The Notion of Scientific Proof in Galileo's Letter to Christina
At the beginning of the letter to the Grand Duchess, Galileo uses the terminology of the Posterior Analytics in explaining to her that his espousal of the Copernican theory is based on particular arguments that refute the Ptolemaic position. These arguments are drawn from "natural effects whose causes could perhaps not otherwise be designated" (effetti naturali, le cause de' quali forse in altro modo non si possono assegnare [311.5-8].  He adds that they, along with other astronomical data ascertained by many new celestial discoveries, "openly confute the Ptolemaic system and are marvelously in accord with the other position and confirm it" (li quali apertamente confutano il sistema Tolemaico e mirabilmente con quest'altra posizione si accordano e la confermano [311.8-12]). The mention of arguments drawn from natural effects whose causes cannot be other than those posited implies the use of demonstration in the Aristotelian sense, where through observation and rigorous reasoning causes are given for effects or effects are traced to their causes. This process of reasoning is the "necessary demonstration" to which Galileo refers so frequently. Galileo himself had begun to work on effect-to-cause reasoning in developing his explanations for sunspots, the phases of Venus, and the tides.  These he thought could be used as demonstrations for the Copernican theory, although they are not developed in the Letter.
With no more than abstract references to the particular astronomical observations he has in mind that would support his position, he goes on to say that Copernicus' De revolutionibus was not even faintly suspect until men intent upon destroying him have tried to discredit the book. This plea for sympathy follows the rhetorical pattern commonly adopted in speeches and letters to capture the good will of the audience.  He makes his plea stronger by adding that this vendetta has come at a time when many discoveries show the doctrines to be "well founded on manifest experiences and necessary demonstrations" (ben fondata sopra manifeste esperienze e necessarie dimostrazioni [312.27-28]) . Here is the first explicit use of the expression "necessary demonstration" in the Letter. Coupled with the previously cited passage, the phrase must mean for Galileo demonstration in the full sense of scientia as developed by Aristotle and carried on in the scholastic tradition.
On the other hand in saying that the doctrine or theory is shown to be "well founded on manifest experiences and necessary demonstrations," Galileo has departed from precise scholastic terms. Perhaps he did so to introduce an almost imperceptible qualification.  For to say that the theory is well founded may mean that it is so in part, that some good observations and some important necessary demonstrations support parts of the complex argumentation. But that is not to say that the Copernican theory is proved entirely by manifest experiences and necessary demonstrations. For example, the supposition that Venus circles the sun is founded on telescopic observations of the planet's crescent phases and its change in size as it recedes or processes. In the same way, the supposition that not all heavenly bodies rotate around the earth is founded on the movement of the satellites around Jupiter. Not so clearly shown, however, are proofs for the supposition that the earth rotates on its axis and revolves around the sun. Today, the movement of the Foucalut pendulum provides a firm foundation for the supposition of the earth's rotation and observations of stellar parallax undergird the supposition of the earth's annual rotation around the sun. But neither of these proofs were available to Galileo. He thought that his argument from the tides might be developed to support the supposition of the earth's rotation and that observations of solar sunspots might be used as the foundation for the supposition of the earth's annual revolution, but these were not "manifest" to other observers nor were the arguments from them "necessary" to his peers. In 1616 he must have believed that these demonstrations simply required further work and that new ones might yet be discovered. The Dialogue shows his later attempt to provide stronger arguments, although for these he does not claim "necessary demonstrations," a point we will examine further below.
Returning now to the first mention of necessary demonstration in the Letter, we may conjecture that Galileo probably had mental reservations about stating that the theory was proved in its entirety, choosing instead to say with more accuracy that it was simply "well founded," partially supported. Before we consider further uses he makes of the terminology of scientia, we should mention yet another possible reason for his equivocal stance at this point in the Letter: his awareness of the position of the Church on the matter of proof for the Copernican theory.
When he composed the final form of his Letter Galileo knew that Cardinal Bellarmine had sent a letter to Father Paolo Antonio Foscarini, a armelite provincial, bearing on this issue and he may have read it at this time, since its arguments are countered in his message to the Grand Duchess.  The Cardinal writes to Father Foscarini, regarding the Carmelite's book defending Copernicanism, that he thinks Foscarini and Galileo acted "prudently in speaking ex supposition and not absolutely" as he believed Copernicus had also. Bellarmine adds that it is all right to say the theory that the earth moves and the sun stands still is a better explanation for the appearances in the heavens than the eccentrics and epicycles of Ptolemy, but to affirm it as a reality would be "very dangerous (cosa molta pericolosa), not only because it would irritate all the philosophers and theologians, but because it would harm the Holy Faith by rendering the Holy Scripture false." 
