Chapter 4
Re-assessing Copernicanism (1609–1616)
The Effect of the Telescopic Discoveries
In 1609, Galileo heard about an optical instrument invented in Holland the year before, consisting of an arrangement of lenses that magnified images three to four times. Without having a prototype in his possession, he was soon able to duplicate the instrument, mostly by trial and error (see Figure 9). He was also able to increase its magnifying power first to 9, then to 20, and, by the end of the year, to 30. Moreover, rather than merely exploiting the instrument for practical applications on Earth, he started using it for systematic observations of the heavens, to learn new truths about the universe.
Figure 9. Galileo’s original telescopes, in a previous display at Museo Galileo, Florence
Within three years, Galileo made several startling discoveries. The Moon had a rough surface full of mountains and valleys, similar to land on the Earth (Figure 10). Innumerable other stars existed besides those visible with the naked eye. The Milky Way and the nebulas were dense collections of large numbers of individual stars. The planet Jupiter had four moons revolving around it at different distances and with different periods. The appearance of the planet Venus, in the course of its orbital revolution, changed regularly in a manner analogous to the phases of the Moon: from a full disc, to half a disc, to crescent, and back to a half and a full disc (Figure 111). And the surface of the Sun was dotted with dark spots that were generated and dissipated in a very irregular fashion and had highly irregular sizes and shapes, like clouds on Earth; but that while they lasted, these spots moved regularly in such a way as to imply that the Sun rotated on its axis with a period of about one month (Figure 12).
Figure 10. Appearance of the Moon through Galileo’s telescope
Figure 11. Galileo’s observation of phases of Venus, from Palmieri (2001)
Figure 12. Sunspots as observed by Galileo on June 2, 1612
There is no question that the telescopic discoveries led Galileo to a significant reappraisal of Copernicanism. Less easy to ascertain is what the change was exactly, how sudden or slow it was, and what the precise motivating reasons were. The first piece of significant evidence here is the book he was quick to publish in 1610, The Sidereal Messenger. There are three relevant passages in this work.
One occurs in the author’s dedication to the Grand Duke of Tuscany. Since this is often taken as the first published evidence that Galileo accepted Copernicanism, it is important to be especially careful. At this point in the dedication Galileo is calling attention to the satellites of Jupiter and to the fact that he had named them Medicean stars, after the ruling family of Tuscany. The paragraph reads as follows:
Behold then, reserved for your famous name four stars, belonging not to the ordinary and less-distinguished multitude of the fixed stars, but to the illustrious order of the wandering stars; like genuine children of Jupiter, they accomplish their orbital revolutions around this most noble star with mutually unequal motions and with marvelous speed, and at the same time all together in common accord they also complete every twelve years great revolutions around the center of the world, certainly around the sun itself.2
The final clause of this passage is the expression often taken as evidence of commitment to Copernicanism. But that is done by translating the original Latin to read, or by interpreting it to mean “around the center of the world, that is the sun.”3 However, I believe it is more correct to have it the way I rendered it above, which embodies a certain ambiguity. That is, a body that follows Jupiter in its orbit is likewise encircling the smaller orbit of the Sun, and in that sense is moving around the Sun; and this is certain, with both sides of the controversy agreeing. But the clause could also mean that Jupiter and its satellites move around the Sun as center, and then we would have some acceptance of Copernicanism. At any rate, this would involve directly only the heliocentricity of planetary motions, and not necessarily the other elements of the system (such as the Earth’s motion). So, although there is no question that Galileo is expressing a favorable attitude toward Copernicanism, and that he is now involved with its astronomical aspects, any more precise definition of his stance is elusive.
This conclusion corresponds very well with the actual content of The Sidereal Messenger, in which we find only two places where the status of Copernicanism is discussed explicitly. They both involve rebuttals of traditional anti-Copernican objections.
In one passage in the middle of the book, Galileo indicates he is now in the position of being able to answer the objection that the Earth cannot be a planet because it is devoid of inherent light; his telescopic discoveries about the optical properties of the Moon enable him to say that the Earth is not essentially different from the Moon in this respect. More generally, he thinks his lunar discoveries are such “that the connection and resemblance between the moon and the earth may appear more plainly”;4 that is, these discoveries show the untenability of the Earth–heaven dichotomy, and so undermine the anti-Copernican objection from the Earth–heaven dichotomy.
The other passage occurs at the end of the book. There, Galileo explains that the ability of Jupiter’s satellites to keep up with Jupiter as it revolves in its orbit allows one to refute the traditional lunar-orbit objection. This was the argument that the Earth cannot revolve around the Sun because the Moon clearly orbits the Earth, and so would be left behind.
Of course, these refutations of objections do not prove Copernicanism; they merely strengthen it. The attitude expressed by Galileo may be described as indicating a more direct, active, and explicit kind of pursuit, and a higher degree of favorable appraisal than before.
Finally, there is an important piece of negative evidence in The Sidereal Messenger, regarding something which Galileo does not say or do. In the printed book he dropped a clause which he had written down in the manuscript draft. This occurs at the end of the discussion of Jupiter’s satellites, near the end of the book. The manuscript draft speaks of “the Copernican system (which above all I judge to be consonant with the truth),”5 but the printed book lacks the parenthetical remark. Clearly, in the printed book, Galileo was being more cautious in his endorsement of Copernicanism.
The next significant document for this period is Galileo’s letter to Giuliano de’ Medici, Tuscan ambassador to Prague, dated January 1, 1611. Its main purpose was to decipher the anagram Galileo had sent him in an earlier letter. When properly transposed the anagram stated that Venus shows phases like the Moon, a phenomenon Galileo had been able to observe with the help of the telescope. The attitude he displays now is not, as some scholars have alleged, complete acceptance of the Copernican system, but rather acceptance of two specific theses in it. In the text this is as clear as his emphasis on empirical accuracy:
From this marvelous observation we have sensible and certain demonstration of two great questions, which so far have been debated by the greatest minds of the world: one is that planets are all dark (since the same thing happens with Mercury as with Venus); the other is that Venus necessarily revolves around the sun, as do also Mercury and all other planets.6
He goes on to describe the change in his attitude as one from belief without, to belief with, empirical proof: “This had indeed been believed by the Pythagoreans, Copernicus, Kepler, and myself, but not sensibly proved, as done now for Venus and Mercury.”7 This change should not be equated with that from pursuit to acceptance; rather it is a change from one kind of acceptance to another, the difference being the grounds for the acceptance; in other words, we have a development from a situation in which acceptance was based on factors other than empirical adequacy, to a situation in which it was based on empirical evidence. On the other hand, non-empirical acceptance should be distinguished from an attitude of nonbelief, pursuit, or exploration. In fact, Galileo makes the latter distinction when he goes on to congratulate Kepler and other Copernicans for their pre-telescopic intuitions, but does not include himself: “Thus, Mr. Kepler and the other Copernicans will have reason to be proud for their having believed and philosophized correctly.”8
The next important milestone in our story is a letter to Prince Federico Cesi, a wealthy aristocrat with an amateur interest in science; he was the founder and head of the Lincean Academy, the first international scientific society, into which Galileo himself was inducted in 1611. On June 20, 1612, Cesi had written to Galileo, expressing his attraction to Copernicanism because of its doing away with epicycles and eccentrics; he also asked Galileo’s opinion on the problem that these seemed unavoidable for the case of the terrestrial and lunar orbits, given the periodic changes in distance between the Earth and the Sun, and between the Earth and the Moon.
