonstration, while Galileo implied that it did. It seems that either Rome had a clearer understanding of what a scientific demonstration should look like than did Galileo, or that Galileo was too close to his tide-argument to see its weaknesses, or perhaps both.

Eventually, however, Galileo saw his mistake: When called to the 1633 trial, Galileo reread his Dialogue and finally admitted that he found the tide argument to be no proof at all, attributing his inability to see this before to the complacency and love one often has for one's own ideas. As he put it, "My error, I confess, has been one of vainglorious ambition, pure ignorance, and inadvertence." So eventually he could see that the skeptics, and Rome in particular, had a point; what he had thought was a scientific demonstration was no such thing, and was probably altogether false. And ironically, in spite of a disastrous judgment about heliocentrism's opposition to the faith, nevertheless, in this particular respect, the Roman Inquisition, not Galileo the scientist, had been on the side of science and the truth about nature.

In conclusion, then, we see that one of the chief elements of the conflict that led both to the 1616 silencing of Galileo and to his 1633 trial pertained to the relationship between how to read Scripture when science appeared to conflict with it. On the one hand, Galileo espoused sound, Augustinian, principles of Scriptural exegesis, especially when Scripture appears to conflict with a rigorously demonstrated physical theory. On the other hand, Rome defensibly insisted that Galileo had no such demonstration of heliocentrism, without which, both sides agreed, Scripture's superficial meaning should not be challenged. To both Galileo and the Holy Office in the 17th century (or at least to Cardinal Bellarmine), these seem to be the only two possible approaches: Demonstrate the theory and then be more careful and open minded in interpreting the relevant passage of Scripture, or stick with the surface sense of the passage.

But might there be a middle path? Let me end with that as a question: Might the evidence backing up a new scientific theory fall short of a demonstration, and appear to conflict with the common reading of certain passages of Scripture, and yet theologians and the Magisterium still be ready to entertain, albeit not insist upon, new interpretations of those same passages as to allow for the theory's being possibly true? My hunch is that, with the proper qualifications, the answer could be "yes." But perhaps we should try to work that out together in the question-and-answer period.