He concludes the letter by saying that were there a "true demonstration" (vera demostratione) that in fact the sun is in the center and the earth moves around it, then Scripture would have to be explained in such a way as to account for this. But he will not believe that there is any such demonstration until it is shown to him. He does admit that a demonstration could exist to show that the appearances are saved in this way, but that is not the same thing, he says, as "a demonstration that in truth (in verita) the sun is in the center and the earth in the heavens." During this period prior to the condemnation of the theory, Galileo composed a point by point answer to the Bellarmine letter, which has been preserved among his papers. Favaro called it and two other notes on the topic, Considerazioni circa l'opinione Copernicana.  He believed that Galileo wrote these at the end of 1615 or near the beginning of 1616, about the same time that he was writing the Letter.  The three brief memoranda in the Considerazioni treat the question of proof in relation to the Copernican system and also the attendant problems with Scriptural references to celestial phenomena.
The first of these memoranda explains the sense in which Copernicus may be said to have argued for his theory ex suppositione. Galileo points out that there are two senses of supposizioni or suppositions. One is primary and based on absolute truth in nature; the other is secondary and constructed imaginatively to explain the appearances in the movements in the stars. It was the second kind, then, that Ptolemy used in attempting to explain the movements of the heavenly bodies. He developed the supposition that there were eccentric and epicyclic circles transcribed by the planets so as to account for the motions that he observed. This was the same kind of supposition that Bellarmine assumed that Copernicus, as well as Foscarini and Galileo, had intended. But Galileo is adamant in the Considerazioni in saying that Copernicus meant his position to be taken as "primary and necessary in nature." 
He goes on to state that the discovery of the changes in size and appearance of Venus shows that it circles the sun in conformity with Copernicus' system and thus removes any doubt that the position is "true and real."  Galileo's conviction that reality lies at the base of this theory and that it is not simply a work of the imagination is clearly stated. Such a conviction would make him determined to find convincing proof for Bellarmine and scientists in the Church. Thus, the ambiguity in his language in the Letter may be a deliberate attempt to urge his ecclesiastical audience to take his statements as a token of complete proof, which he thought lay almost within his grasp. Whatever his intention, only an astute reader would pick up the ambiguity at all. Certainly to the Grand Duchess and most other readers, he is maintaining that the Copernicall' system has the weight of astronomical observation and of reason behind it, as the Ptolemaic does not.
His next reference to necessary demonstration and the thirteen that follow in rapid succession are made in connection with the main point at issue in the Letter: the reason given for "condemning the opinion concerning the mobility of the earth and the stability of the sun is that we may read in many passages in Sacred Scripture that the sun moves and the earth stands still" [315.9-12]. Galileo now attempts to provide the reasons for preferring an opinion that contradicts Scriptural statements. He explains to the Grand Duchess that at times Scripture speaks in the language that men understand, using analogies and popular wisdom, for the Bible is concerned with spiritual truths and not in teaching man about the complexities of the universe. The Scripture is not meant to be taken literally in every sentence. Given that this is true, he suggests that when we dispute about natural problems we should not begin from the authority of Scriptural passages, but "from sensate experiences and from necessary demonstrations" (dalle sensate esperienze e dalle dimostrazioni necessarie [316.42]). He explains the significance of these experiences and demonstrations to her, implying that these are obvious and known. It is "apparent that whatever natural effects either sensate experience sets beforeour eyes or necessary demonstrations conclude for us, these ought not to be ejected nor condemned" (pare che quello degli effetti naturali che o la sensata esperienza ci pone dinanzi agli occhi o le necessarie dimostrazioni ci concludono, non debba in conto alcuno esser revocato in dubbio, non che condennato [316.33-317.3]).
Lest the Grand Duchess fear that such an approach does not show proper respect for the Word, he hastens to reassure her that on the contrary, having come with certainty to some natural conclusions (anzi, venuti in certezza di alcune conclusioni naturali [317.12-13]), one should then use them as the best means of arriving at the true exposition of the Scripture. On propositions that are not matters of faith, the authority of the Holy Scriptures should always be preferred above human writings when the authors do not employ the demonstrative method, but proceed with pure narration or with probable reasons (all'autorità di tutte le scritture umane, scritte non con metodo dimostrativo, rna o con pura narrazione o con anco probabili ragioni... [317.22-24]). He concludes this line of argument by exclaiming that he cannot believe that "the same God who has given us senses, discourse, and reason" would ask us to negate "these natural conclusions which either sensate experiences or necessary demonstrations expose to the eyes and to the intellect" (quelle conclusioni naturali, che o dalle sensate esperienze o dalle necessarie dimostrazioni ci vengono esposte innanzi agli occhi e all'intelletto [317.29-31]).
In this part of the Letter the hint of reservation concerning the proofs available for the Copernican system is abandoned. The Grand Duchess, and especially Galileo's clerical audience, are implored to acknowledge the certainty of natural conclusions that call for a reinterpretation of the Scriptures.
