The Essential Galileo (20 page)

2.
Galilei 1890–1909, 4: 87.21–99.12; translated by Thomas Salusbury (1661–65, 2: 426–36); revised by Finocchiaro for this volume.

3.
Salusbury (1661–65, 2: 426) labels this proposition “theorem V,” and he gives such sequentially numbered labels to other key propositions in this work. Such labeling was retained in Drake's reprint of that translation (Galilei 1960). But there is no such labeling in the original text (Galilei 1890–1909, 4: 63–140). So I follow Drake (1981) in doing without such labeling altogether.

4.
The last four words (“descends to the bottom”) are omitted in Salusbury 1661–65, 2: 434, presumably due to a typographical error.

5.
This sentence and the next are missing from Salisbury 1661–65, 2: 434, presumably due to some typographical error; they correspond to four lines in Galilei 1890–1991, 4: 97.13–17.

CHAPTER 3

From
History and Demonstrations Concerning Sunspots
(1613)
¹

[§
3.1 Solar Rotation and Indifferent Motion]
²

[133] From the things said so far, it seems to me, if I am not mistaken, that we can draw several necessary conclusions. Sunspots are contiguous or extremely near to the body of the sun. They are not permanent or fixed, but variable with respect to shape and density. They also undergo to various degrees some small imprecise and irregular movements. Absolutely all of them are produced and dissipated, some in shorter and others in longer periods. Moreover, it is manifest and indubitable that they turn around the sun.

However, it remains somewhat doubtful whether their turning happens because the solar body rotates and turns around itself thus carrying them along, or whether while the solar body remains motionless there is a turning of the environment that contains them and carries them along; it could be either way. It seems to me much more probable that the motion belongs to the solar body than to the environment.

I am induced to believe this, first, by the certainty that such an environment is very tenuous, fluid, and flexible. My certainty comes from seeing the spots contained in it change shape, combine, and separate so easily, which could not happen in a solid and rigid material (a proposition that will seem very novel to the common philosophy). Now, it seems that for a constant and regular movement such as the one that is shared by all the spots, its root and primary foundation could not lie in a flexible substance made of parts that do not cohere together and are thus subject to the fluctuations and disturbances of many other accidental movements, but rather must lie in a solid and rigid body where the motion of the whole and the parts is necessarily a single one; and it is reasonable to believe that such is the solar 97 body, by contrast to its environment. Such a motion could carry the spots around either by being transmitted to the environment by contact and to the spots through the environment, or by being transferred directly to the spots, also by contact.

Additionally, if someone wanted to claim [134] that the turning of the spots around the sun derived from motion belonging to the environment and not to the sun, I would think that in any case it would be almost necessary that the same environment transmit the same motion also to the solar globe by contact. For I have observed that physical bodies have a natural inclination toward that motion which they undergo by an intrinsic principle, without the need of a particular external mover, whenever they are not impeded by some obstacle (as it happens to heavy bodies moving downwards). Physical bodies also have repugnance toward other motions, and so they never move in such ways unless compelled violently by an external mover (as it happens to heavy bodies with regard to upward motion). Finally, physical bodies are indifferent toward still other motions—for example, heavy bodies toward horizontal motion: these bodies have no inclination toward it because it is not toward the center of the earth, and they have no repugnance for it because it does not make them move away from the same center. Thus, if we remove all external impediments, a heavy body on a spherical surface concentric with the earth will be indifferent to rest and to motion toward any part of the horizon, and it will remain in that state in which it has been placed; that is, if it is placed in a state of rest, it will remain at rest, and if it is placed in motion (e.g., toward the west), it will remain in that motion. For example, if a ship [135] on a calm sea were to receive some impetus just once, it would move continuously around our globe without ever stopping; and if it were placed at rest, it would perpetually remain at rest; as long as in the first case all extrinsic impediments could be removed, and in the second case no external moving cause came about.
³

If this is true, and indeed it is most true, what would a body of ambivalent nature do if it happened to be constantly surrounded by an environment that was moving with a motion to which that physical body was by nature indifferent? I do not think one can doubt that it would move with the motion of the environment. Now the sun, a spherical body suspended and balanced around its own center, cannot fail to follow the motion of its environment since it has neither an intrinsic repugnance nor an external impediment to such rotation. It cannot have an internal repugnance, given that by such rotation the whole is not removed from its location, and the parts are not permuted among themselves and do not change their natural constitution; thus, with regard to the relationship between the whole and the parts, it is as if such a motion did not exist. As regards the external impediments, it seems that no obstacle can hinder without contact (except perhaps the attraction of a loadstone); but in our case everything that touches the sun, that is to say, its environment, not only does not hinder the motion which we are trying to attribute to it, but also moves and transmits that motion as long as there is no resistance; this resistance cannot come from the sun, and hence there are no external impediments.

