Monday, September 06, 2004


Aristotle and Galileo, Part I

This post is a detour from the stated purpose of this blog, which is to talk about the new thinking in evolutionary biology. But I got a new book -- "Galileo in Rome", by William R. Shea and Mariano Artigas. One of the authors is an historian of science, the other is a Catholic priest. They had extensive access to the Vatican archives, which they used to chronicle Galileo's six trips to Rome.

Galileo stands at the middle of one of the most profound sea changes the history of Western thought. The change sort of starts with Copernicus, and is pretty much mature with Newton. Galileo is best known of the three, because of his difficulties with the Catholic church, and I've long had a special interest in him.

But I realized that in order to understand Galileo I would have to understand something more about Aristotle. The philosophy of Aristotle pervaded the intellectual world into which Galileo was born, second in authority only to Holy Scripture. The relationship was always uneasy, as we'll see; but Aristotle was not to be messed with.

One translator says that in order to understand Aristotle you have to read all of him. This is probably true, but I don't have time for that, and anyway I find Aristotle somewhat painful to read. His writing, as we have it, strikes me as disorganized, repetitive, and sometimes contradictory. How much of this is actually Aristotle's fault is unknowable. We have no idea what, if anything, Aristotle actually wrote. Some scholars suggest that the writings we now have may have been lecture notes, maybe by Aristotle himeself, or maybe by a student. After his death in 322 BCE, his manuscripts kicked around Europe for a few hundred years, and were finally put into their present form by Andronicus of Rhodes, in the first century BCE. We don't kow how much is Aristotle, and how much is Andronicus. There are references in antiquity to dialogues of Aristotle, but nobody has any idea about them. Still, we have to take Aristotle as we find him, because that's what Galileo' world did. So what I say about Aristotle is not comprehensive, and some of it may be significantly wrong. But I think maybe I will be able to give a feeling for Aristotelean thought.

You might want to re-read my post "History Through Its Own Eyes" for some background. But keep in mind that I've learned a lot since then.

It is usual to say that such things as that Aristotle believed that celestial bodies moved in perfect circles. This is true, but incomplete: He had reasons for saying what he did, and that's what I want to talk about here. I concentrate on his work "On the Heavens", where Aristotle sets out his theory of astronomy. He says that people will agree with him if they accept his assumptions. But Aristotle never spells out the assumptions, he just throws one in when he needs it. Maybe he spells them out in other works; in any event, a review would have been nice.

As best I can tease them out, the most significant of Aristotle's assumptions are as follows:

1. All material objects are composed of four elements, or essences: earth, air, fire, and water. This was a fairly common starting point in Greek thought. Wood, for instance, is a mixture of earth and fire. If you heat it, the fire escapes, leaving earth. (This reminds me of an old joke in the electronics industry: An integrated circuit -- a computer chip -- works because it has smoke inside. If you put too much voltage on the chip, the smoke gets out, so the chip doesn't work any more.) The essence of something is "what it is"; what we see, its appearance, is an accident. This is not entirely the way we use "accident" in English; for more, see here and here.

2. Every element has its natural motion. Air and fire are light, because it is in their nature to go toward the heavens. Earth and water are part of the earth, so their natural motion is earthward. If a substance is compounded of two elements it has the characteristic motion of the element that predominates. Presumably earth predominates over fire in wood, so the natural motion of wood is to move toward the earth. I don't know whether he addresses floating bodies. Also, he doesn't address the possibility of a body in which more than one element has an influence on the motion. I don't know what improvement this might have made, but it seems to me that he limits himself too much by doing this.

3. Motion is either in a straight line, or else in a circle. Straight-line motion is either up or its contrary, down. Circular motion has no contrary.

4. Everything is generated from its contrary, and is destroyed into its contrary.

5. A body falls with a speed that depends on the quantity of matter it has. (He really ought to say "moves with its characteristic motion".)

From these assumtions, some conclusions follow:

If there is such a thing as circular motion, there must be an element whose natural motion is in a circle. Circular motion has no contrary (as does up/down), so the element whose motion is in a circle must be different from earth, air, fire or water, and must be part of the heavens rather than part of the earth. Motion in a circle must be uniform and eternal, because a circle is perfect.

It also follows that the thing that moves in a circle has no contrary, so it cannot have been generated, thus it must be eternal. It also follows that the heavens must be perfect and eternal, and different from the earth.

This is not exhaustive, but it will, I think, give you a flavor of Aristotle's way of thinking, and of the world that Galileo found himself in.

Aristotle's view of the heavens was intellectually satisfying in its time, but it didn't do anything for the astrologers, who needed to be able to calculate the positions of the planets at specific times in the past, present, and future. In the second century CE, the Egyptian astronomer Claudius Ptolemy published a work now called the The Almagest (a corruption of the title by which it was known in its Arabic translation). Like many manuscripts, this was lost early in the Christian era, and kept alive only by Arab scholars.

The problem with constructing a mathematical model of the motion of a planet is that most planets mostly move east to west across the sky, as does the sun, but sometimes move backwards (this is known as "retrograde motion"). Mars, Jupiter and Saturn behave in this way, but Venus and Mercury always stay close to the sun. Planets move at different speeds at different times, and vary in brightness. To model this, Ptolemy started with the earth in the center of a circle. To get retrograde motion, he put the actual planet on a small circle (the "epicycle") . the center of the epicycle is what moves uniformly on the circle around the sun. With proper sizes of circles and speeds of motion, you get an approximation of the planet's motion. For better accuracy, Ptolemy moved both the earth away from the center of the circle, and also put the line to the epicycle off center. Diagrams are in this article. This fudging away from the center kind of violated Aristotle, but nobody minded much, at least at first.

In Galileo's time, Aristotle's philosophy was seen through a filter of Christian dogma. This was due mainly to St. Thomas Aquinas. Early Christianity knew only the philosphy of Plato, mainly through neoplatonism, as espoused by St. Augustine. Around the year 1100, Aristotle's writings began to appear in Europe, as translations into Latin from Arabic. Thomas tried to merge Christianity with Aristotle, with results that were not totally satisfactory. The main problem was that there were things in Aristotle that contradicted Christian doctrine. For example, Aristotle said, as we've seen, that the universe is eternal and unchanging. Unchanging is OK, but eternal is not: The Church taught that the universe had a beginning, at the Creation, and will have an end, at the Last Judgement. More serious, probably, were differences on the nature of the soul.. In 1277, the Bishop of Paris, charged by the Pope to investigate rumors of heresy at the University of Paris, condemned 219 propositions as heretical. Many of these were derived from Aristotle, some by way of St. Thomas. Many historians consider this to be the beginning of the split between theology and philosophy.

We are now sort of at the end of the preface to modern European thought. Next up: Copernicus.


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