Referring to a point he made earlier regarding the focus of the Scriptures on spiritual matters rather than on scientific ones, Galileo says that if the sacred writers had intended to teach us about astronomy they would certainly not have said so little, "practically nothing in comparison to the infinite and admirable conclusions that are contained and demonstrated in that science" (che e come niente in comparizione delle infinite conclusioni ammirande che in tale scienza si contengono e si dimostrano [318.8-10]).
To add greater weight to the foregoing assertion, he refers to St. Augustine's De Genesi ad literam, a work he cites also at the beginning of the Letter.  In his first reference to De Genesi, he mentions the Saint's admonition to refrain from taking a firm position on a disputed point lest "we conceive a prejudice against something that truth hereafter may reveal to be not contrary in any way to the sacred books of either the Old or the New Testament." 
This quotation he treats almost as a textual theme in a sermon, returning to it often throughout the letter. At this place in the letter he notes a passage in which Augustine says that the authors of Holy Scripture knew the truth regarding the heavens but that the Holy Spirit did not require them to speak of matters that do not relate to salvation.  Then comes the quotation of Cardinal Baronius' clever epigram: "the intention of the Holy Spirit is to teach us how to go to heaven, not how the heavens go." 
Having now established in the minds of his readers that necessary demonstrations supporting the Copernican position exist, and that the Holy Scriptures do not attempt to teach us what is true of nature, he goes on to remind the Grand Duchess and his other readers "how much in natural conclusions they ought to respect necessary demonstrations and sense experiences" (quanto nelle conclusioni naturali si devono stimar le dimostrazioni necessarie e le sensate esperienze [319.29-31]). He strengthens this point also with appeals to authority, selected, he says, from a hundred attestations by scholars and sainted theologians. The first is from the work of Pereira (Pererius), In Genesim. Pereira was a Jesuit and a professor at the Collegia Romano who lectured on natural philosophy there in the 1560's. His opinion might be expected to carry some weight with Galileo's clerical audience. The passage chosen is Pereira's warning that in treating the teaching of Moses we should be wary and avoid "affirming and asserting whatever is refuted by manifest proof and philosophical reasoning ...because with any truth other truths are congruent (quod repugnet manifestis experimentis et rationibus philo sophiae ... namque ... verum omne semper cum vero congruat [320.2-3]). Pereira continues to say that it is not possible for "the truth of Sacred Writings to be contrary to true reasoning and proofs of human science" (non potest Sacrarum Literarum veris rationibus et experimentis humanarum doctrinarum esse contraria [320.4-6]).
A quotation from St. Augustine emphasizes the crucial role of demonstration; "If manifest and certain reasoning are set up against the authority of Holy Scripture, whoever does this is not aware of what he is doing" (Si manifestae certaeque rationi velut Sanctarum Scriptuarum obiicitur authoritas, non intelligit qui hoc facit [320.6-71 ]). Augustine explains that such an act really opposes only what was supposed to be the sense of the Scripture, whose truth had not really been penetrated.
We might note that Galileo's citations of authorities are always in Latin. The shift from the more informal Italian to the language of scholars has the effect of magnifying the importance of the quoted passages, and it also elevates the Letter itself above the genre of familiar epistles.
Drawing the obvious conclusion from his citations, Galileo observes that since "two truths cannot contradict each other, then wise expositors should toil diligently to penetrate the true meaning of Scriptural passages, which indubitably will be in accord with the natural conclusions which manifest sense or necessary demonstrations have first made certain and secure" (che indubitabilmente saranno concordanti con quelle conclusioni naturali, delle quali il senso manifesto o le dimostrazioni necessarie ci avessero prima resi certi e sicuri [320.13-16]).
Then Galileo summarizes the import of his argument for the ecclesiastical hierarchy by stating that, with respect to Scriptural interpretation, it would be more prudent not to "in a certain manner force the meaning in order to sustain as true this or that natural conclusion, for which one day the senses and demonstrative and necessary reasoning would be able to show the contrary (in certo qual modo obbligarli a dover sostener per vere queste o quelle conclusioni naturali, delle quali una volta il senso e le ragioni dimostrative e necessarie ci potessero manifestare il contrario [320.22-25]). The presumptiveness of the advice is excused by the theme previously quoted from St. Augustine. And the phrase "one day" (una volta) follows the meaning of the Saint's reference to what "truth hereafter may reveal" not to be in opposition to the Scriptures. The difficulty, for those who want to discover Galileo's meaning in regard to whether such demonstrations already exist, is in whether he intended the passage to be taken as a general principle, an echo of St. Augustine's when such demonstrations did not exist, or whether he meant that proofs were still not available. The point is not clarified for us in the Letter.
