Three men meet in a Venetian palace. They have come together to discuss "God's wonders in the heavens and on Earth", more specifically which of the two competing "world systems" is right: the Ptolemaic or the Copernican. As truly inquisitive men they have set aside four whole days for their discussions.
The owner of the palace is called Sagredo. He is wise and well informed, not a specialist in science or philosophy, but quick-thinking and well acquainted with the range of positions and views. He has invited a representative of each of the two world philosophies: the Copernican, Salviati, and the solid Aristotelian, Simplicio.
This is the literary structure, the fictional framework if you like, round Galileo's main work. Within it he tried to create a context in which all thinking Italian readers could themselves take a position about the degree of truth in the cosmological discussions, without being distracted by any ecclesiastically conditioned interpretation.
In order for this to succeed, it was not only the scholastic material that had to be convincing. The literary structure had also to grip the reader and preferably hold him captive throughout the lengthy book.
The author in Galileo brings this off. Even though the conversation between the three of them sometimes necessarily assumes the character of a string of deductions, it always remains a conversation. The three are individually drawn, each has his "voice", and they are certainly not reluctant to contribute quick, witty comments and characterisations. After Simplicio has carefully described how all the material in the heavens is unalterably and impenetrably solid (because Aristotle says so), Sagredo exclaims: "What excellent stuff, the sky, for anyone who could get hold of it for building a palace!" But Salviati disagrees:
"Rather, what terrible stuff, being completely invisible because of its extreme transparency. One could not move about the rooms without grave danger of running into the doorposts and breaking one's head."61
Like all writers Galileo took elements of his characters from himself.
The author Italo Calvino has pointed out that Salviati and Sagredo represent two aspects of Galileo's personality: Salviati stands for his careful, methodical reasoning, while Sagredo uses his imagination, draws unexpected conclusions, asks surprising questions: what does life on the moon look like, if it exists? What would happen if the Earth stopped dead in its Copernican revolutions?62
Simplicio, by contrast, is no worthy opponent. He is constantly portrayed in a comic light with his credulous references to Aristotle and his commentators, not to mention contemporary anti-Copernicans. He thinks sluggishly and needs to have Salviati's reasoning thoroughly explained, whereas Sagredo grasps it immediately and often adds perceptive comments. When the others ask if he has read The Assayer or Letters on Sunspots, Simplicio answers that he has flipped through them, but has spent most ofhis time on more solid studies.
There is a fourth person mentioned in the book, but never by name, he is simply called "our mutual friend" or "the academician". This is Galileo himself, and Salviati, it must be said, refers to him almost as Simplicio does to Aristotle.
Clearly, the names are not accidental. Sagredo was Galileo's Venetian friend and benefactor from his years at Padua, Salviati the rich Florentine who owned the Villa delle Selve where Galileo had often lived and worked. He says in his preface that he wants the reputation of these two deceased friends to live on in the pages of his book. Simplicio, on the other hand, is a kind of pseudonym - it stands as it were for the average Aristotelian philosopher:"... whose greatest obstacle in apprehending the truth seemed to be the reputation he had acquired by his interpretations of Aristotle," as Galileo says in his preface.
The actual name Simplicio was in fact taken from a well-known sixth century Aristotelian commentator. But it certainly was not adopted at random - the Italian word semplicione means "unsophisticated person".
The first day's discussions concentrate largely on the relationship between earthly mutability and heavenly perfection. Poor Simplicio is bombarded with information about comets, sunspots and the Moon. He even has Aristotle turned against him, when Salviati ironically notes that it must be far better Aristotelian philosophy to say "Heaven is changeable because my senses tell me so" than "Heaven is immutable because Aristotle worked it out".
As the book unfolds Simplicio - and the reader - are treated to various lessons in the theory of motion, astronomy and optics. The unfortunate philosopher must unwillingly admit that there are one or two things he has not understood - but he defends himself stoutly with the aid of an impressive array of authorities old and new. One such is "a recent little book of hypotheses", which is supposed to refute all Copernican claims.
This book is Father Scheiner's Mathematical Discourses, the little book that the Jesuit had once sent to Galileo many years earlier in the hope of provoking a reply and a discussion. Now he receives his answer - it is much delayed but, to make up for it, pretty clear. To take just one example of Salviati and Sagredo's comments: they assume that the author (who is not mentioned by name) cannot be so foolish as to believe what he himself has written, but is trying to hoodwink people. And as if that were not enough:
"Those who have nets to snare the common people know also how to be the authors of other men's inventions, so long as these are not ancient ones and have not been published in the schools and in the market places so they are more than familiar to everyone."63
In other words it is still the priority to the discovery of sunspots that rankles here.