This can be confirmed even more strongly. For besides what I have already said, it does not seem that a movable body can have repugnance to a motion without having a natural propensity to the opposite (for there is no repugnance in indifference). So whoever wants to attribute to the sun an aversion to the circular motion of its environment would thereby attribute to it a natural propensity to circular motion in the opposite direction to that of the environment. This sounds wrong to any well-balanced intellect.

Therefore, since the apparent rotation of the spots must be attributed to the sun, it is better (for the first reason I gave) to regard solar rotation as natural rather than as acquired by participation.

[§3.2 Heavenly Changes and Aristotelian Empiricism]
⁴

[138] Now, to gather some fruit from the unexpected marvels that have remained hidden until our time, it will be good in the future to reconsider the wise philosophers who judged the heavenly substance differently from Aristotle, and from whom Aristotle himself would not have moved away if [139] he had been in possession of presentday sensory observations. For he not only allowed plain sense experience among the means capable of yielding conclusions about natural phenomena, but he also gave it first place.
⁵
Thus, since he argued for the immutability of the heavens from the fact that no alteration had ever been seen there in past times, it is very reasonable to believe that if the senses had shown to him what they have shown to us, he would have followed the contrary opinion, to which we are now led by such marvelous discoveries.

Indeed I shall go further. I think that by holding the heavenly
material to be alterable (based on the truth of present-day observations), I am opposing Aristotle's doctrine much less than those who would still want to claim it to be inalterable. For I am sure that he never regarded the conclusion of inalterability as certain as the principle that plain sense experience must have priority over any human theory.

Thus, one will philosophize better by giving assent to conclusions dependent on clear observations than by persisting in opinions that are repugnant to the senses and are confirmed only with probable or apparent reasons.

It is not difficult to understand the kind and the number of observed phenomena that lead us to more certain conclusions. Behold, to remove us from any ambiguity, a superior power [140] inspires someone to devise conclusive methods to understand that the generation of comets occurs in the heavenly region; but like a witness who quickly comes and goes, he is opposed by the majority of those who teach to others. Behold, we see new longer-lasting flames, looking like extremely bright stars, being produced and then dissipated in the farthest parts of the heavens; but this is not enough to convince those whose minds do not understand the necessity of geometrical demonstrations. Behold, finally, in the part of the heavens that deserves to be regarded as the purest and most genuine (that is, on the face of the sun itself), one discovers the constant production and quick dissipation of a countless multitude of dark, dense, and smoky spots. Here is a succession of things made and unmade that will not end any time soon; rather, lasting for all future ages, it will give human beings time to observe as much as they please and to learn doctrines that will make them certain about their place.

[§3.3 Knowing Properties vs. Knowing Essences]
⁶

[187] In my estimation, we should not totally refrain from the investigation of things, even if they are very far from us, unless we have first decided it best to postpone any speculative activity to all other occupations of ours. The reason is as follows.

Either we want, by theorizing, to try to penetrate the true and intrinsic essence of natural substances, or we want to limit ourselves to gain information about some of their properties. As for trying to penetrate the essence, I regard it as an undertaking and a job no less impossible and useless for the case of nearby elementary substances than for the case of heavenly and very remote substances. I feel equally ignorant about the substance of the earth and of the moon, of terrestrial clouds and of sunspots.

For understanding these nearby substances, I see no other advantage than the abundance of details; but these are equally not understood, and we keep searching through them with very little or no gain. If I ask what is the essence of clouds and am told that it is a humid vapor, next I will want to know what vapor is. Perhaps I will be told that it is water rarified by the action of heat and transformed accordingly. But equally unclear about what water is, I will ask for this, and finally I will hear that it is the fluid body which flows in rivers and which we constantly handle and deal with. But this information about water is merely more direct and dependent on more senses, but not more intrinsic than my earlier information about clouds. Similarly, I do not understand the true essence of earth or fire any more than that of the moon or the sun; this knowledge is reserved for our understanding when we reach the state of blessedness, [188] not before.

