SALV. I am very glad you find these arguments novel, so that you will not remain of the opinion held by most Peripatetics; they believe that if anyone disagrees with Aristotle’s doctrine, this happens because of not having heard or properly grasped his demonstrations. However, you will certainly hear other novelties, and you will hear the followers of the new system produce against themselves observations, experiments, and reasons much stronger than those produced by Aristotle, Ptolemy, and other opponents of the same [154] conclusions; you will thus establish for yourself that it is not through ignorance or lack of observation that they are induced to follow this opinion.
SAGR. I must take this opportunity to relate to you some things which have happened to me since I began hearing about this opinion. When I was a young man and had just completed the study of philosophy (which I then abandoned to apply myself to other business), it happened that a man from Rostock beyond the Alps (whose name I believe was Christian Wursteisen)38 came into these parts and gave two or three lectures on this subject at an academy; he was a follower of Copernicus and had a large audience, I believe more for the novelty of the subject than anything else. However, I did not go, having acquired the distinct impression that this opinion could be nothing but solemn madness. When I asked some who had attended, they all made fun of it, except one who told me that this business was not altogether ridiculous. Since I regarded him as a very intelligent and very prudent man, I regretted not having gone. From that time on, whenever I met someone who held the Copernican opinion, I began asking whether he had always held it; although I have asked many persons, I have not found a single one who failed to tell me that for a long time he believed the contrary opinion, but that he switched to this one due to the strength of the reasons supporting it; moreover, I examined each one of them to see how well he understood the reasons for the other side, and I found everyone had them at his fingertips; thus, I cannot say that they accepted this opinion out of ignorance or vanity or (as it were) to show off. On the other hand, out of curiosity I also asked many Peripatetics and Ptolemaics how well they had studied Copernicus’ book, and I found very few who had seen it and none who (in my view) had understood it; I also tried to learn from the same followers of the Peripatetic doctrine whether any of them had ever held the other opinion, and similarly I found none who had. Now, let us consider these findings: that everyone who follows Copernicus’ opinion had earlier held the contrary one and is very well informed about the reasons of Aristotle and Ptolemy; and that, on the contrary, no one who follows Aristotle and Ptolemy has in the past held Copernicus’ opinion [155] and abandoned it to accept Aristotle’s. Having considered these findings, I began to believe that when someone abandons an opinion imbibed with mother’s milk and accepted by infinitely many persons, and he does this in order to switch to another one accepted by very few and denied by all the schools (and such that it really does seem a very great paradox), he must be necessarily moved (not to say forced) by stronger reasons.39 Therefore, I have become most curious to go, as it were, to the bottom of this business, and I regard myself very fortunate to have met the two of you; without any great effort I can hear from you all that has been said (and perhaps all that can be said) on this subject, and I am sure that by virtue of your arguments I will lose my doubts and acquire certainty.
SIMP. But beware that your belief and hope will not be frustrated, and that you will not end up being more confused than before.
SAGR. I think I am sure that this cannot happen in any way.
SIMP. Why not? I myself am a good witness that the further we go, the more confused I become.
SAGR. That is an indication that those reasons, which so far seemed conclusive to you and kept you certain of the truth of your opinion, are beginning to feel different in your mind and to gradually let you, if not switch, at least incline toward the contrary one. However, I, who am and have been so far undecided, am very confident to be able to reach a state of serenity and certainty; and you yourself will not deny it, if you want to listen to my reasons for this expectation.
SIMP. I will be glad to listen, and no less glad if the same effect should be produced in me.
SAGR. Please, then, answer my questions. Tell me, first, Simplicio, whether the conclusion whose correctness we are trying to determine is not one of the following: whether one must hold, with Aristotle and Ptolemy, that the earth stands still at the center of the universe and all the heavenly bodies move; or whether the stellar sphere stands still, the sun is placed at the center, and the earth is located off the center and has those motions which appear to belong to the sun and to the fixed stars.
SIMP. These are the conclusions about which we are disputing.
SAGR. Are these two conclusions such that it is necessary for one of them to be true and the other false?
