Aristotle's physical theory of the universe, as consisting of concentric spheres set in motion by the prime mover, was known by the ancients to be incompatible with their observations of the motions of the heavenly bodies. Other astronomers, notably Ptolemy, worked out a mathematical description of the universe. This description had the virtue of being able to generate roughly the right predications about the positions of the heavenly bodies. To do so, however, it was forced to introduce a number of motions in addition to the circular motion of the bodies around the earth.
To account for the looping action of the outer planets, for example, Ptolemy had to introduce epicycles, which are circles whose centers rest on the circumfences of other circles (deferents). The joint motion of the two circles produces a looping action. An equivalent device was the eccentric, a circle whose center is not the center of the earth. In both cases, the motion of a body on the circle is uniform.
Ptolemy was forced to invent a further technique, the equant. The motion introduced by the equant was not uniform with respect to the center of the deferent or the center of the epicycle, but rather to another point within the deferent. This is what Copernicus said is not a "proper" center, and the system containing it thus "seemed neither sufficiently absolute nor sufficiently pleasing to the mind."
To simplify the system of the heavenly bodies and remove the annoyance of the equant, Copernicus proposed that the sun be treated as the center of all celestial motion, indeed as the center of the universe. This was a decisive break with the Aristotelian physical description of the universe, so much so that it has been described as a revolution in thought. The upheaval which was sure to follow did not occur during Copernicus's lifetime, however.
To be sure, Copernicus had many powerful allies in high clerical positions within the Catholic Church. Perhaps more importantly, his system provided a new degree of precision in astrononomical measurement; in particular it was able to settle the question of the exact length of the year, thus allowing for the production of an ecclesiastical calendar.
Of philosophical interest is the fact that Copernicus's system could be regarded as a mere mathematical calculating device. This is an instrumentalist interpretation, one which had been given to the Ptolemaic system because of its conflict with Aristotle's physical description of the universe. (The spurious preface to Copernicus's The Revolutions of the Heavenly Spheres in fact presented him as an instrumentalist, though it appears that Copernicus thought his system to be physically real.)
Copernicus's system was of immense significance to the philosophers of the seventeenth century. A number of themes emerge from his work.
An early seventeenth-century philosopher-scientist who took a decidedly different approach to the investigation of nature was Francis Bacon. In a sense, Bacon was more Aristotelian than was Copernicus. He is considered the founder of the empiricist school, which emphasizes observation and generalization, what Bacon called "true induction," in which investigators are led "to the particulars themselves." He was suspicious of the use of principles which are not drawn from experience; for example, he accused Copernicus of introducing fictions in order to get the right numerical outcomes. He also condemned the notion that the motions of bodies must be explained by perfect circles.
A central characteristic of Bacon's philosophy was his distrust of our ability to avoid various influences which would taint the objectivity of our investigations. Superstitious people ignore contrary evidence, individuals overlook the subjectivity of their perception of the world, followers readily look to authorities rather than checking the facts themselves, people are misled by language. Finally, even the methods which are thought to be the best means to the truth, logic and mathematics, can be abused in a way that taints the results of their use.
One would think that what Bacon advocates is a kind of pure observation, devoid of any evaluation or interpretation. But in one place he describes mere observation and experiment to the mechanical activities of ants. Predictably, he compares the metaphysical systems of pure reasoners to webs spun by spiders from their own bodies. The philosopher, he contends, ought to avoid both extremes, like the bee, which collects pollen and digests it, producing honey. Perhaps it is this metaphor for the co-operation of the senses and reason which led Immanuel Kant in the eighteenth century to dedicate his Critique of Pure Reason to Bacon.
In his early book The Assayer, Galileo revived the old atomist doctrine that held that some of the observed properties of things do not belong to the things themselves, but instead are the product of the interaction of the things with the observer. For example, the Aristotelians observed that certain objects are hot and attributed the feeling of heat to the presence of a hot element, fire. Galileo claimed that heat is nothing but the result of the body's being penetrated by tiny particles. When the action is gentle, we have a pleasant feeling of warmth, and when it is harsh, we feel pain. Heat, then, is a "secondary" property of objects. The "primary" properties are those that can be mathematically quantified, such as size, shape and motion. Descartes would later make the same distinction in a more systematic way.
Another way in which Galileo degraded appearances was in his use of mathematical idealizations. For example, in describing the motions of bodies, he set up thought-experiments involving frictionless surfaces and spheres of perfect roundness. The crudity of ordinary objects obscures their mathematical essence.
The tendency to idealization goes hand-in-hand with Galileo's proclivity for a priori argumentation. In one case, the spokesman for the Aristotelians, Simplico, cites an experiment in which a ball dropped from the mast of a moving ship falls some distance away from the foot of the mast. Galileo's mouthpiece, Salviati, contends that in fact the ball would land at the foot of the mast, just as it would on a ship at rest. He admits that he has never made the experiment himself: "Without experiment, I am sure that the effect will happen as I tell you, because it must happen that way."
Reasoning according to ideal mathematical constructions has no place in Aristotelian explanation, but by the time of Galileo, Aristotle had a rival no less illustrious than himself: his teacher Plato. Galileo was familiar with the newly-found Platonic dialogues, and these no doubt influenced his methodology in favor of mathematical construction and away from induction from appearances.
A problem with a priori reasoning (seen by Descartes) is that the choice of principles can be arbitrary. Galileo had adapted the Greek principle of the uniform circular motion of heavenly bodies to terrestrial phenomena. At one point he tried to argue that a falling object follows a circular path through the air, just as it would if it were attached to a stationary object on a moving earth.
Another way in which Galileo opposed appearances is in his generalized application of the relativity of perception, which had been noted by Copernicus. Any appearance of motion or rest is relative to the frame of reference of the observer. Aristotelians held that the earth is at rest, while the heavens are in motion. One argument for the earth's being at rest was that an object dropped from the top of a tower falls in a straight line to its foot. At least, the line appears to be straight. Galileo pointed out that if the earth moves, then the line would not be straight even though the object falls at the foot of the tower (since the tower is moving as well). Hence, the "straight" drop of the tower does not prove that the earth is stationary, but rather assumes it.
Because appearances are given a diminished role in Galileo's science, he could be a realist with respect to the unobserved motions of bodies. For Galileo, mathematical reasoning was not merely a tool for accurate prediction (as with the instrumentalists), but was instead a means for discovering the true nature of motion. But Galileo's discoveries were piecemeal, lacking in systematic unity. Ulitmately the unification was made, by Newton . But before that, Descartes made his own attempt to place science on a solid foundation. It is to him that we now turn.
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