Ten days later Galileo replied that “we must not desire that nature should accommodate herself to what seems better arranged and ordered to us; rather it is appropriate that we should accommodate our intellect to what she has done, certain that this and nothing else is the best.”9 He then goes on to apply this principle by arguing that if by epicycles we mean orbits not encompassing the Earth, then we must admit their reality, examples being the revolutions of Jupiter’s satellites around that planet, and the orbits of Venus and Mercury around the Sun; and if eccentrics are meant relative to the Earth, then the orbit of Mars encompassing the Earth is a clear example, since the telescope reveals that its apparent magnitude is 60 times greater at certain places of its orbit than at others.
Galileo is here explicitly expressing some theoretical limitations to the principle of simplicity, as well as practically utilizing it in a particular case. This is in accordance with his behavior during the pre-telescopic period. At that time, as we have seen, Galileo did not attach enough weight to the greater simplicity of the Copernican system either to accept it or to ground his pursuit on that. This is not to say that he completely disregarded the criterion of simplicity; only that he did not attach significant or decisive weight to it.
Let us now turn to the History and Demonstrations Concerning Sunspots, which was written in 1612 and published in 1613. This work is widely recognized as containing the strongest endorsement of Copernicanism that Galileo ever published, either before or after this date. What is less well known and seldom discussed is the exact form and context that this endorsement takes. It occurs at the very end, in a passage written on December 1, 1612.
There, Galileo finishes with the topic of sunspots about three pages from the end, by stating his main conclusion that the spots are on the Sun and made of some kind of volatile substances like clouds on Earth. Then he goes on to report some observations about the planet Saturn. Galileo was actually seeing what we now know to be rings around this planet, although he was never able to formulate this correct interpretation. When he first observed them in 1610, he thought he was seeing Saturn as consisting of three distinct but interconnected bodies. However, in this passage, he now reports that Saturn no longer appeared as three-bodied.
Galileo admits his puzzlement and confusion, but goes on to make several very daring and rather precise predictions about the periodic reappearance and re-disappearance of Saturn’s “companions,” up to the summer of 1615. He does not, however, reveal the conjecture on which he says he based these bold predictions, but promises to do so later, after events would confirm or disconfirm him. He concludes his discussion as follows:
As few doubts as I have about their return, I am proceeding with restraint in regard to their particular features, for these are based at present on probable conjecture alone. But whether these things take place precisely in this fashion or in another, I say to Your Lordship that this star, too, and perhaps no less than the emergence of horned Venus, agrees in a wondrous manner with the harmony of the great Copernican system, to whose universal revelation we see such favorable breezes and bright escorts directing us, that we now have little to fear from darkness and cross-winds.10
Two main claims in the last part of this quotation are worth noting. The first is that perhaps Saturn’s behavior too confirms Copernicanism. Despite the fact that Galileo never did reveal the theoretical basis of his prediction and the connection with Copernicanism, and despite the fact that that basis remains a puzzle for scholars, the judgment is obviously based on the criterion of empirical accuracy.
The second claim is that now he thinks all evidence is pointing toward Copernicanism and seems to have little doubt about its correctness. Although Galileo does not explicitly include sunspots in this evidence, the connection is obvious enough that it can easily be attributed to him. For as he will argue later in the Dialogue, sunspots contribute to the empirical undermining of the Earth–heaven dichotomy, and thus to the strengthening of Copernicanism. Yet, the Sunspots book does not contain one of the most powerful pro-Copernican arguments—the argument based on the annual cycle of sunspot paths, which, as we shall see (Chapter 6), is persuasively advanced in the Dialogue. Although at this time (1612–13) Galileo was in a position to observe the annual motion of sunspots, and to formulate the geokinetic explanation of that phenomenon and the corresponding argument, there is no evidence that he did either until 1629–31, while in the process of writing the Dialogue.
In the Postscript that was added to the published version of the Sunspots book, there is another important clue that at about this time Galileo was finding another piece of evidence which could be explained only on the basis of the Earth’s orbital revolution. The phenomenon involved the eclipsing of Jupiter’s satellites and the variations in the duration of these eclipses. The details of the argument were never written up by Galileo, and they are extremely technical. Scholars have tried to identify the relevant documentary evidence and to piece together the main points.11 However, Galileo’s claim in the Postscript is as clear and unambiguous as one could wish, namely that such eclipsing is evidence for the Earth’s annual orbital revolution.
The Sunspots book also contains two other passages that are relevant to understanding Galileo’s re-assessment of Copernicanism. One is the theory that the Sun rotates on its axis, which represents a modification (however slight) of Copernicus’s original system. The other is a criticism of an argument given by Jesuit astronomer Christoph Scheiner to show that Venus revolves around the Sun; of course, Galileo accepts this conclusion, but he finds several faults with Scheiner’s attempt to ground it on the alleged observation of a transit of Venus across the solar disk, rather than on the phases.12
Both passages show a piecemeal attitude that might be called methodological gradualism, as contrasted to methodological holism. That is, Galileo does not hesitate to modify various elements of the Copernican system, or to reject proposed contributions by other supporters, as the case requires. This suggests, it seems, a greater commitment to certain procedures than to any specific physical or cosmological theses; for the case of solar rotation the procedure in question would seem to be related to empirical accuracy, while for Scheiner’s argument about Venus it would seem to involve correct reasoning.