[156] SIMP. That is correct. We are facing a dilemma in which it is necessary that one alternative should be true and the other false. For rest and motion are contradictories, and there is no third alternative such that one might say: “The earth neither moves nor stands still, and the sun and stars neither move nor stand still.”
SAGR. Are the earth, sun, and stars insignificant or substantial bodies in nature?
SIMP. These bodies are the most important, magnificent, huge, substantial, and integral parts of the universe.
SAGR. What kind of phenomena are motion and rest in nature?
SIMP. They are so pervasive and important that nature herself is defined in their terms.40 SAGR. Thus, to be eternally in motion and to be completely immobile are two very significant conditions in nature, especially when attributed to the most important bodies of the universe; as a result of those conditions one can get only very dissimilar occurrences.
SIMP. Certainly.
SAGR. Now, respond to another point. Do you believe that in logic, rhetoric, physics, metaphysics, mathematics, and reasoning in general, there are good arguments proving false as well as true conclusions?
SIMP. No, sir! Instead I firmly believe and am sure that for the proof of a true and necessary conclusion there are in nature not just one but many very powerful demonstrations, that one can discuss and approach it from thousands of points of view without ever encountering any contradiction, and that the more a sophist would want to taint it the clearer its certainty would become. On the contrary, to make a false proposition appear true and to persuade someone of it one can produce nothing but fallacies, sophisms, paralogisms, equivocations, and arguments that are pointless, incoherent, and full of inconsistencies and contradictions.
SAGR. Now, if eternal motion and eternal rest are such important properties in nature and so different that their effects must be very different, especially when attributed to such huge and noteworthy bodies in the universe as the earth and the sun; if it is impossible that one of the two contradictory propositions [157] should not be true and the other false; and if to prove a false proposition one can only produce fallacies, whereas a true one is supportable by all kinds of conclusive and demonstrative arguments; if all this is true, how can it be that someone undertaking to support a true proposition would not be able to persuade me? I would have to have a stupid understanding, a perverse judgment, a dull mind and intellect, and a dim-witted common sense; and I would have to be unable to discern light from darkness, gems from coals, and truth from falsehood.
SIMP. As I have said other times, I tell you that the greatest master from whom to learn how to recognize sophisms, paralogisms, and other fallacies is Aristotle; in this regard, he can never be deceived.
SAGR. You again mention Aristotle, who cannot speak; and I tell you that if Aristotle were here, he would be persuaded by us or he would dissolve our reasons and persuade us with better ones. At any rate, in hearing the gunshot experiments related, did you yourself not admire them and recognize and admit them to be more conclusive than those of Aristotle? Nevertheless, I do not see that Salviati (who has produced them, has undoubtedly examined them, and has probed them most fastidiously) is
admitting being persuaded by them, or even by the stronger ones which he indicates he is about to present. I do not know why you would want to portray nature as having become senile and having forgotten how to produce theoretical intellects, except those who make themselves servants of Aristotle in order to understand with his brain and perceive with his senses. However, let us listen to the remaining reasons favorable to his opinion, and then go on to test them by refining them in the assayer’s crucible and weighing them in his balance.41
SALV. Before proceeding further, I must tell Sagredo that in these discussions I act as a Copernican and play his part with a mask, as it were. However, in regard to the internal effect on me of the reasons I seem to advance in his favor, I do not want to be judged by what I say while we are involved in the [158] enactment of the play, but by what I say after I have put away the costume; for perhaps you will find me different from what you see when I am on stage. Now, let us go on.
Ptolemy and his followers advance another observation, similar to that of projectiles: it concerns things that are separate from the earth and remain at length in the air, such as clouds and birds in flight. Since clouds are not attached to the earth, they cannot be said to be carried by it, and so it does not seem possible that they could keep up with its speed; instead, they should all appear to us to be moving very fast toward the west. And, if we are carried by the earth and in twenty-four hours move along our parallel (which is at least sixteen thousand miles), how could birds keep up with so much drift? On the contrary, we see them fly toward the east as well as toward the west and toward any other direction, without any sensible difference.
