Galileo Dismantles Aristotle's Separation of Earth from the Heavens
Galileo's Dialogue, Day 1
As I wrote in a recent post, I intend to write a post for each of the four days in the Dialogue Comparing the Ptolemaic and Copernican Systems1, the book that got Galileo in trouble with Rome. This post pertains to Day One.
Officially, the Dialogue was written to compare the Ptolemaic and Copernican systems of astronomy. In practice, the book makes a clear stand in favour of the Copernican system. To prepare this stand, Day One focuses on dismantling the worldview in Aristotle’s De Caelo (On the Heavens), which had dominated discourse for close to two thousand years, and upon which the Ptolemaic system was defined.
Day One begins by focusing on the Aristotelean distinction between two substances, the celestial and the elemental. The celestial corresponds to the heavens, including the moon, the sun, the five planets known at the time (Mercury, Venus, Mars, Jupiter and Saturn), and the fixed stars, while the elemental corresponds to the sublunar space, i.e., what happens on earth.
Aristotle argued in favour of this distinction between the celestial and the elemental from both a priori and a posteriori perspectives. Galileo’s representative, Salviati, begins the discussion by addressing the a priori perspectives.
For Aristotle, the number Three was perfect, and the world was perfect because there were three dimensions: length, breadth, depth. Furthermore, there were three kinds of simple motion, all local: circular (around the center), straight down (towards the center), and straight up (away from the center). Here, the center is understood to be both the center of the earth and the center of the universe, which are assumed to coincide. Heaviness and lightness are considered to be two separate attributes, and heavy elements (water and earth) naturally fall straight down, while light elements (fire and air) naturally rise straight up.
Upwards motion and downwards motion are mutual opposites, but circular motion has no opposite, hence it is perfect. The heavens all rotate around the earth in circular orbits, hence are perfect. Because they are perfect, they are ingenerable, incorruptible, inalterable, impenetrable, etc. The elemental, on the hand, is exposed to continual alternation, mutation, etc.
Salviati, representing Galileo, is not convinced by the perfection of the number Three:
I do not understand, let alone believe, that with respect to legs, for example, the number three is more perfect than four or two; neither do I conceive the number four to be any imperfection in the elements, nor that they would be more perfect if they were three.
As this discussion is heating up, Simplicio, representing Aristotle, pipes up:
But I still say, with Aristotle, that in physical (natural) matters one need not always require a mathematical demonstration.
To which Sagredo responds:
Granted, where none is to be had; but when there is one at hand, why do you not wish to use it?
The rôle of Sagredo is to serve as sounding board. For example, Sagredo provides the perfect questions for Salviati to answer:
Why distinguish up and down, when it is just motion in a straight line? Why assume that circular motion only occurs around a single point?
Sagredo’s rôle is also to call out the absurd circular reasoning that is used repeatedly by Simplicio:
A little while ago you would have it that simple and mixed motions would reveal to me which bodies were compound and which were simple. Now you want to use simple and compound bodies to find out which motion is simple, and which is mixed — an excellent rule for never understanding either motions or bodies.
After these exchanges, Salviati makes it clear that he wishes to dismantle the Aristotelean edifice by undermining its foundations:
The whole Aristotelian edifice rests upon the perfection of circular motion, as it is not infinite.
First, Salviati makes it clear that although a mathematical line can go on forever at both extremities, that in fact, motion in a straight line in the real world can only go for a finite distance, until the object has reached its natural position (this is a term that would naturally be used by the Peripatetics, the name for the supporters of Aristotle).
Second, he demonstrates that if an object is thrown upwards, it will reach all of the intermediate velocities upwards from its original upwards velocity until it reaches the apex of its trajectory, and then all of the intermediate velocities downward until its maximum velocity when it reaches the ground.
Third, he cites Plato, giving the possibility that many circular motions begin with straight motion until the object has reached its natural place, then continues with circular motion.
These three points lead to the understanding that the different motions are not completely separate, as an object can pass from one to another.
After these exchanges, Salviati makes it clear that many aspects of Aristotle’s reasoning are based on tenuous assumptions:
I might add that neither Aristotle nor you can ever prove that the earth is de facto the center of the universe; if any center may be assigned to the universe, we shall rather find the sun to be placed there, as you will understand in due course.
All this time, Sagredo is encouraging Salviati to go on with his explanations, and Simplicio is getting more and more confused. At some point, Salviati goes for the jugular [emphasis mine]:
I answer that none of the conditions by which Aristotle distinguishes celestial from elemental bodies has any other foundation than what he deduces from the difference in natural motion between the former and the latter. In that case, if it is denied that circular motion is peculiar to celestial bodies, and affirmed to belong to all naturally movable bodies, then one must choose one of two necessary consequences. Either the attributes of generable-ingenerable, alterable-inalterable, divisible-indivisible, etc. suit equally and commonly all world bodies — as much the celestial as the elemental — or Aristotle has wrongly and erroneously deduced, from circular motion, those attributes which he has assigned to celestial bodies.
The discussion of a priori arguments ends with the following hilarious exchange. Simplicio says:
For a beginning, then, here are two powerful demonstrations proving the earth to be very different from celestial bodies. First, bodies that are generable, corruptible, alterable, etc., are quire different from those that are ingenerable, incorruptible, inalterable, etc. The earth is generable, corruptible, alterable, etc., while celestial bodies are ingenerable, incorruptible, inalterable, etc. Therefore the earth is very different from the celestial bodies.