Galileo’s re-assessment of Copernicanism comes to a climax in 1614, as we can see in his letter to Giovanni Battista Baliani dated March 12. Baliani was a government official of the Republic of Genoa who was engaged in serious research on the physics of falling bodies, and his contributions overlapped somewhat with Galileo’s. In this letter, we find for the first time a genuine expression of certainty, together with a summary of his reasons, as well as a reasoned rejection of Tycho’s theory. The crucial passage is the following:
As regards Copernicus’s opinion, I really hold it as certain, and not only because of the observations of Venus, of sunspots, and of the Medicean planets, but because of his other reasons, and because of many other particular reasons of mine, which seem conclusive to me … In Tycho’s opinion I still find all the very great difficulties which make me abandon Ptolemy, whereas in Copernicus I find nothing which gives me the least scruple, and least of all the objections which Tycho makes to the earth’s motion in certain letters of his.13
There is no question that we have here an endorsement of Copernicanism stronger than any we have seen up to this date, and stronger than we find anywhere else subsequently. And it is clear that Galileo uses labels that are epistemically loaded: “certain” to characterize the position, and “conclusive” to describe the supporting arguments. Nevertheless, I do not think we have here a qualitative jump from what came immediately before; it is a change of degree. Besides, the notions of certainty and conclusiveness are used loosely in this kind of informal context. And in any case Galileo’s words must be balanced against other considerations.
For a start, this is a private letter, and there is no analogous expression in any of Galileo’s published works. Also, Galileo was not at this time writing the work on the “System of the World” which he had promised in The Sidereal Messenger. Although there were many causes for this delay, including external ones, one contributing factor may have been that he did not have yet all the arguments and evidence required to be really sure, as he was claiming in this letter; so the crucial words here are perhaps merely that—words.
And there is another, more complex consideration. This alleged certainty and conclusiveness may be no more literally true than is Galileo’s assertion in the same passage that he finds nothing in Copernicus which gives him “the least scruple.” In fact, Galileo had several qualms about Copernicanism. One scruple was that, as we have just seen, solar rotation represents a departure (however minor) from Copernicus. Moreover, and more significantly, Galileo never accepted Copernicus’s “third motion,” according to which the terrestrial axis was supposed to “precess” (wobble) with an annual period in order to compensate for the orbital revolution and thus enable the axis to remain always parallel to itself; instead, as the later Dialogue clearly and plausibly explains, the constant parallelism of the Earth’s axis is an instance of rest (or inertia, as we might say). Another qualm was that, as the Dialogue also shows (and as we shall see later), Galileo never refuted the objection from stellar parallax, but rather accepted the difficulty, and suggested a way of testing for it.
Despite all these provisos, the endorsement neither can nor should be ignored. So let us look at the motivating reasons. Notice that Galileo speaks of three groups of reasons: first, the observational ones depending on the telescope and involving the phases of Venus, sunspots, and Jupiter’s satellites; second, Copernicus’s own reasons; and third, Galileo’s own “other particular reasons.” The first group were the chronologically latest ones which had been accumulating in the past several years since the telescope, and which seem to have caused a qualitative change in Galileo’s attitude, from something like indirect pursuit or qualified rejection to something like direct pursuit or qualified acceptance. These reasons relate intimately to the principle of empirical accuracy.
The second group (Copernicus’s own reasons) were never, not even in the Dialogue, discussed by Galileo in any great detail. We do know, as we saw above, that for Galileo they were not primarily simplicity considerations, at least not in any simple sense of the notion of simplicity. Instead, I believe that they are best seen as considerations of explanatory coherence or against ad-hocness. Galileo was sensitive to the importance of explanatory coherence, however insufficient he may have regarded it as a basis of acceptance, or even as a sole basis of pursuit. And in view of Galileo’s qualms about the naïve application of the principle of simplicity, notice that this conception of Copernicus’s reasons is also different from the simplicity interpretation.
Finally, Galileo also had a third group of reasons, and these he had had for a very long time. Collectively, they reduced, as we have seen already, to the fruitfulness of his theory of motion, a theory quite compatible with Copernicanism, and quite at odds with the Aristotelian–Ptolemaic system. Specifically, they included not only the argument from neutral motion (discussed in Chapter 3), but also the argument based on a geokinetic explanation of the tides (which we will consider later). Since Galileo claims he has “many” such arguments, one wonders what else he had in mind.
One clue to what they might have been lies in Galileo’s remark about the Tychonic system. He says he has the same basic difficulties with it as with the Ptolemaic system. These can only be problems stemming from his theory of motion, or as we would say, dynamical difficulties. They are sketched and summarized at the beginning of the Second Day of the Dialogue. Two points are especially pertinent here. First, these objections apply with equal force to both the Tychonic and the Ptolemaic versions of the geostatic system, and so the often-heard criticism that Galileo is guilty of neglecting the Tychonic system is without foundation. Galileo was not insensitive to the need to appraise theories in the light of rival alternatives, and his behavior shows that he appraised them not just vis-à-vis empirical data, but also on a comparative basis. Second, those anti-geostatic objections are explicitly labeled as probable and not conclusive by Galileo, and this helps us to resolve one last question about the present passage.
Here Galileo judges the strength of his reasons as “conclusive.” The actual expression and punctuation he uses (“which seem conclusive to me”) is ambiguous, since the conclusiveness could be referring to his third group of reasons individually, or his third group of reasons as a whole, or all his three groups of reasons collectively. I believe that the last one is meant, as suggested by the just-mentioned passage in the Dialogue (beginning of the Second day). But one difficulty now remains. It is this. Since the tidal argument is obviously included in the third group, could he not here be attributing conclusiveness to the tidal argument? It is conceivable that he might, but in fact he is not. It can be argued that the tidal argument is presented in the Dialogue as an inductive, probable, hypothetical, non-necessitating argument, but the Dialogue was written after the anti-Copernican decree of 1616, so Galileo had external motives for such a presentation. We need to examine the tidal argument in the version found in his “Discourse on the Tides,” written in January 1616, two months before the Decree of the Index.
An important but often overlooked fact about the “Discourse on the Tides” is that it is not just about tides, but also about winds. In other words, we have not one but two arguments for the Earth’s motion, the first based on the tides, the second on prevailing easterly winds, the so-called trade winds. This immediately suggests that neither one is considered to be absolutely conclusive, for the argument from the trade winds would be superfluous if the tidal argument were conclusive, and vice versa. It is true that in the twenty pages or so of this essay, only about two at the end are devoted to the wind argument, but that only means that the topic of tides involves more details and that the argument has more complications. So if length were at all relevant, that might actually weaken it by exposing it to more potential difficulties or errors. Moreover, degrees of strength may be a function of length, but absolute conclusiveness is not because it would lead to the absurdity of labeling the tidal argument both absolutely conclusive and also nine times stronger than the wind argument.
