Indeed, not until the work of Darwin and Wallace in the nineteenth century did naturalists came to understand that although bodily organs serve various purposes, there is no purpose underlying their evolution. They are what they are because they have been naturally selected over millions of years of undirected inheritable variations. And of course, long before Darwin, physicists had learned to study matter and force without asking about the purpose they serve.
Aristotle’s early concern with zoology may also have inspired his strong emphasis on taxonomy, on sorting things out in categories. We still use some of this, for instance the Aristotelian classification of governments into monarchies, aristocracies, and not democracies but constitutional governments. But much of it seems pointless. I can imagine how Aristotle might have classified fruits: All fruits come in three varieties—there are apples, and oranges, and fruits that are neither apples nor oranges.
One of Aristotle’s classifications was pervasive in his work, and became an obstacle for the future of science. He insisted on the distinction between the natural and the artificial. He begins Book II of Physics4 with “Of things that exist, some exist by nature, some from other causes.” It was only the natural that was worthy of his attention. Perhaps it was this distinction between the natural and the artificial that kept Aristotle and his followers from being interested in experimentation. What is the good of creating an artificial situation when what are really interesting are natural phenomena?
It is not that Aristotle neglected the observation of natural phenomena. From the delay between seeing lightning and hearing thunder, or seeing oars on a distant trireme striking the water and hearing the sound they make, he concluded that sound travels at a finite speed.5 We will see that he also made good use of observation in reaching conclusions about the shape of the Earth and about the cause of rainbows. But this was all casual observation of natural phenomena, not the creation of artificial circumstances for the purpose of experimentation.
The distinction between the natural and artificial played a large role in Aristotle’s thought about a problem of great importance in the history of science—the motion of falling bodies. Aristotle taught that solid bodies fall down because the natural place of the element earth is downward, toward the center of the cosmos, and sparks fly upward because the natural place of fire is in the heavens. The Earth is nearly a sphere, with its center at the center of the cosmos, because this allows the greatest proportion of earth to approach that center. Also, allowed to fall naturally, a falling body has a speed proportional to its weight. As we read in On the Heavens,6 according to Aristotle, “A given weight moves a given distance in a given time; a weight which is as great and more moves the same distance in a less time, the times being in inverse proportion to the weights. For instance, if one weight is twice another, it will take half as long over a given movement.”
Aristotle can’t be accused of entirely ignoring the observation of falling bodies. Though he did not know the reason, the resistance of air or any other medium surrounding a falling body has the effect that the speed eventually approaches a constant value, the terminal velocity, which does increase with the falling body’s weight. (See Technical Note 6.) Probably more important to Aristotle, the observation that the speed of a falling body increases with its weight fitted in well with his notion that the body falls because the natural place of its material is toward the center of the world.
For Aristotle, the presence of air or some other medium was essential in understanding motion. He thought that without any resistance, bodies would move at infinite speed, an absurdity that led him to deny the possibility of empty space. In Physics, he argues, “Let us explain that there is no void existing separately, as some maintain.”7 But in fact it is only the terminal velocity of a falling body that is inversely proportional to the resistance. The terminal velocity would indeed be infinite in the absence of all resistance, but in that case a falling body would never reach terminal velocity.
In the same chapter Aristotle gives a more sophisticated argument, that in a void there would be nothing to which motion could be relative: “in the void things must be at rest; for there is no place to which things can move more or less than to another; since the void in so far as it is void admits no difference.”8 But this is an argument against only an infinite void; otherwise motion in a void can be relative to whatever is outside the void.
Because Aristotle was acquainted with motion only in the presence of resistance, he believed that all motion has a cause.* (Aristotle distinguished four kinds of cause: material, formal, efficient, and final, of which the final cause is teleological—it is the purpose of the change.) That cause must itself be caused by something else, and so on, but the sequence of causes cannot go on forever. We read in Physics,9 “Since everything that is in motion must be moved by something, let us take the case in which a thing is in locomotion and is moved by something that is itself in motion, and that again is moved by something else that is in motion, and that by something else, and so on continually; then the series cannot go on to infinity, but there must be some first mover.” The doctrine of a first mover later provided Christianity and Islam with an argument for the existence of God. But as we will see, in the Middle Ages the conclusion that God could not make a void raised troubles for followers of Aristotle in both Islam and Christianity.
Aristotle was not bothered by the fact that bodies do not always move toward their natural place. A stone held in the hand does not fall, but for Aristotle this just showed the effect of artificial interference with the natural order. But he was seriously worried over the fact that a stone thrown upward continues for a while to rise, away from the Earth, even after it has left the hand. His explanation, really no explanation, was that the stone continues upward for a while because of the motion given to it by the air. In Book III of On the Heavens, he explains that “the force transmits the movement to the body by first, as it were, tying it up in the air. That is why a body moved by constraint continues to move even when that which gave it the impulse ceases to accompany it.”10 As we will see, this notion was frequently discussed and rejected in ancient and medieval times.
Aristotle’s writing on falling bodies is typical at least of his physics—elaborate though non-mathematical reasoning based on assumed first principles, which are themselves based on only the most casual observation of nature, with no effort to test them.