Furthermore, when we run on horseback we feel the air strike very hard against our face, and so what a wind should we constantly feel blowing from the east if we are carried with such rapid motion against the air? Yet, no such effect is felt.
Here is another very ingenious argument, taken from the following observation; it is this: circular motion has the property of extruding, scattering, and throwing away from its center the parts of the moving body whenever the motion is not very slow or the parts are not attached together very firmly. For example, consider those huge treadmill wheels designed so that the walking of a few men on their inner surface causes them to move very great weights, such as the massive rollers of a calender press or loaded barges dragged overland to move them from one river to another; now, if we made one of these huge wheels turn very rapidly and its parts were not very firmly put together, they would all be scattered along with any rocks or other material substances however strongly tied to its external surface; nothing could resist the impetus which would throw them with great force in various directions away from the wheel, and consequently away from its center. If, then, the earth were rotating with a very much greater speed, what weight and what strength of mortar or cement would keep rocks, buildings, and entire cities from being hurled toward the sky by such a reckless turning? And think of people and animals, which are not attached to the earth at all; how would they resist so much impetus? On the contrary, we see them and other things with much less resistance (pebbles, sand, [159] leaves) rest very calmly on the earth and fall back to it even when their motion is very slow.
Here, Simplicio, are the very powerful reasons taken from terrestrial things, so to speak. We are left with the other kind, namely, those that relate to heavenly phenomena. Actually, those reasons tend to demonstrate instead that the earth is at the center of the universe and consequently lacks the annual motion around it, which Copernicus attributed to the earth; since they deal with a somewhat different subject, they can be produced after we have examined the strength of the ones presented so far.
SAGR. What do you say, Simplicio? Does it seem that Salviati knows and can explain the Ptolemaic and Aristotelian reasons? Do you think that any Peripatetic is equally knowledgeable of the Copernican demonstrations?
SIMP. If the discussions so far had not produced in me such a high opinion of Salviati’s well-founded understanding and of Sagredo’s sharp intelligence, I (with their permission) would be ready to leave without listening to anything else. For it seems to me impossible that one can contradict such palpable observations; moreover, I would like to keep my old opinion without having to hear anything else, because it seems to me that even if it were false, the fact that it is supported by such likely reasons would render it excusable. If these are fallacies, what true demonstrations were ever so beautiful?
SAGR. Still, it will be good to hear Salviati’s answers. If these should be true, they must be even more beautiful and infinitely more beautiful, and those others must be ugly, indeed very ugly; this would follow if there is truth in the metaphysical proposition that truth and beauty are the same thing, as falsehood and ugliness also are. However, Salviati, let us not lose any more time.
[§8.5 Day II:Vertical Fall, Conservation of Motion,
and the Role of Experiments]42
[164] SALV. So we can now go on to the fourth argument, which should be discussed at great length since it is based on an observation from which most of the remaining arguments then derive their strength. Aristotle says43 that a most certain argument for the earth’s immobility is based on the fact that we see bodies which have been cast upwards return perpendicularly by the same line to the same place from which they were thrown, and that this happens even when the motion reaches a great height; this could not happen if the [165] earth were moving because, while the projectile moves up and down separated from the earth, the place of ejection would advance a long way toward the east due to the earth’s turning, and in falling the projectile would strike the ground that much distance away from the said place. Here we may also include the argument from the cannon ball shot upwards, as well as another one used by Aristotle and Ptolemy, namely, that one sees bodies falling from great heights move in a straight line perpendicular to the earth’s surface. Now, to begin to untie these knots, I ask Simplicio how Aristotle and Ptolemy would prove, if someone denied it, that bodies falling freely from on high move in a straight and perpendicular line, namely, in the direction of the center.
SIMP. By means of the senses: they assure us that the tower is straight and perpendicular; they show us that the falling rock grazes it without inclining so much as a hairbreadth to one side or the other; and they show that the rock lands at the foot of the tower exactly under the place from which it was dropped.