This is reinforced by frequent explicit remarks on Galileo’s part. For example, after listing several possible causes of the motion of water in general, he introduces the connection between tides and the Earth’s motion with words that indicate tentativeness on his part. This tentativeness is typical of the rest of this essay, but we shall limit ourselves to a concluding note on the last page of the essay, which is simply too revealing to be ignored:
I could propose many other considerations if I wanted to delve into finer details. Many, many more could be advanced if we had abundant, clear, and truthful empirical reports of observations made by competent and diligent men in various places of the earth; for by comparing and collating them with the assumed hypothesis we could decide more firmly and ascertain more correctly the things that pertain to this very obscure subject. At the moment I only claim to have given something of a sketch, suitable at least for stimulating students of nature to reflect on this new idea of mine. I hope, however, that it does not turn out to be delusive, like a dream which gives a brief image of truth followed by an immediate certainty of falsity. This I submit to the judgment of intelligent investigators.14
In conclusion, there is no doubt that the certainty expressed by Galileo in the letter to Baliani was an inductive and practical, rather than an absolute, kind of certainty; it was based on the practical conclusiveness of all the arguments taken together, physical, telescopic, and Copernicus’s original ones. Certainly he did not think that any one individual argument or piece of evidence was absolutely conclusive.
This analysis also fits very well with another series of relevant remarks in another key document of the period, Galileo’s Letter to the Grand Duchess Christina (1615). The main purpose of this letter, as we shall see later, was to try to defend the Copernican system from the scriptural objection; in other words, to explore whether Copernicanism is really incompatible with Scripture. However, in the letter’s introductory part, to set the stage for the discussion, Galileo describes his attitude toward Copernicanism.
Galileo tells us, “in my astronomical and philosophical studies, on the question of the constitution of the world’s parts, I hold that the sun is located at the center of the revolutions of the heavenly orbs and does not change place, and that the earth rotates on itself and moves around it.”15 This is a clear and explicit statement of endorsement, but the strength and nature of this endorsement must be inferred from other statements. There are two sets of relevant statements: those that are meant to clarify his relationship to Copernicus, and those that are intended to explain how Galileo’s cosmological position relates to his own astronomical discoveries.
By the latter it is obvious that Galileo is referring to such things as the lunar mountains, Jupiter’s satellites, the phases of Venus, and sunspots. Although these were questioned at first, he now regards their existence and main features as conclusively proved, for he notes with pride that “then it developed that the passage of time disclosed to everyone the truths I had first pointed out.”16 By contrast, about the geokinetic hypothesis, Galileo says that
I confirm this view not only by refuting Ptolemy’s and Aristotle’s arguments, but also by producing many for the other side, especially some pertaining to physical effects whose causes perhaps cannot be determined in any other way, and other astronomical ones dependent on many features of the new celestial discoveries; these discoveries clearly confute the Ptolemaic system, and they agree admirably with this other position and confirm it.17
The key notion here is that of confirmation. He seems to regard the Copernican position as confirmed. What does this mean?
That it does not mean conclusively proved is shown by Galileo’s understanding of his relationship to Copernicus. On the next page of the letter’s introductory part, in the context of discrediting some of his opponents who thought that the geokinetic idea was Galileo’s invention, he clarifies that “Copernicus was its author, or rather its reformer and confirmer.”18 The same terminology of confirmation is used. Thus, it is obvious that Galileo thinks he is doing more of the same of what Copernicus did. There is no claim of a breakthrough from Copernicus’s mere confirmation to his own strict demonstration.
There is, of course, a strengthening of the position, which Galileo describes with the words that now “one is discovering how well-founded upon clear observations and necessary demonstrations this doctrine is.”19 He does not say that the Earth’s motion is now clearly observed and necessarily demonstrated, but that it is well founded on them. The necessary demonstrations referred to must be those that prove the truth of his celestial discoveries mentioned earlier. There is no problem, of course, about a long and complex probable proof, such as that supporting the Earth’s motion was for him at that time, consisting partly of segments that are necessary demonstrations or clear observations, because the final conclusion would be only as strong as the weakest supporting subargument. And Galileo clearly realizes this since, apropos of Copernicus, one remark he makes is that parts of his work too consist of clear observations and necessary demonstrations. That is, rather than getting involved in biblical interpretation, Copernicus “always limits himself to physical conclusions pertaining to celestial motions, and he treats of them with astronomical and geometrical demonstrations based above all on sense experience and very accurate observations.”20
To summarize, in the post-telescopic period, Galileo came to regard the geokinetic theory, compared to the geostatic theory, as much better supported by astronomical, physical, and philosophical arguments and evidence. This re-assessment represented a kind of reversal of his pre-telescopic judgment. The main factor that tipped the balance was the telescopic astronomical discoveries, which provided answers to almost all astronomical objections to Copernicanism and some positive support for the Earth’s motion. However, even after the telescope he was not insensitive to the existence of some unanswerable objections to Copernicanism and to the fact that it had not yet been conclusively proved, although he regarded the case in favor of the Earth’s motion as very strong and increasingly more convincing, and the case for the Earth’s rest as vanishingly weak. Similarly, in the pre-telescopic period, although he regarded the case for Copernicanism very weak, and the case for the geostatic system as overwhelming, he was not blind to the existence of some good pro-Copernican arguments, such as the physical arguments of his own invention.
For a complete re-assessment of Copernicanism, besides the astronomical, mechanical, and epistemological arguments, one had to examine the scriptural and theological objections. Galileo was well aware of this, but had little inclination to do so, for various reasons. However, little by little, and willy-nilly, he was dragged into the latter discussion as well, with fateful consequences.
Scripture vs. Copernicanism
Scriptural criticism of Copernicus’s theory was immediate. We now know this from the censure of his Revolutions (1543) written by Dominican friar Giovanni Maria Tolosani in 1546–7. Indeed, such criticism even antedated the publication of Copernicus’s masterpiece, since his ideas had circulated earlier through his own unpublished writings and the publications of some followers. For example, in an incidental remark in 1539, Martin Luther criticized Copernicanism as incompatible with the biblical passage in Joshua 10:12–13.21 An even more negative assessment is found in the preface to the second edition (1541) of a Copernican book by a friend and former student of Copernicus’s named Georg Joachim Rheticus; in that preface, Rheticus included a letter by one of his own friends praising the Copernican theory, but also expressing the worry that clergymen will judge it to be heretical.
It is also well known that the criticism continued, for scriptural objections were usually included in discussions of the status of heliocentrism. However, it was not until Galileo’s telescopic discoveries in 1609–12 that the problem became a crisis. I believe the key reason for this crisis was that, as we have seen, these discoveries entailed a major re-assessment of Copernicanism: they suggested that the Earth’s motion and its off-center “heavenly” location could now be regarded as real possibilities and not merely convenient instruments of astronomical calculations and predictions, and indeed as more likely to be true than the alternatives, although they did not provide a conclusive demonstration of the physical truth of the Copernican theory.