However, if we want to limit ourselves to knowledge of some properties, I do not think we should despair of being able to ascertain them in bodies that are extremely far from us as well as in those next to us; on the contrary, sometimes by chance we know more precisely a property of the former than one of the latter. Who does not know the periods of the motions of planets better than those of seawater? Who does not know that the spherical shape of the body of the moon was understood much earlier and more quickly than that of the earth? And is it not still controversial whether the earth remains motionless or goes wandering, whereas we are most certain about the motions of quite a few stars?

Thus, I want to conclude that although it would be fruitless to undertake the investigation of the essence of sunspots, it does not follow that we cannot know some of their properties, such as their location, motion, shape, size, opacity, mutability, production, and dissipation. These can then enable us to philosophize better about other more controversial questions regarding natural substances. Finally, lifting us to the final purpose of our efforts, namely, the love of the Divine Architect, they can sustain our hope of learning all other truths from Him, source of light and truth.

1.
For the historical background, see the Introduction, especially §0.6.

2.
Galilei 1890–1909, 5: 133.5–135.33; newly translated by Finocchiaro.

3.
Galileo's principle of indifferent motion in this paragraph should be compared and contrasted to the law of inertia, or Isaac Newton's first law of motion: “Every body perseveres in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by forces impressed” (Newton 1999, 416). As Drake (1957, 113 n. 8) notes, this paragraph contains an approximate formulation, as well as two applications “to the cases of (1) rotating bodies and (2) heavy bodies moving freely upon smooth spheres concentric with the earth.” Cf. also the discussion in Galileo's critique of the ship analogy argument, in Day II of the
Dialogue
, below in §8.5 (pp. 229–33).

4.
Galilei 1890–1909, 5: 138.24–140.16; newly translated by Finocchiaro.

5.
Cf. Aristotle,
On the Generation of Animals
, III, 10, 750b27 and 760b31.

6.
Galilei 1890–1909, 5: 187.8–188.19; newly translated by Finocchiaro.

CHAPTER 4
¹

Letters on Copernicanism and Scripture (1613–15)
²

§4.1 Letter to Castelli (1613)
³

[281] Very Reverend Father and My Very Respectable Sir:

Yesterday Mr. Niccolò Arrighetti came to visit me and told me about you. Thus I took infinite pleasure in hearing about what I did not doubt at all, namely, about the great satisfaction you have been giving to the whole University, to its administrators as well as to the professors themselves and to the students from all countries. This approval has not increased the number of your rivals, as it usually happens in similar cases, but rather they have been reduced to very few; and these few too will have to acquiesce unless they want this competition (which is sometimes called a virtue) to degenerate and to change into a blameworthy and harmful feeling, harmful ultimately more to those who practice it than to anyone else. However, the seal of my pleasure was to hear him relate the arguments which, through the great kindness of their Most Serene Highnesses, you had the occasion of advancing at their table and then of continuing in the chambers of the Most Serene Ladyship, in the presence also of the Grand Duke and the Most Serene Archduchess, the Most Illustrious and Excellent Don Antonio and Don Paolo Giordano, and some of the very excellent philosophers there. What greater fortune can you wish than to see their Highnesses themselves enjoying discussing with you, putting forth doubts, listening to your solutions, and finally remaining satisfied with your answers?

[282] After Mr. Arrighetti related the details you had mentioned, they gave me the occasion to go back to examine some general questions about the use of the Holy Scripture in disputes involving physical conclusions and some particular other ones about Joshua's passage,
⁴
which was presented in opposition to the earth's motion and sun's stability by the Grand Duchess Dowager with some support by the Most Serene Archduchess.

In regard to the first general point of the Most Serene Ladyship, it seems to me very prudent of her to propose and of you to concede and to agree that the Holy Scripture can never lie or err, and that its declarations are absolutely and inviolably true. I should have added only that, though Scripture cannot err, nevertheless some of its interpreters and expositors can sometimes err in various ways. One of these would be very serious and very frequent, namely, to want to limit oneself always to the literal meaning of the words; for there would thus emerge not only various contradictions but also serious heresies and blasphemies, and it would be necessary to attribute to God feet, hands, and eyes, as well as bodily and human feelings like anger, regret, hate, and sometimes even forgetfulness of things past and ignorance of future ones. Thus in Scripture one finds many propositions which look different from the truth if one goes by the literal meaning of the words but which are expressed in this manner to accommodate the incapacity of common people; likewise, for the few who deserve to be separated from the masses, it is necessary that wise interpreters produce their true meaning and indicate the particular reasons why they have been expressed by means of such words.