After remarking that the Scripture itself declares that God has given the world over to disputations, and that it is difficult to discern the work of his hands from beginning to end (Ecclesiastes 3.11), he mentions that many distinguished philosophers throughout history have believed in the stability of the sun and the mobility of the earth. But is was Copernicus who "finally amplified and confirmed it with many observations and demonstrations" (e finalmente ampliata e con molte osservazioni e dimostrazioni confermata) [321.15-16]). Although there is an ambiguity in the words "amplified and confirmed," the tone conveys the impression that there is no question of Copernicus' accomplishment . And so Galileo adds that, in view of this evidence, Church authorities should be cautious about adding to the articles of faith unnecessarily, especially at the request of people who have not the capacity to understand, and less to challenge, the demonstrations with which the more acute sciences proceed to confirm such conclusions (le dimostrazioni con le quali le acutissime scienze procedono nel confermare simili conclusioni [321.27-28]).
Galileo then inveighs against lay persons who presume to interpret the Scriptures on physical matters, and some who support their own pet theories with citations from the Scriptures. He expresses his gratitude that God has given authority in such matters to men who are led by the Holy Spirit. Then with the preliminary remark that he does not wish to include some theologians whose learning he admires among these lay writers, he launches into a lengthy criticism of the methods employed by theologians in approaching the problem of contradictions between Scripture and science. In this passage he grounds his criticism upon the sanctity of proofs by demonstration.
He is annoyed, he says, by those who would force people to follow an opinion that is in accord with the Scripture in disputes concerning nature, and then these same men think they are under no obligation to answer the reasoning and experiences of those who have a contrary view. These tyrants argue that since theology is queen of the sciences, she need not accomodate herself to the lesser sciences. And further, they believe that "in the inferior sciences if any conclusion be taken as certain by the force of demonstrations or of experiences" (nell'inferiore scienza si avesse alcuna conclusione per sicura, in vigor di dimostrazioni o di esperienze) and they find in the Scripture other conclusions that contradict the demonstrations, then they would have "the professors of these sciences find the means to annul these demonstrations and discover the fallacies in their own experiences (gli stessi professori di quella scienza procurar per se medesimi di scioglier le lor dimostrazioni e scoprir le fallacie delle proprie esperienze) without recourse to the theologians and Scripture scholars [324.7-12]." The particular import of this line of criticism becomes clearer ad he expresses his dismay: "to command these same professors of astronomy to procure for themselves a defense for their own observations and demonstrations (aile proprie osservazioni e dimostrazioni), as if these could be no more than fallacies and sophisms, this is to command them to do an impossible deed [325.27-301]."
Speaking almost directly to the prelates he says, "I would like to entreat these most prudent Fathers" to consider carefully "the difference between doctrines of opinion and of demonstration" (la diffe renza che e tra le dottrine opinabili e le dimostrative [326.9-11]. Seeing the strength of logical deductions, they would understand all the more that "it is not in the power of professors of demonstrative sciences to change their opinions, applying them now to this and now to that (come non e in potesta de' professori delle scienze dimostrative il mutar l'opinioni a voglia loro applicandosi ora a questa ed ora quell a [326.12-15]). Then he adds an example: commanding a mathematician or a philosopher is very different from ordering a lawyer or a merchant to do something. They cannot be expected with the same facility "to change conclusions demonstrated in things of nature and the heavens, as to change opinions concerning what is lawful in a contract, in a bargain, or an exchange (mutare le conclusioni dimostrate circa le cose della natura e del cielo, che le opinioni circa a quello che sia lecito o no in un contratto, in un censo, o in un cambio [326.17-20]).
Thus, Galileo underscores his entreaty that the proofs for the Copernican system be regarded as proofs in the realm of scientia, not in that of dialetic or rhetoric. A quotation from St. Augustine legitimizes his request of the theologians:
This is to be held without doubt, that whatever the savants of this world would be able to demonstrate truly of the nature of things (natura rerum veraciter demonstrare potuerint) we should not show to be contrary to our Scripture. Moreover, whatever they teach in their books contrary to the Holy Scripture, without doubt we believe would be false, and in the best manner we can we should show it as such; and so we should hold to our faith in our Lord, in whom are hidden all the treasures of knowledge and never be seduced by the loquacity of false philosophy or be frightened by the superstition of simulated religion [327.4-11].
He boldly draws a conclusion from the above statements and in the process all tentativeness concerning the extent of the proofs under consideration is relinquished. From the words of St. Augustine, he says, he derives the doctrine that the writings of the wise of this world "contain some things demonstrated truly of nature and others that simply explain" (alcune cose della natura dimostrate veracemente, ed altre semplicemente insegnate), and that it is the "primary duty of wise theologians to show that the former are not contrary to the Sacred Scripture, while the latter, which explain but do not demonstrate necessarily (insegnate ma non necessariamente dimostrate), if these are at variance with the Sacred Word, they ought to be regarded as indubitably false and in every possible manner ought to be so demonstrated" (dimostrare) [327.12-19].