I don’t mean to say that Aristotle’s philosophy was seen by his followers and successors as an alternative to science. There was no conception in the ancient or medieval world of science as something distinct from philosophy. Thinking about the natural world was philosophy. As late as the nineteenth century, when German universities instituted a doctoral degree for scholars of the arts and sciences to give them equal status with doctors of theology, law, and medicine, they invented the title “doctor of philosophy.” When philosophy had earlier been compared with some other way of thinking about nature, it was contrasted not with science, but with mathematics.
No one in the history of philosophy has been as influential as Aristotle. As we will see in Chapter 9, he was greatly admired by some Arab philosophers, even slavishly so by Averroes. Chapter 10 tells how Aristotle became influential in Christian Europe in the 1200s, when his thought was reconciled with Christianity by Thomas Aquinas. In the high Middle Ages Aristotle was known simply as “The Philosopher,” and Averroes as “The Commentator.” After Aquinas the study of Aristotle became the center of university education. In the Prologue to Chaucer’s Canterbury Tales, we are introduced to an Oxford scholar:
A Clerk there was of Oxenford also . . .
For he would rather have at his bed’s head
Twenty books, clad in black or red,
Of Aristotle, and his philosophy,
Than robes rich, or fiddle, or gay psaltery.
Of course, things are different now. It was essential in the discovery of science to separate science from what is now called philosophy. There is ac
tive and interesting work on the philosophy of science, but it has very little effect on scientific research.
The precocious scientific revolution that began in the fourteenth century and is described in Chapter 10 was largely a revolt against Aristotelianism. In recent years students of Aristotle have mounted something of a counterrevolution. The very influential historian Thomas Kuhn described how he was converted from disparagement to admiration of Aristotle:11
About motion, in particular, his writings seemed to me full of egregious errors, both of logic and of observation. These conclusions were, I felt, unlikely. Aristotle, after all, had been the much-admired codifier of ancient logic. For almost two millennia after his death, his work played the same role in logic that Euclid’s played in geometry. . . . How could his characteristic talent have deserted him so systematically when he turned to the study of motion and mechanics? Equally, why had his writings in physics been taken so seriously for so many centuries after his death? . . . Suddenly the fragments in my head sorted themselves out in a new way, and fell in place together. My jaw dropped with surprise, for all at once Aristotle seemed a very good physicist indeed, but of a sort I’d never dreamed possible. . . . I had suddenly found the way to read Aristotelian texts.
I heard Kuhn make these remarks when we both received honorary degrees from the University of Padua, and later asked him to explain. He replied, “What was altered by my own first reading of [Aristotle’s writings on physics] was my understanding, not my evaluation, of what they achieved.” I didn’t understand this: “a very good physicist indeed” seemed to me like an evaluation.
Regarding Aristotle’s lack of interest in experiment: the historian David Lindberg12 remarked, “Aristotle’s scientific practice is not to be explained, therefore, as a result of stupidity or deficiency on his part—failure to perceive an obvious procedural improvement—but as a method compatible with the world as he perceived it and well suited to the questions that interest him.” On the larger issue of how to judge Aristotle’s success, Lindberg added, “It would be unfair and pointless to judge Aristotle’s success by the degree to which he anticipated modern science (as though his goal was to answer our questions, rather than his own).” And in a second edition of the same work:13 “The proper measure of a philosophical system or a scientific theory is not the degree to which it anticipated modern thought, but its degree of success in treating the philosophical and scientific problems of its own day.”
I don’t buy it. What is important in science (I leave philosophy to others) is not the solution of some popular scientific problems of one’s own day, but understanding the world. In the course of this work, one finds out what sort of explanations are possible, and what sort of problems can lead to those explanations. The progress of science has been largely a matter of discovering what questions should be asked.
Of course, one has to try to understand the historical context of scientific discoveries. Beyond that, the task of a historian depends on what he or she is trying to accomplish. If the historian’s aim is only to re-create the past, to understand “how it actually was,” then it may not be helpful to judge a past scientist’s success by modern standards. But this sort of judgment is indispensable if what one wants is to understand how science progressed from its past to its present.
This progress has been something objective, not just an evolution of fashion. Is it possible to doubt that Newton understood more about motion than Aristotle, or that we understand more than Newton? It never was fruitful to ask what motions are natural, or what is the purpose of this or that physical phenomenon.
I agree with Lindberg that it would be unfair to conclude that Aristotle was stupid. My purpose here in judging the past by the standards of the present is to come to an understanding of how difficult it was for even very intelligent persons like Aristotle to learn how to learn about nature. Nothing about the practice of modern science is obvious to someone who has never seen it done.
Aristotle left Athens at the death of Alexander in 323 BC, and died shortly afterward, in 322 BC. According to Michael Matthews,14 this was “a death that signaled the twilight of one of the brightest intellectual periods in human history.” It was indeed the end of the Classical era, but as we shall see, it was also the dawn of an age far brighter scientifically: the era of the Hellenistic.