Thus, given that in many places Scripture is not only capable but necessarily in need of interpretations different from the apparent meaning of the words, it seems to me that in disputes about natural phenomena it should be reserved to the last place. For the Holy Scripture and nature both equally derive from the divine Word, the former as the dictation of the Holy Spirit, the latter as the most obedient executrix of God's commands; moreover, in order to adapt itself to the understanding of all people, it was appropriate for Scripture to say many things [283] which are different from absolute truth in appearance and in regard to the meaning of the words; on the other hand, nature is inexorable and immutable, and she does not care at all whether or not her recondite reasons and modes of operations are revealed to human understanding, and so she never transgresses the terms of the laws imposed on her; therefore, whatever sensory experience places before our eyes or necessary demonstrations prove to us concerning natural effects should not in any way be called into question on account of scriptural passages whose words appear to have a different meaning, since not every statement of Scripture is bound to obligations as severely as each effect of nature. Indeed, because of the aim of adapting itself to the capacity of unrefined and undisciplined peoples, Scripture has not abstained from somewhat concealing its most basic dogmas, thus attributing to God himself properties contrary to and very far from his essence; so who will categorically maintain that, in speaking even incidentally of the earth or the sun or other creatures, it abandoned this aim and chose to restrict itself rigorously within the limited and narrow meanings of the words? This would have been especially problematic when saying about these creatures things which are very far from the primary function of the Holy Writ, indeed things which, if said and put forth in their naked and unadorned truth, would more likely harm its primary intention and make people more resistant to persuasion about the articles pertaining to salvation.

Given this, and moreover it being obvious that two truths can never contradict each other, the task of wise interpreters is to strive to find the true meanings of scriptural passages agreeing with those physical conclusions of which we are already certain and sure from clear sensory experience or from necessary demonstrations. Furthermore, as I already said, though Scripture was inspired by the Holy Spirit, because of the mentioned reasons many passages admit of interpretations far removed from the literal meaning, and also we cannot assert with certainty that all interpreters speak by divine inspiration; hence, I should believe that it would be prudent not to allow anyone to oblige [284] scriptural passages to have to maintain the truth of any physical conclusions whose contrary could ever be shown to us by the senses and demonstrative and necessary reasons. Who wants to fix a limit for the human mind? Who wants to assert that everything which is knowable in the world is already known? Because of this, it would be most advisable not to add anything beyond necessity to the articles concerning salvation and the definition of the Faith, which are firm enough that there is no danger of any valid and effective doctrine ever rising against them. If this is so, what greater disorder would result from adding them upon request by persons of whom we do not know whether they speak with celestial inspiration, and of whom also we see clearly that they are completely lacking in the intelligence needed to understand, let alone to criticize, the demonstrations by means of which the most exact sciences proceed in the confirmation of some of their conclusions?

I should believe that the authority of the Holy Writ has merely the aim of persuading men of those articles and propositions which are necessary for their salvation and surpass all human reason, and so could not become credible through some other science or any other means except the mouth of the Holy Spirit itself. However, I do not think it necessary to believe that the same God who has furnished us with senses, language, and intellect would want to bypass their use and give us by other means the information we can obtain with them. This applies especially to those sciences about which one can read only very small phrases and scattered conclusions in Scripture, as is particularly the case for astronomy, of which it contains such a small portion that one does not even find in it the names of all the planets; but if the first sacred writers had been thinking of persuading the people about the arrangement and the movements of the heavenly bodies, they would not have treated of them so sparsely, which is to say almost [285] nothing in comparison to the infinity of very lofty and admirable conclusions contained in such a science.

So you see, if I am not mistaken, how disorderly is the procedure of those who in disputes about natural phenomena that do not directly involve the Faith give first place to scriptural passages, which they quite often misunderstand anyway. However, if these people really believe to have grasped the true meaning of a particular scriptural passage, and if they consequently feel sure of possessing the absolute truth on the question they intend to dispute about, then let them sincerely tell me whether they think that in a natural dispute someone who happens to maintain the truth has a great advantage over another who happens to maintain the false.
⁵
I know they will answer yes, and that the one who supports the true side will be able to provide a thousand experiments and a thousand necessary demonstrations for his side, whereas the other person can have nothing but sophisms, paralogisms, and fallacies. But if they know they have such an advantage over their opponents as long as the discussion is limited to physical questions and only philosophical weapons are used, why is it that when they come to the meeting they immediately introduce an irresistible and terrible weapon, the mere sight of which terrifies even the most skillful and expert champion? If I must tell the truth, I believe it is they who are the most terrified, and that they are trying to find a way of not letting the opponent approach because they feel unable to resist his assaults. However, consider that, as I just said, whoever has truth on his side has a great, indeed the greatest, advantage over the opponent, and that it is impossible for two truths to contradict each other; it follows therefore that we must not fear any assaults launched against us by anyone, as long as we are allowed to speak and to be heard by competent persons who are not excessively upset by their own emotions and interests.

