Astronomers labored to conform their planetary hypotheses to Plato's paradigm of uniform circular motion. Another body of beliefs, more all-encompassing and possibly even stronger than that of uniform circular motion, was Aristotle's physics.

Supposedly, astronomers were not equipped as physicists were to contemplate physical causes and their effects. Nor were astronomers required to derive from the essence of bodies, or from the nature of things, explanations for why other things occurred. In his sixth-century-a.d. commentary on Aristotle's Physics, Simplicius wrote about the need for astronomy to conform to geometry and about the difference between astronomy and physics:

It happens frequently that the astronomer and the physicist take up the same subject

. . . But in such case they do not proceed in the same way . . . Often the physicist will fasten on the cause and direct his attention to the power that produces the effect he is studying . . . The astronomer is not equipped to contemplate causes . . . He feels obliged to posit certain hypothetical modes of being . . . Whether one assumes that the circles described by the stars are eccentric or that each star is carried along by the revolution of an epicycle, on either supposition the apparent irregularity of their course is saved. The astronomer must therefore maintain that the appearances may be produced by either of these modes of being. (Duhem, To Save the Phenomena, 9)

Simplicius concluded:

This is the reason for Heraclides Ponticus's contention that one can save the apparent irregularity of the motion of the Sun by assuming that the Sun stays fixed and that the Earth moves in a certain way. The knowledge of what is by nature at rest and what properties the things that move have is quite beyond the purview of the astronomer. He posits, hypothetically, that such and such bodies are immobile, certain others in motion, and then examines with what [additional] suppositions the celestial appearances agree. His principles, namely, that the movements of the planets are regular, uniform, and constant, he receives from the physicist. By means of these principles he then explains the revolutions of all the planets. (Duhem, To Save the Phenomena, 9)

Astronomers concerned only with saving the appearances could easily have interchanged the Sun and the Earth in their schemes. According to Simplicius, Heraclides (a student at Plato's Academy and also an attendee at lectures by Aristotle) had done so.

Simplicius could also have mentioned Aristarchus. Again, Ptolemy provides a date: Aristarchus observed the summer solstice at the end of the fiftieth year of the First Kallipic Cycle (279 b.c.). According to Archimedes, who lived in Syracuse (in what is now Sicily) in the third century b.c. and probably studied geometry in Alexandria, "Aristarchus put out a tract of certain hypotheses . . . His hypotheses are that the fixed stars and the Sun remain unmoved, that the Earth revolves about the Sun in the circumference of a circle, the Sun lying in the middle of the orbit. . . ." (Heath, Aristarchus of Samos, 302). The tract may have been a book or a written outline or merely explanatory drawings; it may have included some kind of geometric proof or merely stated hypotheses.

Aristarchus has been hailed as an "ancient Copernicus" by antiquarians searching the past for purported promulgations of theories now judged correct. His hypothesis was a remarkable anticipation of the Copernican hypothesis established 1,800 years later. Why, then, wasn't it adopted earlier?

Aristarchus's heliocentric hypothesis claimed no advantage over geocentric hypotheses. And it suffered a considerable potential disadvantage, notwithstanding the distinction being urged by philosophers between astronomy and physics. Any heliocentric hypothesis was incompatible with Aristotelian physics.

The standard interpretation of Aristotle's thought is that he began very close to Plato's intellectual position and only gradually departed from it. Ambiguities in the dating of Aristotle's writings encourage such an analysis, because the resulting pattern can then be used to determine the chronological order of undated passages.


The Greek philosopher Plutarch (first—second century a.d.) reported that the earlier Greek philosopher Cleanthes (fourth—third century b.c.) had "thought it was the duty of Greeks to indict Aristarchus of Samos on the charge of impiety for putting in motion the Hearth of the Universe, this being the effect of his attempt to save the phenomena by supposing the heaven to remain at rest and the Earth to revolve in an oblique circle, while it rotates, at the same time, about its own axis" (Plutarch, On the Face in the Orb of the Moon, VI 922f-923a).