At this point, taking the cue from St. Augustine, Galileo carries the meaning of the Saint much further than Augustine obviously intended, and attempts to seize an advantage over the theologians. He says that if natural conclusions, truly demonstrated (le conclusioni naturali, dimostrate veracemente) are not to be deferred to the Scriptures, but rather the passages are to be shown as not contrary to the conclusions, then what is needed (adunque bisogna), is that whoever condemns a proposition of nature first should show that it is not demonstrated necessarily (mostrar ch' ella non sia dimostrata necessariamente). Moreover, "this is to be done not by those who hold it to be true, but by those who regard it as false." [327.19-24)
The demand that theologians be given the responsibility of proving the conclusions of astronomers and mathematicians to be based on inadequate demonstrations was an unprecedented provocation in this emerging battle between science and theology. Certainly it would be regarded by theologians as an insolent and empty challenge, seeing that they were not trained in the requisite knowledge for refuting demonstrations in the physical sciences. But even more, the placement of science above the Scripture in the development of his challenge to them would be a source of fear as well as consternation. The bastions of faith had already felt the tremors of the Protestant revolt, and those called to guard the edifice would be anxious that these new rumblings be quickly stilled.
Now we have come to the climax of his carefully argued case for the acceptance of Copernicanism and his solicitation for a reinterpretation of Scriptural passages that imply a contradictory view of nature. Taking into consideration our earlier discussion of what must be accounted for in a complete demonstration of the system and presuming the values of the theologians who must be convinced by his argumentation, we will see that what Galileo must have thought to be the most compelling passage in his defense of Copernicanism is really its weakest part.
He begins this section of the letter by saying he wishes to answer those who think the Scriptures should be received in the literal sense without gloss or interpretation when a subject is always treated in the same way and when the Holy Fathers have always been in agreement on it. They believe that the stability of the earth and the motion of the sun has been so regarded and so it should he accepted as an article of faith. (The idea that whatever the Fathers of the Church agreed on unanimously should be considered as binding on Christians was adopted as an article at the Council of Trent and was mentioned by Bellarmine in his letter to Foscarini referred to above) .
Galileo replies to this position by first distinguishing among physical propositions. Some of these he says are of the kind to which human reason can "provide no more than probable opinion and seemingly true reasoning" (piu presto qualche probabile opinione e verisimil coniettura) instead of a "sure and demonstrated science (una sicura e dimostrata scienza), as for example, whether the stars are animated." Other propositions are "those of which one has, or which one firmly believes could have, undoubted certainty based on experience, long observation, and necessary demonstration (altre sono, delle quali o si ha, o si puo credere fermamente che aver si possa, con esperienze, con lunghe osservazioni e con necessarie dimostrazioni, indubitata certezza [330.17-19)]." "These are the kinds of proposition that would treat of the mobility of the earth and the sun and whether the earth is a sphere." In the case of the first sort of proposition "where human reason cannot reach, and for which, consequently, it is not possible to have a science, but only opinion and faith," no doubt one should "piously conform absolutely to the pure sense of the Scriptures." But in regard to the last type, he believes that "first we need to ascertain the fact" and then we will be led to the "true sense of the Scriptures." The authentic interpretation will be completely "in accord with the demonstrated fact (concordi col fatto dimostrato), even though the words may seem to say the opposite: for two truths cannot be contradictory" [330.6-29]. Here is a point that Galileo returns to repeatedly in the Letter. Each time the import is that the truths have been demonstrated on the Copernican side so completely that a reinterpretation of Scripture is warranted.
Galileo reinforces his position with another reference to St. Augustine who warns us not to be concerned when the Bible contradicts astronomers. For we should believe the Bible if these astro nomers are not speaking the truth, but if they demonstrate their position beyond question we should look for another interpretation. Galileo emphasizes the fact that the Saint does not ask the astronomers "to dissolve their demonstrations and declare their conclusions false" (solvendo le lor dimostrazioni, dichiarino la lor conclusione per falsa [331.10-11]). Then he returns to the Letter's opening quotation from St. Augustine, expanding and restating his admonition against taking an obdurate position on a questionable point lest it keep us from seeing e truth that may be shown not contrary to the Bible after all.
Galileo concludes this appeal with a sweeping endorsement by the Holy Fathers whom he explains agree that "for natural questions that are not matters of faith, one should first consider whether things are indubitably demonstrated or known by sensate experiences or whether such knowledge and demonstrations are possible" (nelle questioni naturali e che non son de Fide prima si deva considerar se elle sono indubitabilmente dimostrate o con esperienze sensate conosciute, o vero se una tal cognizione e dimostrazione aver si possa); if this knowledge is available to us we should regard it as "a gift from God" and use it to discover "the true sense of the Scripture where the passages appear to show a different meaning [332.3-9]." This passage is the strongest statement of the basic demand or petition of the Letter, and it clearly is meant to leave the impression that the requisite conditions laid out by the Holy Fathers have been met. It goes without saying that they would have expected the Copernican system to have been fully demonstrated, not just partially.