Galileo’s Sidereal Messenger left the printing press in March 1610. Three months thereafter, Martin Horky published A Very Short Peregrination Against the Sidereal Messenger. A few months after that, Ludovico delle Colombe compiled an essay, “Against the Earth’s Motion,” that included theological objections; it circulated widely, but was left unpublished. The following year, Francesco Sizzi published in Venice a book objecting on scriptural grounds to Galileo’s discovery of the satellites of Jupiter. In 1612, Giulio Cesare Lagalla, professor of philosophy at the University of Rome, published a book disputing Galileo’s lunar discoveries.
By the summer of that year, Galileo was worried enough that he asked Cardinal Carlo Conti for advice on whether Scripture really favors Aristotelian natural philosophy and contradicts Copernicanism. Conti was an influential churchman in Rome and replied promptly in two thoughtful letters. His views can be summarized as follows. It is the Aristotelian doctrine of heavenly unchangeability that contradicts Scripture and the common opinion of Church Fathers. However, it will take some time to determine whether the new discoveries establish heavenly changeability since, for example, some will try to explain sunspots in terms of swarms of small planets circling the Sun. The scriptural contradiction of the Aristotelian thesis of the eternity of the universe is even more obvious. In regard to the Earth’s motion, if one is talking about straight motion downwards, there is no difficulty with Scripture. If one is talking about the Pythagorean or Copernican circular motions, it is less conforming to Scripture, although passages attributing stability to the Earth could be interpreted as attributing perpetuity to it. If one is referring to the Sun and heavens not moving, then the scriptural passages stating the opposite could only be interpreted as accommodating the popular manner of speaking, but such an interpretation should not be adopted “without great necessity.”
On November 2, 1612, in a private conversation, Dominican friar Niccolò Lorini attacked Galileo for being inclined to heresy by believing ideas, such as that the Earth moves, which contradict Scripture. However, on November 5, Lorini wrote Galileo a letter of apology. In the fall of 1613, Ulisse Albergotti published a book, Dialogue … in Which It Is Held … That the Moon Is Intrinsically Luminous …, containing biblical criticism of Galileo’s views.
In December 1613, Christina of Lorraine, the grand duchess dowager and mother of Grand Duke Cosimo II, questioned Galileo’s disciple Benedetto Castelli about the compatibility of Galileo’s ideas with Scripture. The duchess had been incited by Cosimo Boscaglia, special professor of philosophy at the University of Pisa, who was also present at the meeting. Castelli gave satisfactory answers, but informed Galileo of the incident. Thus, on December 21, 1613, Galileo felt the need to write a long letter to Castelli giving a multifaceted refutation of the scriptural objection to Copernicanism. The details of Galileo’s criticism will be discussed later, when we come to the letter which he wrote two years later addressed to the grand duchess Christina herself; in fact, the Letter to Christina is an expansion and elaboration of the “Letter to Castelli.” But a brief summary will be useful here.
In the “Letter to Castelli,” Galileo argued that the scriptural objection to Copernicanism has three fatal flaws. First, it attempts to prove a conclusion (the Earth’s rest) on the basis of a premise (the Bible’s commitment to the geostatic system) which can only be ascertained with a knowledge of that conclusion in the first place; in other words, the scriptural objection puts the cart before the horse, or in technical terminology, it begs the question. Second, in the biblical objection, the conclusion is derived from the key premise by means of a logically invalid inference, insofar as the Bible is not a scientific authority, and hence its saying something about a natural phenomenon does not make it so, and its statements do not constitute valid reasons for drawing corresponding scientific conclusions; in other words, the objection is a non sequitur, or inferentially invalid. Third, it is questionable whether the Earth’s motion really contradicts the Bible, and an analysis of the Joshua passage shows that it cannot be easily interpreted in accordance with the geostatic theory, but accords better with the geokinetic view; in other words, the objection’s key premise is false anyway.
Exactly a year after Galileo’s “Letter to Castelli,” on December 21, 1614, at the Church of Santa Maria Novella in Florence, Dominican friar Tommaso Caccini preached a sermon against mathematicians in general and Galileo in particular; their beliefs and practices allegedly contradicted the Bible and were thus heretical. Caccini illustrated his point by explaining that the biblical passage on the Joshua miracle contradicts the Earth’s motion and thus renders belief in it heretical. (Some writers have claimed that Caccini also discussed the suggestive verse in Acts 1:10, “Ye men of Galilee, why stand ye gazing up into heaven?” But there is no contemporaneous evidence that this particular verse was actually mentioned; instead, the story was first told in the eighteenth century.)
Caccini was not the only preacher who attacked Galileo from the pulpit. Another one was Raffaello delle Colombe, also a Dominican friar preaching in Florence, and brother of the philosopher Ludovico delle Colombe, mentioned earlier. Raffaello’s sermons were less direct and explicit, insofar as they did not mention Galileo by name; instead they criticized unmistakably Galilean contributions, such as the telescope and sunspots, and they mentioned Galileo implicitly by describing him as “our brilliant Florentine Mathematician.” However, Raffaello’s sermons were more numerous and persistent, spanning the period 1608–15; and they were more durable, insofar as they were included in a monumental edition of several volumes of sermons on all sorts of topics, edited by Raffaello himself during his own lifetime.22
Foscarini’s Theological Re-assessment
Not all clergymen held that Copernicanism was incompatible with Scripture, and thus false or heretical. One who did not was the Italian Carmelite friar Paolo Antonio Foscarini (1580–1616), who in 1615 published a book containing a theological defense of Copernicanism. Foscarini was the provincial head of the Carmelites in Calabria and had an ambitious agenda of works in philosophy and theology that tended to be encyclopedic in scope. For the Lent of 1615, he had been invited to preach at the Church of Traspontina in Rome. On his way there from Calabria, he stopped in Naples in January 1615 to supervise the publication of his book. It was written in the form of a letter to the general of the Carmelite order and was entitled Letter on the Opinion, Held by Pythagoreans and by Copernicus, of the Earth’s Motion and Sun’s Stability and of the New Pythagorean World System. As it will emerge, Foscarini’s views are similar, although not identical, to those expressed by Galileo in his “Letter to Castelli” and Letter to the Grand Duchess Christina.
Foscarini is explicit that his aim is to give a theological defense of the Earth’s motion, that is a defense of the geokinetic proposition from the objection that it is contrary to Scripture. He is equally clear that he is in no position to mount the following direct defense: that since Copernicanism is physically true, and since two truths cannot contradict each other, Copernicanism is not contrary to Scripture. This defense is not feasible because the Earth’s motion has not been proved with certainty. However, Foscarini is also at pains to repeat frequently the assertion that Copernicanism is probable or likely true, indeed more probable than the Ptolemaic system, and that this probability is largely the result of Galileo’s telescopic discoveries.
