From Aristotle to Early Modern Science (for PHIL 103)

This page last modified on October 24, 2012

Aristotle (384-322 BCE)
Ptolemy of Alexandria (85-165 CE)
Thomas Aquinas (1224-1274)
Nicholas Copernicus (1473-1543)
Giordano Bruno (1548-1600)
Johannes Kepler (1571-1630)
Galileo Galilei (1564-1642)
Isaac Newton (1642/3- 1727)


For Aristotle (384-322 BCE), Earth is in center of the physical universe, and stationary. Sun, moon and what we today call planets (the ones then known, Mercury, Venus, Mars, Jupiter, Saturn) orbit the center. Their orbits are determined by rotating spheres to which they are attached. Since planetary motion appears to be somewhat irregular, more than one sphere is required to explain the motion of a single planetary body. (Secondary spheres are attached to the primary ones; sometimes tertiary spheres are attached to secondary ones.)

Aristotle says there are 55 spheres corresponding to the "planets" (which includes the Sun and the Moon).

The sphere of fixed stars (bringing the total to 56) at periphery of the universe. Sometimes this sphere is simply called the kosmos, i.e., universe or world. There is no "place" and nothing material beyond this sphere.

There are five material elements. Things below the level of the moon--in the sublunary order--are composed of terrestrial matter, and contain the four EWAF elements. Each has its natural place, to which it returns if not impeded--fire tends to rise, earth to move to the center, while water and air are by nature intermediate.

Terrestrial matter moves discontinuously, tends to come to a stop upon reaching its natural place. Terrestrial bodies come to be and pass away; they are corruptible.

The celestial spheres and celestial bodies are composed of an altogether different kind of matter, the fifth element. By nature celestial bodies move continuously; their motion is circular, everlasting, without beginning or end. The heavenly bodies, including the mostly invisible spheres, are incorruptible.

Celestial Bodies are Characterized by     Terrestrial Bodies are Characterized by
Continous MotionDiscontinuous Motion
Circular MotionLinear Motion
Perfect Shape (Sphericity)Irregular Shape

For each of the spheres there is an intelligence, an unmoved mover: there are 56 such intelligences. The motions of the subordinate spheres are caused partly by the intelligences associated with them, partly by the motions of the spheres adjacent to them but farther from Earth. Aristotle's view seems to be that planetary motions are partly independent, and partly dependent on the motion of other spheres.

The intelligence associated with the sphere of fixed stars is an unmoved mover. Nothing moves him, but he somehow causes the sphere of fixed stars to move eternally. Since the motion of this sphere is entirely independent of the motion of other spheres, its mover is the first or prime (unmoved) mover.

The unmoved mover is an immaterial mind. Its only activity is to think about itself (To think of anything other than itself would be to think of something less perfect.) It does not cause the motion of the stars as a driver's hand causes the motion of a steering wheel she is turning but in a different way; rather it does so as a loved object, simply through its appearing beautiful, causes the motion of the lover.

The unmoved movers are divine (theios) but Aristotle does not regard them as gods.*

Natural movement must be distinguished from unnatural movement. When things move naturally, there is a cause of movement internal to them. The maturation of animals and plants are natural movements. Unnatural movement involves external causes; works of deliberate production, therefore, are products of unnatural movement.

Ptolemy and the Middle Ages

The later ancient astronomer Ptolemy of Alexandria (85-165 CE) refined the picture of the heavens, what we would call astronomy. The Aristotelian-Ptolemaic synthesis continued to be taken seriously until the scientific revolution in the 16th century. It was supplemented in various ways, to keep the theory close to observed data, but not abandoned.

After the mid-13th century it was supported by the theology and metaphysics of Catholic philosopher and theologian Thomas Aquinas (1224-1274) and his followers, who revived and "modernized" the study of Aristotle and affirmed most of Aristotle's views in a way that did not contradict the teachings of Catholic Christianity.

But the detailed model of the ancient astronomical system, with epicycles upon epicycles (spheres attached to spheres attached to spheres) began to strike natural philosophers as too messy. If the creator of the universe is an intelligent designer, surely He would have had a more beautiful and elegant model in mind.

Thinkers who were attracted by the beauty and elegance of mathematical thinking began to look for new ways of understanding the cosmos.

The Copernican Revolution

Nicholas Copernicus (1473-1543) was one of those natural philosophers who was dissatisfied with the Aristotelian Ptolemaic system. He attempted to explain the observed motions of the heavenly bodies by hypothesizing circular orbits surrounding the Sun. In other words, he replaced the geocentric system inherited from the ancients with a heliocentric system. This was an improvement in elegance, but it still did not fit observed evidence perfectly. What is more, it apparently contradicted everyday experience according to which the Sun goes around the Earth and the planets seem to obey laws of nature different from the laws governing bodies close to the surface of the Earth.

Copernicus' model made the Earth a planet (it had not been a planet in the Ptolemaic model), thus implying that the Aristotelian theory of two basically distinct types of matter, celestial and terrestrial, was wrong. Those attracted by his views starting trying to work out new mathematical formulations of the laws of motion so that the observed patterns of motion near the earth and the observed patterns of motion of astronomical bodies could be understood as applications of a single fundamental set of natural laws.

This reformulation would not have been possible if Europe, which used Roman numerals until well into the Middle Ages, had not adopted the Indo-Arabic numeral system in the 15th century and had learned algebra (refined by Arabic-speaking scholars during the Middle Ages).

After Copernicus

Johannes Kepler (1571-1630) abandons the circular planetary orbits still preserved by Copernicus in his heliocentric model, replacing them with elliptical orbits.

For Giordano Bruno (1548-1600), the universe is one, infinite, immobile; the stars are not all at the same distance from the Earth or the Sun. All places are equally close to God. The four elements (EWAF elements) are everywhere, in the so-called heavens as well as "below the Moon." The Sun, finally, is just another star. (The idea of the center of the universe no longer makes sense; the sphere of fixed stars is abandoned.)

Galileo Galilei (1564-1642), building on work by Copernicus and Kepler, begins to develop the mathematical expression of natural laws for motion of bodies on earth compatible with natural laws that also apply to what used to be called celestial bodies. Isaac Newton (1642/3-1727) and others will continue and refine Galileo's work.

For a more detailed account see the following.

Note. A lot of commentators get this wrong; Aristotle held that the unmoved mover is similar in various ways to "the god" or gods, but, unlike Thomas Aquinas, he did not claim that an unmoved mover (or the prime unmoved mover) is a god--he was reasoning by analogy. He was not inventing a theology to substitute for pagan religion.

However, Aristotle did accept the existence of gods. Like Plato, he conceived of them as immortal living beings, with incorruptible bodies; they are normally invisible to us but can reveal themselves to us when they choose. Probably some gods concern themselves with the destinies of individuals and city-states. It is likely that some individuals, say, philosophers, are "beloved by the gods." However, we cannot have strictly scientific knowledge of the gods.