Having dealt with the problem of physical propositions that contradict the Scriptures, he turns to the second point in his answer to the theologians who would retain the literal sense of the Scripture in questions that relate to the mobility of the earth. These theologians demand that the literal words of the Bible be adhered to when all the passages on a topic say the same thing and when the Fathers of the Church agree that the "nude sense" be followed. Galileo first undercuts reliance on the actual words of the text by returning to a point he had made earlier: Scripture speaks of the popular conceptions of ordinary men in common language. But this does not mean that we should take common opinions as truth statements. The arguments of astronomers, on the other hand, are "fine observations and subtle demonstrations based on abstractions that call for a very energetic imagination" (esquite osservazioni e sottili dimostrazioni, appoggiate sopra astrazioni, che ad esser concepite richieggon troppo gagliarda imaginativa [332.29-333-1]). The common people are convinced by "simple appearances and vain and ridiculous impressions" while those of finer understanding who believe differently from them are persuaded by "experiences and demonstrations" (esperienze e dimostrazioni [333.13-15]).
Quoting St. Thomas Aquinas in this matter, he shows how that eminent doctor of the Church explained that a passage in the Book of Job simply accomodates to common conceptions when it refers to the earth as being hung above nothing, implying that space is a vacuum or nothing, while we know it to be filled with air [333.30-334.9].
Copernicus, too, bowed to the language of ordinary men, "after first demonstrating the movements (prima dimostrato che i movimenti) that appear to take place in the sun, or the firmament, are truly the earth's", he then turned to speak of the sunrise and sunset and other such things used in common discourse [334.21-335.6].
Galileo next challenges the theologian's assertion that all the Church Fathers agree on the stability of the earth. He points out that the topic was really never the subject of debate, and therefore we cannot be obligated to accept a doctrine that they did not impose. The view that the earth moves was not condemned. In fact, only recently have theologians begun to treat it, he says. One of these, Diego de Zuniga, in his Commentaries on Job finds that a text from Job corroborates the Copernican thesis: "Who moves the earth from its place." [336.14-19] 
In any event, Galileo doubts that the Council of Trent, in calling the common assent of the Church Fathers on matters binding, was referring to physical matters. He thinks, rather, that the Council was referring to questions of faith or morals, where people have perverted or distorted the sense of the Scriptures. Certainly, the mobility of the earth is not a matter of faith or morals, nor have those who have written on it distorted or misused the Bible to support their position.
Concluding this part of his argument, he says that even if we concede that in the Copernican question we must submit to the opinions of wise theologians, then the wise theologians of this age must argue it, since the Holy Fathers did not debate the issue. These deliberations should take place after "first having heard the experiences, observations, reasonings, and demonstrations of philosophers and astronomers for the one and the other side (prima l'esperienze, l'osservazioni, le ragioni e le dimostrazioni de' filosofi ed astronomi per l'una e per l'altra parte) because the controversy is of natural problems and of necessary dilemmas" (la controversia e di problemi naturali e di dilemmi necessarii) and impossibile to be decided otherwise than in these two ways. The theologians will then be able to determine the matter absolutely with the aid of divine inspiration. [338.7-12].
The concession of which Galileo conjectures was of course effected by the Church at the time of his writing. It had assumed that it had the right to make a decision on the Copernican issue. He hoped to sway their opinion by his petition that the Scriptures be reinterpreted. To the theologians and the philosophers in the service of the Church, only a complete and necessary series of demonstrations for the Copernican system could force a reversal of Scriptural interpretations that were in opposition to it. Throughout the Letter to Christina, Galileo exploits the initial ambiguity of his first statements regarding the extent and nature of the proofs presented by Copernicus and the supporting evidence of his own discoveries. Repeatedly, he speaks of the demonstrations that exist for the system, and rarely qualifies his meaning. An example of his rhetorical treatment of the concept of proof occurs in one of the remaining references to demonstration. He points out that the Holy Fathers knew it would be "prejudicial, and against the primary rules of the Catholic Church," to use the Scripture to determine conclusions about nature of which either experience or necessary demonstration might in time demonstrate the contrary of the sense of the bare words (conclusioni naturali, delle quali o con esperienze o con dimostrazioni necessarie, si potrebbe in qualche tempo dimostrare il contrario di quel che suonan le nude parole [338.30-339.1]). The passage reechoes his theme from St. Augustine and, like the earlier similar passage, may be variously interpreted. If he means that demonstrations have shown already the Scripture's bare words to be in need of reinterpretation, that of course supports his case. If he means that he expects in time to have such demonstrations, he hopes that this probability will have the same effect.