This passage has tempted some historians to imagine a clash between science and religion in the same league as the much later Galileo affair. There is no evidence, however, that anyone ever acted on the purported suggestion to indict Aristarchus for heresy—for disrespecting religious tenets concerning Hestia's Fire or the Earth as a Divine Being. Aristarchus may have been an ancient Copernicus, but he was no ancient Galileo.

A different interpretation of Aristotle's thinking characterizes his philosophy as governed by the interests of a biologist. He continually analyzed and classified, as if what were necessary to understand a subject was to divide it into categories. In his Poetics, he divided poetry into tragedy and comedy and analyzed tragedy into six factors. In his Politics, he classified the species of government and named their normal and perverted forms (one of which was democracy). In his scientific books, Aristotle took a similar approach.

Aristotle was a member of Plato's Academy for 20 years, yet his philosophy developed very differently. Certainly, he experienced more pleasant encounters with the world of the senses. His statement that the ideal age for marrying is 37 years for the man and 18 for the woman correlates with his own biographical details: at age 37 he married the 18-year-old niece and adopted daughter of the ruler of a small area in Asia Minor.

His father-in-law funded Aristotle's new academy. A few years later, in 342 b.c. , Aristotle returned to Macedon, just north of Greece. There he tutored a young prince, Alexander, whose father, Philip, completed his conquest of Greece in 338 b.c. In 336 b.c. Philip was assassinated. Alexander now ruled Greece, and soon much of the Near East. Alexander the Great was more appreciative of Aristotle's tutoring than had been the young Dionysius II of Plato's teaching, and from the Near East sent back a flood of new plants and animals, and also Babylonian eclipse records, to Aristotle in Athens.

A major strength of Aristotle's physical worldview was its completeness. Every part followed logically from the other parts. To understand Aristotle's astronomy, it is necessary to understand his physics. His definitions of motion and space, his conception of what constitutes a cause, and his criterion for an acceptable answer to the question, why? are all essential to an understanding of his view of the physical universe.

In Aristotelian physics, motion was not solely change of position, or locomotion, as it was called by Aristotle. He defined motion more broadly as the fulfillment of potentiality. It involved the concept of purposeful action. Aristotle's science is animistic: he places the cause of motion within an object, rather than explain motion in terms of outside forces.

In his search for causes, Aristotle often seems content merely to restate effects as undefined powers producing the effects. In Moliere's satirical play Le Malade Imaginaire, a quack doctor slavishly and stupidly following Aristotelian philosophy explains the sleep-inducing power of opium as due to virtus dormitiva, its dormative potency.

Aristotle's sense of motion led him to a particular understanding of place. Place encompassed both motion and potential. Each of the four elements (earth, water, air, and fire) had its natural place. Moved away from its natural place, each element had a natural tendency to return to its natural place.

To explain motion, Aristotle attributed it to a final cause or purpose. An object moved because it had a tendency to return to its natural place, its proper place in the universe. Fire moves upward toward its natural place, and earth falls downward toward its natural place.

Natural motion of a body is movement toward that body's natural place in the universe without the interposition of any force other than the natural tendency to move toward the natural place. The opposite of natural motion was forced or violent motion. It was the result of continuous contact between the moved and a mover.

In his Physics, Aristotle discussed the one form of locomotion that could be continuous. Locomotion was either rotatory, rectilinear, or a combination of both. Only rotatory motion could be continuous. Furthermore, rotation was the primary locomotion because it was more simple and complete than rectilinear motion. This value judgment regarding the relative merits of circular and rectilinear motion would affect the subsequent development of planetary astronomy.

Aristotle's views on the organization and structure of the universe are found in his book De caelo (On the Heavens). It may have been written before his Physics, but whatever the chronological order, the logical order is physics followed by astronomy.

Aristotle repeated his belief that locomotion was either straight, circular, or a combination of the two. Bodies were either simple (composed of a single element) or compound. The element of fire and bodies composed of fire had a natural movement upward. Bodies composed of earth had a natural movement downward, toward the center of the universe. Hence, the Earth must be at the center of the Aristotelian universe.