4
Hellenistic Physics and Technology
Following Alexander’s death his empire split into several successor states. Of these, the most important for the history of science was Egypt. Egypt was ruled by a succession of Greek kings, starting with Ptolemy I, who had been one of Alexander’s generals, and ending with Ptolemy XV, the son of Cleopatra and (perhaps) Julius Caesar. This last Ptolemy was murdered soon after the defeat of Antony and Cleopatra at Actium in 31 BC, when Egypt was absorbed into the Roman Empire.
This age, from Alexander to Actium,1 is commonly known as the Hellenistic period, a term (in German, Hellenismus) coined in the 1830s by Johann Gustav Droysen. I don’t know if this was intended by Droysen, but to my ear there is something pejorative about the English suffix “istic.” Just as “archaistic,” for instance, is used to describe an imitation of the archaic, the suffix seems to imply that Hellenistic culture was not fully Hellenic, that it was a mere imitation of the achievements of the Classical age of the fifth and fourth centuries BC. Those achievements were very great, especially in geometry, drama, historiography, architecture, and sculpture, and perhaps in other arts whose Classical productions have not survived, such as music and painting. But in the Hellenistic age science was brought to heights that not only dwarfed the scientific accomplishments of the Classical era but were not matched until the scientific revolution of the sixteenth and seventeenth centuries.
The vital center of Hellenistic science was Alexandria, the capital city of the Ptolemies, laid out by Alexander at one mouth of the Nile. Alexandria became the greatest city in the Greek world; and later, in the Roman Empire, it was second only to Rome in size and wealth.
Around 300 BC Ptolemy I founded the Museum of Alexandria, as part of his royal palace. It was originally intended as a center of literary and philological studies, dedicated to the nine Muses. But after the accession of Ptolemy II in 285 BC the Museum also became a center of scientific research. Literary studies continued at the Museum and Library of Alexandria, but now at the Museum the eight artistic Muses were outshone by their one scientific sister—Urania, the Muse of astronomy. The Museum and Greek science outlasted the kingdom of the Ptolemies, and, as we shall see, some of the greatest achievements of ancient science occurred in the Greek half of the Roman Empire, and largely in Alexandria.
The intellectual relations between Egypt and the Greek homeland in Hellenistic times were something like the connections between America and Europe in the twentieth century.2 The riches of Egypt and the generous support of at least the first three Ptolemies brought to Alexandria scholars who had made their names in Athens, just as European scholars flocked to America from the 1930s on. Starting around 300 BC, a former member of the Lyceum, Demetrius of Phaleron, became the first director of the Museum, bringing his library with him from Athens. At around the same time Strato of Lampsacus, another member of the Lyceum, was called to Alexandria by Ptolemy I to serve as tutor to his son, and may have been responsible for the turn of the Museum toward science when that son succeeded to the throne of Egypt.
The sailing time between Athens and Alexandria during the Hellenistic and Roman periods was similar to the time it took for a steamship to go between Liverpool and New York in the twentieth century, and there was a great deal of coming and going between Egypt and Greece. For instance, Strato did not stay in Egypt; he returned to Athens to become the third director of the Lyceum.
Strato was a perceptive observer. He was able to conclude that falling bodies accelerate downward, by observing how drops of water falling from a roof become farther apart as they fall, a continuous stream of water breaking up into separating drops. This is be
cause the drops that have fallen farthest have also been falling longest, and since they are accelerating this means that they are traveling faster than drops following them, which have been falling for a shorter time. (See Technical Note 7.) Strato noted also that when a body falls a very short distance the impact on the ground is negligible, but if it falls from a great height it makes a powerful impact, showing that its speed increases as it falls.3
It is probably no coincidence that centers of Greek natural philosophy like Alexandria as well as Miletus and Athens were also centers of commerce. A lively market brings together people from different cultures, and relieves the monotony of agriculture. The commerce of Alexandria was far-ranging: seaborne cargoes being taken from India to the Mediterranean world would cross the Arabian Sea, go up the Red Sea, then go overland to the Nile and down the Nile to Alexandria.
But there were great differences in the intellectual climates of Alexandria and Athens. For one thing, the scholars of the Museum generally did not pursue the kind of all-embracing theories that had preoccupied the Greeks from Thales to Aristotle. As Floris Cohen has remarked,4 “Athenian thought was comprehensive, Alexandrian piecemeal.” The Alexandrians concentrated on understanding specific phenomena, where real progress could be made. These topics included optics and hydrostatics, and above all astronomy, the subject of Part II.
It was no failing of the Hellenistic Greeks that they retreated from the effort to formulate a general theory of everything. Again and again, it has been an essential feature of scientific progress to understand which problems are ripe for study and which are not. For instance, leading physicists at the turn of the twentieth century, including Hendrik Lorentz and Max Abraham, devoted themselves to understanding the structure of the recently discovered electron. It was hopeless; no one could have made progress in understanding the nature of the electron before the advent of quantum mechanics some two decades later. The development of the special theory of relativity by Albert Einstein was made possible by Einstein’s refusal to worry about what electrons are. Instead he worried about how observations of anything (including electrons) depend on the motion of the observer. Then Einstein himself in his later years addressed the problem of the unification of the forces of nature, and made no progress because no one at the time knew enough about these forces.
To Explain the World: The Discovery of Modern Science Page 4