SALV. But if by chance the terrestrial globe were rotating and consequently were also carrying the tower along with it, and if the falling rock were still seen to graze the edge of the tower, what would its motion have to be?
SIMP. In that case one would rather have to speak of “its motions”; for there would be one that would take it from above downwards, and it would have to have another in order to follow the course of the tower.
SALV. Therefore, its motion would be a compound of two, namely, one with which it grazes the edge of the tower, and another one with which it follows the tower; the result of this compound would be that the rock would no longer describe a simple straight and perpendicular line, but rather an inclined, and perhaps not straight, one.44
SIMP. I am not sure about its not being straight; but I understand well that it would have to be inclined and different from the straight perpendicular one it would describe on a motionless earth.
SALV. Therefore, from just seeing the falling rock graze the tower, you cannot affirm with certainty that it describes a straight and perpendicular line unless you first assume the earth to be standing still.
SIMP. That is correct; for if the earth were moving, the rock’s motion would be inclined and not perpendicular.
[166] SALV. Here, then, is the paralogism of Aristotle and Ptolemy made clear and evident, and discovered by yourself; the argument is assuming as known what it is trying to prove.
SIMP. In what way? To me it seems to be a syllogism in proper form and not a fallacy of question begging.
SALV. Here is how. Tell me: does not the demonstration
regard the conclusion as unknown?
SIMP. Yes, unknown, for otherwise it would be superfluous to demonstrate it.
SALV. But, should not the middle term be known?
SIMP. That is necessary, for otherwise it would be an attempt to prove the unknown by means of what is equally unknown.
SALV. Is not the conclusion to be proved, and which is unknown, the proposition that the earth stands still?
SIMP. It is.
SALV. Is not the middle term, which must be already known, the straight and perpendicular fall of the rock?
SIMP. That is the middle term.
SALV. But, did we not just conclude that we can have no knowledge that this fall is straight and perpendicular unless we first know that the earth is standing still? Therefore, in your syllogism the certainty of the middle term is inferred from the uncertain conclusion. So you see the type and the seriousness of the paralogism.
SAGR. On behalf of Simplicio, I should like to defend Aristotle, if possible, or at least to understand better the strength of your inference. You say: seeing the rock graze the tower is not enough to become certain that its motion is perpendicular (which is the middle term of the syllogism) unless one assumes that the earth stands still (which is the conclusion to be proved); for, if the tower were moving together with the earth and the rock grazed it, the rock’s motion would be inclined and not perpendicular. However, I will answer that, if the tower were moving, it would be impossible for the falling rock to graze it; hence, from seeing the falling rock graze it one infers that the earth is motionless.
SIMP. That is correct. For, if the falling rock should graze the tower while the latter was carried along by the earth, the rock would have to have two natural motions (namely, straight toward the center and circular around the center); and this is impossible.
SALV. Therefore, Aristotle’s defense consists in its being impossible, [167] or at least in his having regarded it as impossible, that the rock could move with a motion mixed of straight and circular; for, if he had not regarded it as impossible that the rock could move simultaneously toward the center and around the center, he would have understood that it could happen that the falling rock could graze the tower when it is moving as well as when it is standing still; consequently, he would have realized that from this grazing nothing could be inferred regarding the motion or the rest of the earth. However, this does not in any way excuse Aristotle, because he should have said so if he had had this thought in mind, it being such a key point in his argument; moreover, one cannot say either that this effect is impossible or that Aristotle regarded it as impossible. The first cannot be said, because I will soon show that it is not only possible but necessary. Nor can one say the second, for Aristotle himself grants45 that fire goes naturally upward in a straight line and turns by participation with the diurnal motion, which is transferred by the heavens to all of the element fire and to most of the air; if, then, he did not regard it as impossible to mix straight upward motion with the circular one communicated to fire and air by the inside of the lunar orb, much less should he regard it as impossible to mix the rock’s straight downward with the circular one that would be natural for the whole terrestrial globe of which the rock is a part.
The Essential Galileo Page 28