sun, not the earth, be considered the center of the universe. (He retained the notion of circular orbits for the earth and other planets, however.) This new axiom allowed a much simpler explanation of the observed motions of heavenly bodies.  Yet the Copernican axiom of a moving earth was far less "self-evident" than the Greek axiom of a motionless earth, and so it is not surprising that it took nearly a century for the Copernican theory to be accepted.
In a sense, the Copernican system itself was not a crucial change.  Copernicus had merely switched axioms; and Aristarchus of Samos haO already anticipated this switch to the sun as the center 2,000 years earlier.  This is not to say that the changing of an axiom is a minor mat-ter.  When mathematicians of the nineteenth century challenged Euclid's axioms and developed "non-Euclidean geometries" based on other assumptions, they influenced thought on many matters in a most profound way: today the very history and form of the universe are thought to conform to a non-Euclidean (Riemannian) geometry rather than the "commonsense" geometry of Euclid.  But the revolution in-itiated by Copernicus entailed not just a shift in axioms but eventually involved a whole new approach to nature.  This revolution was carried through in the person of the Italian Galileo Galilei.

The Greeks, by and large, had been satisfied to accept the "obvious" facts of nature as starting points for their reasoning.  It is not on record that Aristotle ever dropped two stones of different weight to test his as-sumption that the speed of fall was proportional to an object's weight.  To the Greeks, experimentation seemed irrelevant.  It interfered with and detracted from the beauty of pure deduction.  Besides, if an experi-ment disagreed with a deduction, could one be certain that the experi-ment was correct?  Was it likely that the imperfect world of reality would agree completely with the perfect world of abstract ideas, and, if it did not, ought one to adjust the perfect to the demands of the imperfect?  To test a perfect theory with imperfect instruments did not impress the Greek philosophers as a valid way to gain knowledge.
Experimentation began to become philosophically respectable in Europe with the support of such philosophers as Roger Bacon (a con-temporary of Thomas Aquinas) and his later namesake Francis Bacon.  But it was Galileo who overthrew the Greek view and effected the revolution.  He was a convincing logician and a genius as a publicist.  He described his experiments and his point of view so clearly and so dramati-cally that he won over the European learned community.  And they ac-cepted his methods along with his results.
According to the best-known story about him, Galileo tested Aristotle's theories of falling bodies by asking the question of nature in such a way that all Europe could hear the answer.  He is supposed to have climbed to the top of the Leaning Tower of Pisa and dropped a ten-pound sphere and a one-pound sphere simultaneously; the thump of the two balls hitting the ground in the same split second killed Aristotelian physics. Actually Galileo probably did not perform this particular experiment, but the story is so typical of his dramatic methods that it is no
wonder it has been widely believed throut the centuries..Galileo undeniably did roll balls down inclined planes and measured the distance that they traveled in given times.  He was the first to conduct time experiments, the first to use measurement in a systematic way.
His revolution consisted in elevating "induction" above deduction
as the logical method of science.  Instead of building conclusions on an assumed set of generalizations, the inductive method starts with obser-

vations and derives generalizations (axioms, if you will) from them.  Of course, even the Greeks obtained their axioms from observation; Euclid's axiom that a straight line is the shortest distance between two points was an intuitive 'udgment based on experience.  But whereas the Greek philosopher mini ) mized the role played by induction, the modern scientist looks on induction as the essential process of gaining knowledge, the only way of justifying generalizations.  Moreover, he realizes that no generalization can be allowed to stand unless it is repeatedly tested by newer and still newer experiments-unless it withstands the continuing test of further induction.
The present general viewpoint is just the reverse of the Greeks.  Far from considering the real world an imperfect representation of ideal truth, we consider generalizations to be only imperfect representatives of the real world.  No amount of inductive testing can render a generaliza-tion completely and absolutely  valid.  Even though billions of observers tend to bear out a generalization, a single observation that contradicts or is inconsistent with it must force its modification.  And no matter how many times a theory meets its tests successfully, there can be no certainty that it will not be overthrown by the next observation.
This, then, is a cornerstone of modern natural philosophy.  It makes no claim of attaining ultimate truth.  In fact, the phrase "ultimate truth" becomes meaningless, because there is no way in which enough observa-tions can be made to make truth certain, and therefore "ultimate." The Greek philosophers recognized no such limitation.  Moreover, they saw no difficulty in applying exactly the same method of reasoning to the question "What is 'ustice?" as to the question "What is matter?" Mod-ern science, on the I other hand, makes a sharp distinction between the two types of question.  The inductive method cannot make generalizations about what it cannot observe, and, since the nature of the human soul, for example, is not observable by any direct means yet known, this sub'ect lies outside the realm of the inductive method.