To which Sagredo responds, in his sarcastic style:
With your first argument, you bring back to the table what has been standing there all day and has just now been carried away.
The discussion then moves on to a posteriori arguments. Simplicio begins:
Sensible experience shows that on earth there are continual generations, corruptions, alterations, etc., the like of which neither our senses nor the traditions or memories of our ancestors have ever detected in heaven; hence heaven is inalterable, etc., and the earth alterable, etc., and therefore different from the heavens.
The second argument I take from a principal and essential property, which is this: whatever body is naturally dark and devoid of light is different from luminous and resplendent bodies; the earth is dark and without light, and celestial bodies are splendid and full of light; therefore, etc.
Salviati picks up the gauntlet and completely takes apart both arguments put forward by Simplicio. With respect to the first argument, Salviati brings up a number of celestial phenomena, clearly showing mutable heavens, that had been noted in the heavens in the previous decades, including the “new stars” (supernovae) sighted in 1572 and 1604 and the three comets that appeared in 1608. Simplicio replies by quoting from Anti-Tycho, a text written by Scipione Chiaramonti in 1621 against Tycho Brahe, the Danish astronomer; Chiaramonti had simply denied against all evidence that these were new phenomena in the sky. Salviati adds that Chiaramonti, a staunch defender of the Aristotelian orthodoxy, simply ignored the sunspots that Galileo and others observed. Salviati further explains why the sunspots had to be attached to the sun, and were not simply mini-satellites of the sun, and Simplicio replies that for the moment, he does not know how to respond, but one day some learned Peripatetic will come up with the correct response.
Salviati’s response is memorable:
But in the natural sciences, whose conclusions are true and necessary and have nothing to do with human will, one must take care not to place oneself in the defense of error; for here a thousand Demostheneses and a thousand Aristotles would be left in the lurch by every mediocre wit who happened to hit upon the truth for himself. Therefore, Simplicio, give up this idea and this hope of yours that there may be men so much more learned, erudite, and well-read than the rest of us as to be able to make that which is false become true in defiance of nature.
In order to deal with the second argument, Galileo uses Sagredo to voice a crucial idea, namely that a mutable world is better than a static one, as it is living:
I cannot without great astonishment — I might say without great insult to my intelligence — hear it attributed as a prime perfection and nobility of the natural and integral bodies of the universe that they are invariant, immutable, inalterable, etc., while on the other hand it is called a great imperfection to be alterable, generable, mutable, etc. For my part I consider the earth very noble and admirable precisely because of the diverse alterations, changes, generations, etc. that occur in it incessantly.
The discussion then moves on to the nature of the moon. Salviati makes seven points:
The moon is round, like the earth, and this can be determined by the way that the moon reflects the light of the sun and by the variance through the month of the lunar phases.
The moon is dark and opaque, allowing reflection of the light from the sun.
The moon must be solid, as it has mountains visible through the telescope.
There is a sharp distinction between the bright and dark parts of the moon.
The earth seen from the moon would have phases similar to those of the moon seen from the earth.
The ashen light on the moon in its quarter is a reflection from the earth.
A lunar eclipse and solar eclipse are reciprocal.
An extensive discussion follows, as Simplicio insists that the light from the moon is its own, and not the reflection of light from the sun. Furthermore, he insists that the moon is smooth and polished, and that the earth is incapable of reflecting light from the sun to create the ashen light during the quarter, as the earth has too rough a surface.
The response from Salviati is crucial. It is precisely those surfaces that are rough that can be seen from afar, because the light that is reflected from those surfaces is diffused, and hence can be seen to be bright, no matter from where one is looking. He demonstrates this fact by having a mirror placed on an outer wall, facing the sun. Apart from the spot where the sun’s light is exactly reflected by the sun, everywhere else, the wall appears brighter than does the mirror. And for a convex mirror, as Simplicio claims the moon to be, it would not even be visible.
With respect to the reflection of sunlight by the earth, it is not the sea, which has no mountains, that is providing light to the moon, but the land, for precisely the same reasons given above. Furthermore, the earth’s reflection of sunlight should be much brighter than the moon’s reflection.
The day’s discussion ends with a more explicit philosophical bent. Salviati mentions that he had considered the possibility of civilization on the moon, but ultimately concluded that it was impossible, since the lunar days were too long. This prompts a discussion about how much we can know. Salviati recalls that Socrates had been named “the wisest of the Greeks,” yet he always said that he knew nothing. The two statements could be both true, because, compared to other men, Socrates did know so much, but in the absolute sense, he knew nothing.
Nevertheless, Salviati insists that although the human mind is not of being directly compared to the Divine mind, it is possible for the human mind to completely understand certain topics, as in geometry, and that this is for him proof that “the human mind is a work of God’s, and one of the most excellent.”2 The day concludes with Sagredo praising some of the great achievements of humanity, including the works of Michelangelo, Raphael and Titian, and those who discovered the intervals in music, but most importantly, the one who invented writing.
To conclude, Day One of the Dialogue, by bringing the celestial and elemental together, had two objectives. First, of course, was to bring the heavens down to Earth. The second, no less important, was to elevate humanity, and specifically, the human mind, to the heavens.
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Galileo Galilei. Dialogue concerning the two chief world systems — Ptolemaic and Copernican. Translated by Stillman Drake, foreword by Albert Einstein. University of California Press, 2nd ed., 1967.
It should be noted that the fact that Galileo “asserted some equality between the Divine and the human mind in geometrical matters” was one of the points for which Galileo had problems with Rome. Ibid, p.474.