Circular movement was natural to some substance other than the four elements (earth, fire, air, and water). This fifth element was more divine than the other four, because circular motion is prior to straight movement. The fifth element constituted a region beyond the region of the Earth.

Thomas Kuhn has characterized Aristotle's conceptual scheme as a "two-sphere universe" (Kuhn, Copernican Revolution, 28). There was a huge sphere of the stars and a tiny sphere of the Earth. The region of change had the Earth in its center, surrounded by water, air, and fire. This region extended up to the Moon. Beyond were the heavenly bodies, in circular motion, with different laws of physics. It was of a superior glory to our region.

And the celestial region was without change. Aristotle wrote: "It is equally reasonable to assume that this body will be ungenerated and indestructible and exempt from increase and alteration . . . The reasons why the primary body is eternal and not subject to increase or diminution, but unchanging and unalterable and unmodified, will be clear from what has been said to any one who believes in our assumptions" (De caelo, I3, 269b23-270b16). Aristotle's reasoning may not appeal to modern readers, but his conclusions correctly follow logically from his assumptions. He explained: "If then this body can have no contrary, because there can be no contrary motion to the circular, nature seems justly to have exempted from contraries the body which was to be ungenerated and indestructible, for it is on contraries that generation and decay depend" (De caelo, I3, 269b23-270b16).

Comets and new stars, when observed many centuries later, would be placed below the Moon, and any evidence that they were beyond the Moon would pose a challenge to the entire Aristotelian worldview.

Aristotle proceeded to argue that the heavens rotated and that the Earth was stationary in the center. The shape of the heavens was necessarily spherical, because that was the shape most appropriate to its substance and also by nature primary. The heavens also had a smooth finish. The circular motions of the heavens were regular: "Circular movement, having no beginning or limit or middle . . . has neither whence nor whither nor middle; for in time it is eternal, and in length it returns upon itself without a break. If then its movement has no maximum, it can have no irregularity, since irregularity is produced by retardation and acceleration" (De caelo, II6, 288a14-289a10).

Aristotle had sought to combine physical dynamics explaining the causes of planetary motions with mathematical kinematics describing planetary motions. This goal proved too ambitious. Neither Eudoxus nor Callippus nor Aristotle succeeded in developing a satisfactory model of concentric spheres. Saving the phenomena mathematically was difficult enough, without complicating the task further with an insistence on a plausible physical mechanism. Greek geometric astronomers generally ignored the direction suggested by Aristotle. Instead, they looked to ingenious and beautiful geometric schemes to save the phenomena, without any confusing physical mechanisms. Still, Greek astronomers were constrained, or guided, by Aristotle's physical theory, especially when choosing between observationally equivalent models. The criterion of which geometric model better conformed to the nature of things was often present, implicitly if not explicitly.

Demonstrative Syllogisms: The Logical Foundation of Aristotle's Science

Aristotle's science was not a science of discovery. It was, rather, a science of demonstration. It was much like a Platonic dialog, the conclusion present from the beginning, if not articulated initially. The goal was to present arguments or phrase questions forcing readers to accept or admit the preordained conclusion.

Aristotle's method of proof was the syllogism. He explained that a deduction is a discourse in which, certain things being stated, something other than what is stated follows of necessity from their being so. The following is an example of a syllogism:

All organisms are mortal;

All men are organisms;

Therefore all men are mortal.

The first two propositions are premises. From them necessarily (logically) follows the third proposition, the conclusion. The form is all A are B, and all C are A; therefore all C are B. Demonstrative syllogisms derive facts already known, not new facts. Aristotle established the use of syllogisms in logical presentations.

The best of Greek astronomy was geometric, almost inevitably; geometry was one of the finest intellectual achievements of the Greeks. On the other hand, the Greeks never understood, and in fact distrusted, observation and experiment. Only mathematics, Ptolemy wrote, and he could have included Aristotle's syllogisms, can provide sure and unshakable knowledge. But syllogisms cannot provide new knowledge.

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