In repeating the scholastic terminology and by stating in many places that demonstrations now exist, and thus implying complete proof of the system, he has attempted to persuade his audience that the theory is not heretical simply because it contradicts passages of Scripture. The rhetorical element of his treatment lies not only in his capitalizing on the ambiguity of his having proof, but it figures prominently also in the passages where he implies he has not, and is basing his request for reinterpretation on the probability of proof becoming available. We must remember that in the seventeenth century scientia dealt with certainties, rhetoric with probabilities. Thus, his use of the passage from St. Augustine as a theme serves him well. The Saint was of course speaking of situations concerning heavenly bodies where proof was not yet forthcoming, but could be in time. Since this might one day come about, one should not take a firm position on Scriptural interpretations so as to prejudice oneself against the new sense of the Word. The manner in which Galileo handles the fuller explication of the theme from De Genesi actually allows for an explicit admission that a complete demonstration is still to be effected. But he never explicitly makes that admission. On the contrary, he presents his arguments so as to yield the opposite impression - that it has been accomplished. By doing so he has covered himself in case those who have examined the existing proofs and found them inconclusive should press him further. As I have explained in a previous article, many scientists of the day preferred the Tychonian explanation of the movement of the heavens, for it allowed the stability of the earth and the movement of the planets around the sun and of both planets and sun around the earth .
Ten remaining passages refer to demonstration, as the term continues to figure importantly throughout Galileo's closing arguments. In brief, all of these references are made in connection with his final appeal to the theologians to recognize the primacy of physical evidence in determining the proper sense of the Scriptures. During this last part of the Letter his tone becomes quite forceful and his assumptions of proof frequent. He explains to the Grand Duchess that one would have a false idea of Scriptural texts if they disagreed with "demonstrated truths" (le verita dimostrate), but with the help of a "true demonstration'" (vero dimostrato) one can find "the sure sense of the Scripture." To accept the bare words alone would be, in a manner of speaking, "to force nature and negate experiences and necessary demonstrations" (sforzar la natura e negare l'esperienze e le dimostrazioni necessarie [339.15-18]. Pressing the point more strongly, he quotes St. Augustine who speaks of a truth of nature that seems to contradict a text: "When a truth is demonstrated by certain reasoning (Si autem hoc verum esse certa ratio demonstraverit), it is not certain the writer intended this sense or another no less true [331.25-26]." St. Augustine warns that often Christians may not fully understand the movement of the heavens and not expound it correctly, yet claim that it is a Christian teaching. This is dangerous for the Church and the Scriptures, he says, for infidels may understand these matters better and thereby conceive contempt for the Bible. Who would believe its teachings "on the resurrection of the body, eternal life, and the kingdom of heaven, when on things that can be perceived through experience and indubitable reasoning they find it instructing falsely?" (quando de his rebus quas iam experiri vel indubitatis rationibus percipere potuerunt, fallaciter putaverint esse conscriptos [340.29-32]). Men who are imprudent and rash in these matters thus bring great sorrow to the Church if they impose their own purposes on the Scriptures. Galileo then turns the point on his adversaries, saying that such are the men "who are not willing or not able to understand the demonstrations and observations (le dimostrazioni ed esperienze) with which the author and his followers confirm their position [341.12-14]."
Developing the meaning, Galileo says that "whoever sustains the true interpretation must have many sensate experiences and many necessary demonstrations on his side" (molte esperienze sensate e molte dimostrazioni necessarie per la parte sua [341.32-33]. The Scriptures, he reiterates, can never oppose "manifest experiences and necessary demonstrations" (le manifeste esperienze o le necessarie dimostrazioni [342.12-13]).
He then returns to the provocative suggestion he made earlier: Those who would argue that the Copernican view is false should occupy themselves "in demonstrating its falsity (in dimostrar la falsita di quella [342.24-25])." He adds that it would be better first to be assured of the necessary and immutable truth of the fact (della necessaria ed immutabili verita del fatto). "In summary, if a conclusion must not be declared heretical when it might be true, then vain are the efforts of those who would aspire to condemn the mobility of the earth and the stability of the sun, if they have not first demonstrated it to be impossible and false (prima non la dimostrano essere impossibile e falsa [343.6-15])." This is the last of the texts asserting or implying proofs for the Copernican system in the Letter.
N.B.: the numbers of the footnotes are those present in the original text
 The references in square brackets are to Favaro's critical edition of the Letter, Opere, vol. V. Since so many references to this volume are made in this part of the essay, I have omitted "5" from each of these citations and simply listed page numbers followed by line numbers. The original Italian or Latin is quoted only for those parts specifically pertaining to scientific proof. The English translations are my own.