To show that Copernicanism is not contrary to Scripture, Foscarini gives several arguments. One of his most important ones is based on the principle of accommodation, which he takes to be uncontroversial and universally accepted: “whenever Sacred Scripture attributes to God or to some creature anything which is otherwise known to be problematic or improper, then it is interpreted and explicated in one of the following four ways”;23 these amount to saying that Scripture is speaking metaphorically or analogically, or is accommodating itself to the common or popular manner of speaking, thinking, perceiving, describing, or believing. Foscarini illustrates this principle with scriptural statements that attribute to God physical attributes such as walking and hands, and emotional states like anger and regret; also with statements that attribute to the Earth ends and foundations; and with those that speak of light and night and day having been created before anything else, of the six “days” of creation, and of the Sun and Moon as the two great luminaries. Moreover, Foscarini is careful to formulate the conclusion of this particular argument by saying that “if the Pythagorean opinion were otherwise true, then it could easily be reconciled with the passages of Sacred Scripture that appear contrary to it … by saying that there Scripture speaks in accordance with our manner of understanding, with the appearances, and with our point of view.”24
Indeed, such a conditional and relatively weak conclusion is all that follows from the principle of accommodation as stated by Foscarini, which is contingent on a scriptural attribution that is otherwise known to be literally incorrect. Thus, by means of the principle of accommodation, Foscarini does not show, and does not pretend to show, that Copernicanism is indeed compatible with Scripture, but only that if we knew that Copernicanism were true then we could unproblematically reinterpret geostatic statements in Scripture.
Another key argument is based on the principle of limited scriptural authority. Paraphrasing various scriptural passages, Foscarini claims that
Sacred Scripture … does not instruct men in the truth of the secrets of nature … because [God] has already allowed and decided that the world be occupied with disputations, quarrels, and controversies and be subject to uncertainty in everything (as stated in Ecclesiastes), and that the answer will only come at the end … Thus, its intention is now only to teach us the true road to eternal life.25
Here, the conclusion he reaches is that “so consequently we see how and why from the passages already mentioned we cannot derive any certain resolutions in such subjects, and how with this principle we can easily avoid the hits from the first and second group of passages and from any other allegation derived from Sacred Scripture against the Pythagorean and Copernican opinion.”26
This argument seems a more direct line of reasoning in support of his claim that Copernicanism does not contradict Scripture. For Foscarini is saying that, since Scripture is not an authority on the secrets of nature, scriptural allegations about the Earth’s rest and Sun’s motion do not entitle us to infer that the Earth is motionless and the Sun moves; thus, we are in no position to assert the Earth’s rest on scriptural grounds, and hence the conflict with the Copernican opinion evaporates. In other words, the Earth’s motion is not contrary to Scripture because Scripture is not a philosophical (or scientific) authority, and so scriptural assertions that the Earth is motionless do not entail that the Earth is really motionless.
A third argument involves what Foscarini calls the principle of “extrinsic denomination” and the passage in Joshua 10:12–13. This, recall, is the passage where Joshua prays to God to stop the Sun and prevent it from setting, so that the Israelites can have more time of daylight to finish winning a battle against the Amorites; God did the miracle and the Sun stood still for a whole day. The principle states that “many times one says commonly and most properly that a motionless agent moves not because it really moves but by extrinsic denomination, namely because with the motion of the subject that receives its influence and action, what also moves is some property which the agent causes in the subject.”27 Applied to the Joshua miracle, we get the following analysis: if the Earth moves and the Sun stands still, sunlight would still move over the Earth’s surface, and so it would be proper to say, by extrinsic denomination, that the cause of this moving sunlight itself moves. The Earth’s motion can thus be reconciled with the Joshua passage.
Most of the rest of Foscarini’s Letter consists of arguments attempting to show that various specific scriptural passages that have been alleged to be contrary to Copernicanism can be reconciled with it for various reasons and in various ways.
Foscarini’s Letter attracted the attention of the Inquisition. By March 1615, the Inquisition had ordered an evaluation of it, and a consultant had written a very critical opinion. Foscarini must have learned something about this censure, and so he wrote a defense of his Letter and sent both to Cardinal Robert Bellarmine. On April 12, Bellarmine replied with a gracious but firmly critical letter, which explicitly named Galileo as holding the same position as Foscarini and thus as being liable to the same criticism. Soon thereafter, Foscarini left Rome and returned home with the intention of revising his Letter to take such criticism into account. This revision never materialized, because the Inquisition proceedings against Galileo had already started, and they climaxed on March 5, 1616 with an Index decree that condemned and completely prohibited Foscarini’s book. Foscarini died on June 10, 1616 “perhaps from a heartbreak,” according to one scholar’s speculation.28
Galileo’s Criticism of the Biblical Objection
Although encouraged by Foscarini’s book, Galileo was also increasingly concerned with the attacks against his views, especially with the scriptural objections, and especially with the criticism emanating from the pulpit. Moreover, he received a copy of Bellarmine’s letter to Foscarini, and from this and other sources he got an inkling that he and Foscarini were being investigated by the Inquisition. So Galileo decided to expand his “Letter to Castelli”; he wrote his essay in the form of a letter to the grand duchess Christina.
The Letter to Christina consists of a brief introductory part explaining its origin and purpose; a long central part that takes up in turn a number of distinct questions about the relationship between scriptural interpretation and scientific investigation; and a brief final part in which Galileo engages in some scriptural exegesis meant to show that the Earth’s motion is not contrary to Scripture.
In the introductory part, we are told that the letter originated from some unprovoked attacks against Galileo charging him with heresy because he believed in the Earth’s motion, and that in it he plans to defend himself from this accusation. It is important to stress the apologetic and defensive character of the letter. Galileo states:
Now, in matters of religion and of reputation I have the greatest regard for how common people judge and view me; so, because of the false aspersions my enemies so unjustly try to cast upon me, I have thought it necessary to justify myself by discussing the details of what they produce to detest and to abolish this opinion, in short, to declare it not just false but heretical.29
The apologia takes the form of the criticism of what we may call the scriptural argument against Copernicanism, and he concludes this part of the letter with the following clear and incisive statement of the objection:
So the reason they advance to condemn the opinion of the earth’s mobility and sun’s stability is this: since in many places in the Holy Scripture one reads that the sun moves and the earth stands still, and since Scripture can never lie or err, it follows as a necessary consequence that the opinion of those who want to assert the sun to be motionless and the earth moving is erroneous and damnable.30
In the central part of the letter, Galileo addresses himself to the major premise of this argument, that Scripture cannot err. He objects that this proposition is true but irrelevant, because what is relevant is the interpretation of what Scripture says, and scriptural interpretations can indeed err. So the question becomes that of what kind of interpretation, or whose interpretation, if any, is correct. In the various sections of the letter’s central part Galileo takes up, in turn, literal interpretation, the interpretation by professional theologians, the interpretation in accordance with the principle of scriptural consensus, the unanimous opinion of Church Fathers, and the official interpretation of the Church (from a pronouncement of the pope speaking ex cathedra or from a decision reached by an ecumenical council). A main conclusion here is that scriptural interpretations often presuppose philosophical or scientific claims. Moreover, Galileo distinguishes between questions of faith and morals and questions about the physical universe; he points out that, although Scripture cannot err about the former, when we come to physical questions, it is not so much false as improper to say that Scripture cannot err; the reason is that it is not meant to provide scientific information, and hence it would be equally improper to say that Scripture can be wrong. A central thesis here is that Scripture is not a scientific (or philosophical) authority.