Towards the end of the first day, Salviati makes some comments on the relationship between human and divine understanding. He says that it is true that human knowledge is nothing compared to God's, for the latter's is infinite, and even something is nothing compared to the infinite. But as regards the few things about which man can achieve true knowledge, his knowledge is qualitatively as certain as God's, if it is underpinned with definite proof - there is no extra degree of certainty above and beyond that which can be demonstrated incontrovertibly. This only applies to limited aspects of arithmetic and geometry, but the assertion still causes Simplicio to exclaim:
"This speech strikes me as very bold and daring."64
Far from it, Salviati replies - these are perfectly ordinary statements. But on this one point unfortunately, Simplicio is right.
The discussions on the second day take up the largest part of the book. Here, the most difficult part of the Copernican theory is aired. If the Earth really does revolve completely on its own axis in the course of a day, how can it be that we who live there, do not experience the least sensation of it?
Galileo was quite used to meeting such arguments in discussions, and his elucidation is therefore a pedagogic masterpiece. Taking dozens of examples from daily life, Salviati hammers home the tenets of motion. The most important of these is that all motion is relative. When we are on a ship travelling at a constant speed, we only notice motion in relation to the water, to islands, other boats etc. - not in relation to other objects on the ship, which are moving at precisely the same speed as ourselves. It is the same with the Earth, because the planet and everything on it, including us human beings, are making the same journey.
Salviati's Copernican defence is so full of power and conviction that it feels almost painful when Galileo suddenly recalls the conditions under which he is writing. Then, quickly, he interjects a little aside
"[I] who am impartial between these two opinions and masquerade as
Copernicus only as an actor in these plays of ours... "65
But it gets worse. On the third day the discussion becomes more technical and astronomical, dealing mainly with the Earth's annual movement around the Sun, Copernicus' quintessential point: which heavenly bodies move and which stand still. Painstakingly, Salviati describes all the seemingly extraordinary phenomena Ptolemy must explain, but which vanish if one reverses the system and gives the Earth an orbit:
"The illnesses are in Ptolemy, and the cure for them in Copernicus."66
This is all to do with the planets and the so-called retrograde motions.
But here, too, sunspots are dealt with. And as Salviati makes clear:
"our Lyncean Academician" discovered them in 1610, in Padua. Furthermore, he "spoke about them to many people here in Venice, some of whom are yet living."67
This, to put it mildly, is definitely a lapse of memory.
In a letter from Galileo to Maffeo Barberini dated 2 July 1612 he wrote that he saw the sunspots "about eighteen months ago", so about New Year 1611; not when he was living in Padua in the spring of 1610. The difference of nine months in the new dating may seem insignificant - but it is just enough to pre-empt Scheiner in the discovery. This is then thoroughly rubbed in. Galileo is, according to Salviati:
"The original discoverer and observer of the solar spots (as indeed of all other novelties in the skies)."68
Sunspots describe what looks like gently curving trajectories across the face of the Sun. If one assumes that the Earth moves in a plane that is not absolutely vertical to the Sun's axis, the sunspots' motions will appear just like this from the Earth. This is presented as an argument in favour of Copernicus, based on observations Galileo was said to have made. There is no trace of these observations in Galileo's notes. Of course, it is possible that they have been lost, but what is certain is that Scheiner published just such observations in Rosa Ursina. It is not surprising that, having read the Dialogue, he believed that Galileo had simply used his own painstaking work of many years.
Then Salviati begins to demolish Scheiner's various anti-Copernican arguments. This is done without mention of his name or that of his book, but with phrases like "apish puerelities",69 "trifling tomfooleries",70 "gigantic fallacy".71 It all culminates in Salviati addressing Scheiner directly: "O foolish man!"72
On the fourth day the three debaters finally tackle the tides, the original theme of the Dialogue. This chapter is shorter than the others and lacks their supple digressions; it chiefly consists of a continuous lecture from Salviati in which he lays out Galileo's complicated reasoning concerning the interplay between the Earth's rotation and its orbit in space. Kepler is given a friendly kick for his medieval superstitiousness about the Moon's influence - but otherwise the tone is much more subdued. It is as if part of Galileo senses that his much beloved tidal theory really is not convincing enough to crush its opponents.
But then the discussion must be brought to a close. And it cannot happen in the way that all of the preceding pages - close on 500 of them - have been building up to, with the obvious conclusion that everything supports Copernicus: the Sun is motionless, the Earth moves in its orbit and on its own axis. Quite the opposite, as Salviati suddenly says:
"I do not claim and have not claimed from others that assent which I myself do not give to this invention, which may very easily turn out to be a most foolish hallucination and a majestic paradox."73
And so Simplicio is left with his conclusion:
"Keeping always before my mind's eye a most solid doctrine that I once heard from a most eminent and learned person, and before which one must fall silent, I know that if I asked whether God in His infinite power and wisdom could have conferred upon the watery element its observed reciprocating motion using some other means than moving its containing vessels, both of you would reply that He could have, and that He would have known how to do this in many ways that are unthinkable to our minds."74
Both the others are in heartfelt agreement. And how could they be otherwise? For the eminent and learned person whose pet argument is here being rehearsed by the play's Pantaloon, is His Holiness himself, Galileo's intimate friend of many years, Urban VIII Barberini.
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