 See the Letters on Sunspots (1613), and the Discourse on the Tides (1616), Opere, V, 71-260, 377-395.
 The five parts of the rhetorical pattern according to the teachings of the dictatores in Ars dictaminis were salutatio, captatio benevolentiae, narratio, petitio, and conclusio. For discussions of Renaissance survivals of the art see Paul Oskar Kristeller, "The Scholar and his Public in the Late Middle Ages and the Renaissance," Medieval Aspects of Renaissance Learning: Three Essays by Paul Oskar Kristeller, Edward P. Mahoney, ed. and. tr. (Durham, North Carolina: Duke University Press, 1974) and Kristeller's "Humanism and Scholasticism in the Italian Renaissance," in his collected essays, Renaissance Thought and its Sources, Michael Mooney, ed. (New York: Harper and Row, 1979), pp. 85-105. Jerrold E. Seigel discusses it in his Rhetoric and Philosophy in Renaissance Humanism (Princeton: Prince ton University Press, 1968), ch. 7. See also Ronald Witt, "Medieval 'Ars Dictaminis' and the Beginnings of Humanism: A New Construction of the Problem," Renaissance Quarterly, 35 (Spring 1982), 1-35; and James J. Murphy , Rhetoric in the Middle Ages (Berkeley, University of California Press, 1974), ch. 5.
 On the use of the terms "sense experience" and other related scholastic terminology see Luigi Olivieri, "La Problematica del Rapporto senso-discorso, tra Aristotele, Aristotelismo e Galilei," Aristotelismo Veneto e Scienza Moderna II, Luigi Olivieri, ed. (Padova: Editrice Antenore, 1983).
 The ambiguity of Galileo's language here was touched upon in Jean Dietz Moss, op. cit. (see reference 3), p. 564, n. 34.
 Not only does the substance of the Letter evince a knowledge of Bellarmine's letter to Foscarini, but Galileo stated at the trial that he had a copy of it. See Favaro's comments in Opere, V, 277
 Opere, XII, 171.10-16.
 Ibid., 172.32-38.
 The transcription by Favaro is in Opere V, 351-376.
 Opere, V, 277. In a letter to Piero Dini, 23 March 1615, Galileo speaks of having compiled the arguments for Copernicanism along with many other considerations to make his reasoning on the issue as clear as possible. These he says he plans to offer as guidance for the Church in arriving at a decision (Ibid., 300).
 Ibid., 357.34-35.
 Ibid., 362.34-37.
 For his use of St. Augustine's writings see Francois Russo, "Lettre a Christine de Lorraine Grande-Duchesse de Toscane (1615)," Revue d'historie des sciences, 17 (1964), 337, n. 1. In his comments on this translation of the letter into French, Russo says that the references to St. Augustine's De Genesi ad literam outnumber all the rest of the citations, presumably because the commentary on Genesis is the one most sympathetic to Galileo's point of view.
 Opere, V, 310.4-8. This is Drake's translation in Discoveries, pp. 175-176 (see Reference 3).
 Opere, V, 318.26-29.
 Ibid., V, 318.26-28.
 "Confirmation" is a term more commonly used in a rhetorical sense at this time. It was not applied to scientific reasoning ordinarily, but rather to the proof of a thesis in a rhetorical argument. The confirmatio was the fourth of six parts of a classical argument where proofs are marshalled for a position or thesis. See Cicero, De Inventione, I, XXIV, Loeb Classical Library edition (Cambridge, Mass.: Harvard University Press, 1976), pp. 69-70. De Inventione was one of two major texts used in teaching rhetoric throughout the Middle Ages and into the Renaissance. That Galileo intends the rhetorical sense of the term is quite possibile, but the distinction again would be lost on readers not trained in philosophy.
 Opere, XII, 172.20-31. Only after completing this essay was my attention drawn to the excellent study of Olaf Pederson on the significance of Galileo's attempt to work out the appropriate implications for science of the decrees of the Council of Trent, "Galileo and the Council of Trent," Studi Galileiani, I, no. I, Vatican Observatory Publications, 1983, 1-29.
 As I have pointed out in Jean Dietz Moss, op. cit., p. 572, n. 42 (see Ref. 3), this was an unfortunate reference, for Zuniga had been reprimanded for his view, and the text of his writing was ordered to be corrected in the 1616 decree.
 Jean Dietz Moss, op. cit., pp. 568-569 and. n. 38 (see Ref. 3).
J. Dietz Moss, The Rhetoric Of Proof In Galileo's Writings On The Copernican System, in “The Galileo Affair: A Meeting of Faith and Science”, ed. By G.V. Coyne, M. Heller, J. Zycinsky (Vatican City State: Specola Vaticana 1985), pp. 42-56.