In other words, in this central part of the letter, Galileo interprets the scriptural argument against Copernicanism as essentially an argument from authority to the effect that it is erroneous to believe in the Earth’s motion because Scripture says so. He objects that Scripture is not a scientific authority, and therefore even if Scripture does endorse the geostatic thesis, it does not follow that it is true and the geokinetic thesis is false; that is, the reason given for the conclusion is inadequate, even if it were true. He also objects that generally speaking, to know what Scripture really says about physical questions, one has to know the scientific truth about them; this means that to know whether this reason is true, we would have to know whether the conclusion is true, or, as we might say, the argument ultimately begs the question.
The brief final part of the letter may be interpreted as a criticism of truth of the minor premise of the scriptural argument. Galileo tries to show that it is questionable whether Scripture says that the Earth stands still and the Sun moves. He does this by an analysis of several passages that were typically given to support the contrariety thesis. The Joshua miracle (Joshua 10:12–13) is discussed at great length.
Galileo argues that the Joshua passage contradicts the geostatic system, whereas it could be given a literal interpretation from the Copernican viewpoint. The passage says that, in response to Joshua’s prayer to prolong daylight, God ordered the Sun to stop, and the Sun stood still for a whole day, needed by the Israelites to defeat the Amorites. Galileo points out that in any system, to lengthen the day the diurnal motion must be stopped. Unfortunately, in the geostatic system the diurnal motion belongs not to the Sun, but to the outermost sphere in the universe, either the celestial sphere or the hypothesized sphere called the primum mobile. The proper motion that belongs to the Sun is the annual motion, which, being opposite in direction to the diurnal motion, would shorten the day if stopped, making the Sun set that much sooner. It follows that if we take the Scripture literally, the miracle is physically impossible in the geostatic system, whereas if God did the miracle, he should have ordered the primum mobile to stop.
By contrast, Galileo argues that in the geokinetic system the miracle could have happened as follows. First, he refers to his own discovery that the Sun is not completely motionless but rotates on its axis with a period of about a month; thus, it makes sense, to begin with, to stop the Sun from moving. To this Galileo adds the speculation that solar rotation probably causes the planetary revolutions, one of which is the Earth’s own annual orbital motion; and further that this terrestrial orbital motion is probably connected with the Earth’s axial rotation. All of this makes some sense because all these motions are in the same direction in the heliocentric system. Thus, by stopping the Sun’s rotation, God could have stopped the Earth’s diurnal motion and thus lengthened Joshua’s day.
Before we move on to more details of the central part of the Letter to Christina, let me summarize my view of its overall conceptual structure (which also corresponds to the basic structure of the “Letter to Castelli,” highlighted earlier). This structure amounts to a threefold criticism of the argument that Copernicanism is wrong because Scripture says so: first, Scripture’s saying so would not make it so; second, to know what Scripture really says about the physical universe one normally has to know what is physically true; and third, it is questionable whether Scripture does in fact say so.
Galileo begins the central argument of the letter by elaborating several uncontroversial points. The first is that the literal interpretation of Scripture is not always correct since, for example, some scriptural statements about God state or imply that He has eyes, ears, and so on, and we know that it is not literally true. The second point is that the literal interpretation of Scripture is incorrect when it conflicts with physical truths that have been conclusively proved. The third is an explanation for this priority of proved scientific truths over literal scriptural meaning, and it was also universally accepted; the explanation is that, whereas Scripture is the Word of God, which was meant “to teach us how one goes to heaven and not how heaven goes,”31 the physical universe and the human senses and mind are the Work of God, and hence one cannot doubt the truth of physical conclusions grounded on sense experience and conclusive arguments.
From these three points, Galileo thinks it plausibly follows that the literal interpretation of Scripture is not binding when we are dealing with physical propositions that are capable of being conclusively proved (even if not proved yet); this consequence follows because doing so would be the more prudent policy, and because what we know is a minute part of what we do not know. Galileo’s own words make clear the tentativeness and prudential character of his conclusion: “I should think it would be very prudent not to allow anyone to commit and in a way oblige scriptural passages to have to maintain the truth of any physical conclusions whose contrary could ever be proved to us by the senses or demonstrative and necessary reasons.”32
Galileo next undertakes an explicit criticism of theological authority. He argues that theology is not the queen of the sciences because its principles do not provide the logical foundations of the knowledge formulated in other sciences, the way that, for example, geometry does for surveying. Moreover, theologians cannot dictate physical conclusions from the above (i.e., without themselves actually getting involved in physical investigations), any more than a king who is not a physician can prescribe cures for the sick. Nor can theologians tell scientists to undo their own observations and proofs because this is an inherently impossible or self-defeating task. Rather, theologians can and should follow two courses. The first corresponds to already established practice: apropos of conclusively established physical truths, they should strive to show that they are not contrary to Scripture by an appropriate interpretation of the latter. The second would be a rule of interdisciplinary communication. Theologians should presume scientific ideas that are not conclusively proved but contrary to Scripture to be false, and accordingly should try to give a scientific disproof of them; this is desirable because the inadequacies of an idea can be discovered more easily by those who reject it. This ingenious but plausible rule is this section’s main methodological conclusion.
Next, Galileo questions the traditional principle that used scriptural consensus combined with the unanimity of the Church Fathers to require acceptance of the literal meaning of physical statements. In other words, he criticizes what earlier I called the anti-Copernican objection from the consensus of Church Fathers. Once again, he makes his fundamental distinction between physical propositions that are and those that are not capable of conclusive proof. For the latter the principle makes sense, but for the former the previous considerations suggest that it is not sound. Two new points emerge in this discussion. First, scriptural consensus is not a sign that physical statements are meant to be taken as literally and descriptively true, but rather it is the result of Scripture’s desire for consistency, its appeal to common people, and the need to reflect the opinions of the time. Second, the unanimity of Church Fathers is not binding unless it is explicit, unless it is the result of reasoned discussion, and unless it refers to matters of faith and morals.