To confirm this I now come to examining the specific passage of Joshua, concerning which you put forth three theses for their Most Serene Highnesses. I take the third one, which you advanced as mine (as indeed it is), but I add some other consideration that I do not believe I have ever told you.

Let us then assume and concede to the opponent that the words [286] of the sacred text should be taken precisely in their literal meaning, namely, that in answer to Joshua's prayers God made the sun stop and lengthened the day, so that as a result he achieved victory; but I request that the same rule should apply to both, so that the opponent should not pretend to tie me and to leave himself free to change or modify the meanings of the words. Given this, I say that this passage shows clearly the falsity and impossibility of the Aristotelian and Ptolemaic world system, and on the other hand agrees very well with the Copernican one.

I first ask the opponent whether he knows with how many motions the sun moves. If he knows, he must answer that it moves with two motions, namely, with the annual motion from west to east and with the diurnal motion in the opposite direction from east to west.

Then, secondly, I ask him whether these two motions, so different and almost contrary to each other, belong to the sun and are its own to an equal extent. The answer must be no, but that only one is specifically its own, namely, the annual motion, whereas the other is not but belongs to the highest heaven, I mean the Prime Mobile; the latter carries along with it the sun as well as the other planets and the stellar sphere, forcing them to make a revolution around the earth in twenty-four hours, with a motion, as I said, almost contrary to their own natural motion.

Coming to the third question, I ask him with which of these two motions the sun produces night and day, that is, whether with its own motion or else with that of the Prime Mobile. The answer must be that night and day are effects of the motion of the Prime Mobile, and that what depends on the sun's own motion is not night and day but the various seasons and the year itself.

Now, if the day derives not from the sun's motion but from that of the Prime Mobile, who does not see that to lengthen the day one must stop the Prime Mobile and not the sun? Indeed, is there anyone who understands these first elements of astronomy and does not know that, if God had stopped the sun's motion, He would have cut and shortened the day instead of lengthening it? For, the sun's motion being [287] contrary to the diurnal turning, the more the sun moves toward the east the more its progression toward the west is slowed down, whereas by its motion being diminished or annihilated the sun would set that much sooner; this phenomenon is observed in the moon, whose diurnal revolutions are slower than those of the sun inasmuch as its own motion is faster than that of the sun. It follows that it is absolutely impossible to stop the sun and lengthen the day in the system of Ptolemy and Aristotle, and therefore either the motions must not be arranged as Ptolemy says or we must modify the meaning of the words of Scripture; we would have to claim that, when it says that God stopped the sun, it meant to say that He stopped the Prime Mobile, and that it said the contrary of what it would have said if speaking to educated men in order to adapt itself to the capacity of those who are barely able to understand the rising and setting of the sun.

Add to this that it is not believable that God would stop only the sun, letting the other spheres proceed; for He would have unnecessarily altered and upset all the order, appearances, and arrangements of the other stars in relation to the sun, and would have greatly disturbed the whole system of nature. On the other hand, it is believable that He would stop the whole system of celestial spheres, which could then together return to their operations without any confusion or change after the period of intervening rest.

However, we have already agreed not to change the meaning of the words in the text; therefore it is necessary to resort to another arrangement of the parts of the world, and to see whether the literal meaning of the words flows directly and without obstacle from its point of view. This is in fact what we see happening.

For I have discovered and conclusively demonstrated that the [288] solar globe turns on itself, completing an entire rotation in about one lunar month, in exactly the same direction as all the other heavenly revolutions; moreover, it is very probable and reasonable that, as the chief instrument and minister of nature and almost the heart of the world, the sun gives not only light (as it obviously does) but also motion to all the planets that revolve around it; hence, if in conformity with Copernicus' position the diurnal motion is attributed to the earth, anyone can see that it suffices stopping the sun to stop the whole system, and thus to lengthen the period of the diurnal illumination without altering in any way the rest of the mutual relationships of the planets; and that is exactly how the words of the sacred text sound. Here then is the manner in which by stopping the sun one can lengthen the day on the earth, without introducing any confusion among the parts of the world and without altering the words of Scripture.