To Explain the World: The Discovery of Modern Science (6 page)

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 Lindberg
¹²
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 because 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.

Another important difference between Hellenistic scientists and their Classical predecessors is that the Hellenistic era was less afflicted by a snobbish distinction between knowledge for its own sake and knowledge for use—in Greek,
episteme
versus
techne
(or in Latin,
scientia
versus
ars
). Throughout history, many philosophers have viewed inventors in much the same way that the court chamberlain Philostrate in
A Midsummer Night’s Dream
described Peter Quince and his actors: “Hard-handed men, who work now in Athens, and never yet labor’d with their minds.” As a physicist whose research is on subjects like elementary particles and cosmology that have no immediate practical application, I am certainly not going to say anything against knowledge for its own sake, but doing scientific research to fill human needs has a wonderful way of forcing the scientist to stop versifying and to confront reality.
⁵

Of course, people have been interested in technological improvement since early humans learned how to use fire to cook food and how to make simple tools by banging one stone on another. But the persistent intellectual snobbery of the Classical intelligentsia kept philosophers like Plato and Aristotle from directing their theories toward technological applications.

Though this prejudice did not disappear in Hellenistic times,
it became less influential. Indeed, people, even those of ordinary birth, could become famous as inventors. A good example is Ctesibius of Alexandria, a barber’s son, who around 250 BC invented suction and force pumps and a water clock that kept time more accurately than earlier water clocks by keeping a constant level of water in the vessel from which the water flowed. Ctesibius was famous enough to be remembered two centuries later by the Roman Vitruvius in his treatise
On Architecture.

It is important that some technology in the Hellenistic age was developed by scholars who were also concerned with systematic scientific inquiries, inquiries that were sometimes themselves used in aid of technology. For instance, Philo of Byzantium, who spent time in Alexandria around 250 BC, was a military engineer who in
Mechanice syntaxism
wrote about harbors, fortifications, sieges, and catapults (work based in part on that of Ctesibius). But in
Pneumatics
, Philo also gave experimental arguments supporting the view of Anaximenes, Aristotle, and Strato that air is real. For instance, if an empty bottle is submerged in water with its mouth open but facing downward, no water will flow into it, because there is nowhere for the air in the bottle to go; but if a hole is opened so that air is allowed to leave the bottle, then water will flow in and fill the bottle.
⁶