Finally, the authority of the Church herself comes under discussion. Galileo admits that she does have the power to condemn an idea as heretical, but he notes that “it is not always useful to do all that one can do.”33 Moreover, to make ideas heretical is not the same as making them false; indeed, “no creature has the power of making them be true or false, contrary to what they happen to be by nature and de facto.”34 At any rate, the Church should not be hasty in her condemnation; he hopes that she is not “about to make rash decisions.”35 Before condemning a physical idea, she should examine all the evidence and listen to all the arguments on both sides of the issue, and she should rigorously prove that her interpretation of the relevant scriptural passages is correct. For example, such a rigorous proof should use all the cautious advice elaborated by St. Augustine (quoted below). To avoid potential embarrassment, it might be best to wait until the physical idea is conclusively refuted before declaring it heretical.
One of the most striking features of this central part of the letter is the negative tone of its component conclusions: that the literal interpretation of Scripture is not binding in scientific investigation; that theology is not the queen of the sciences; that scriptural consensus is not a sufficient condition for a literal interpretation; that the unanimity of Church Fathers is not necessarily decisive in physical questions; and that the authority of the Church should not be hastily applied. This negativity corresponds to the apologetic and critical purpose of the letter, and the general suggestion is, as mentioned earlier, a denial of the scientific (or philosophical) authority of Scripture. But there is an underlying positive idea: the principle of autonomy, according to which scientific investigation can and should proceed independently of Scripture. And from the point of view of the enterprise of understanding Scripture, we get another constructive idea underlying these negative conclusions: that scriptural interpretation often depends on the results of scientific investigation.
A second striking theme is that of prudence and caution, which he adopts from St. Augustine and elaborates further. Galileo’s explicit admonitions are, of course, against haste in condemning Copernicanism. But it would also extend to the question of accepting the theory or judging the conclusiveness of its supporting arguments.
Equally striking is the theme involving the distinction between physical propositions that are and those that are not capable or susceptible of conclusive proof. This is obviously the main epistemological distinction, rather than that between propositions that have and those that have not been conclusively proved already. The central issue concerns the former distinction, and Galileo tries to resolve it by arguing that no physical proposition capable of conclusive proof should ever be condemned. The priority of established scientific knowledge that has already been conclusively proved over scriptural statements is a non-issue. From the viewpoint of this uncontroversial principle, there would have been no reason for him to write an essay on the methodology of scriptural interpretation and scientific investigation; rather, the only thing to do would have been to produce or search for the conclusive demonstration. The very fact that he writes this methodological essay indicates that he wants to advocate a (relatively) novel principle.
Besides the argumentative content of this letter and the very fact of writing it, there is a third indication of Galileo’s stress on potential demonstrability, as distinct from accomplished demonstration. In 1636, the Letter to Christina was published for the first time by some foreign friends but with his cooperation; the edition contained both Galileo’s original Italian text and a Latin translation by one of the editors. Now, the stress on demonstrability was explicitly incorporated in the long Latin title of the book, which can be translated as New and Old Doctrine of the Most Holy Fathers and Esteemed Theologians on Preventing the Reckless Use of the Testimony of the Sacred Scripture in Purely Natural Conclusions That Can Be Established by Sense Experience and Necessary Demonstrations. Here the crucial phrase is “that can be established,” which obviously is not equivalent to “that have been established.”
Let me finish with two further considerations on Galileo’s Letter to Christina. The first involves an aspect of the letter which so far I have largely ignored: the letter is full of references to and quotations from the patristic and theological tradition, such as St. Jerome, St. Thomas Aquinas, and especially St. Augustine. This aspect of the letter could be reconstructed as an argument from authority, or a series of such arguments. This is important for two reasons. Galileo was aware that regardless of how cogent his methodological argument was, his main conclusion (denying scriptural authority for demonstrable physical claims) could be taken to be so radical that its novelty needed to be toned down by trying to root it in tradition. Thus, in the just quoted title of the 1636 edition of the letter, the initial part of the title (“New and Old,” in Latin a single word “Nov-antiqua”) stresses precisely the two-fold aspect of being partly radical and partly traditional. And secondly, the passages quoted from Augustine are so crucial that they played a significant role in the subsequent history of scriptural hermeneutics. For example, in 1893, Pope Leo XIII’s encyclical Providentissimus Deus, without even mentioning Galileo, put forth a Galilean view of the role of Scripture in scientific investigation, and appealed to the same Augustinian passages as had been quoted by Galileo.
Two passages serve to give a flavor of Galileo’s appeal to St. Augustine. One is Augustine’s version of the principle of nonscientific authority of Scripture: “it should be said that our authors did know the truth about the shape of heaven, but that the Spirit of God, which was speaking through them, did not want to teach men these things which are of no use to salvation.”36 The other is Augustine’s versions of the principle of the priority of demonstrated physical truth: “whenever the experts of this world can truly demonstrate something about natural phenomena, we should show it not to be contrary to our Scriptures.”37
The second observation is that in the Letter to Christina there are several passages that undeniably appear to be inconsistent with the principle that Scripture is not an authority in astronomy or natural philosophy. Some scholars stress this inconsistency and end up attributing to Galileo an incoherent position, which includes a principle of priority of Scripture. But in my view the alleged Galilean inconsistencies are more apparent than real, and Galileo’s alleged incoherence reflects the inadequate and insufficiently deep analysis of such scholarly accounts.
If such Galilean assertions were expressions of the principle of the general priority of Scripture, they would seriously undermine the apologetic purpose of the letter, which was after all to refute the scriptural objection to Copernicanism by arguing (among other criticisms) that the objection is a non sequitur because scriptural statements about the Earth’s or Sun’s motion or rest do not entail that the Earth or Sun really moves or rests. However, if these passages were regarded essentially as attempts to define more precisely the proper scope of the principle of non-scientific authority of Scripture, then such an interpretation would conform with the apologetic purpose of the letter, and so would be preferable to the alternatives that undermine that purpose. I believe such an interpretation would be along the following lines.38
The first of the problematic passages declares that scriptural assertions have priority over other assertions in regard to historical questions. The second passage states that Scripture has priority over unprovable assertions in regard to physical and natural phenomena. The third claims that, for theologians, scriptural assertions have priority over unsupported assertions in all other writings. In other words, Scripture is a superior authority regarding (1) historical questions that depend on balancing probabilities of testimony; (2) undecidable questions about physical reality; and (3) unsupported assertions on any topic in any book. So although Galileo denies the scientific (astronomical, or philosophical) authority of Scripture, he accepts its authority not only for questions of faith and morals, but also for the weighing of probable testimony in history, for undecidable questions in natural philosophy, and for questions of presumption of truth for unsupported claims. These are important nuances, complications, and qualifications in Galileo’s position, but none of this undermines his criticism of the scriptural objection to Copernicanism or his principle of scriptural irrelevance in astronomical research.