Aristotelian Physics - Ancient Concepts

Ancient Concepts

Some concepts involved in Aristotle's physics are:

1. Teleology: Aristotle observes that natural things tend toward definite goals or ends insofar as they are natural. Regularities manifest a rudimentary kind of teleology.
2. Natural motion: Terrestrial objects tend toward a different part of the universe according to their composition of the four elements. For example, earth, the heaviest element, tends toward the center of the universe—hence the reason for the Earth being at the center. At the opposite extreme the lightest element, fire, tends upward, away from the center. The relative proportion of the four elements composing an object determines its motion. The elements are not proper substances in Aristotelian theory or the modern sense of the word. Refining an arbitrarily pure sample of an element isn't possible; They were abstractions; one might consider an arbitrarily pure sample of a terrestrial substance having a large ratio of one element relative to the others.
3. Terrestrial motion: Terrestrial objects move downward or upward toward their natural place. Motion from side to side results from the turbulent collision and sliding of the objects as well as transformations between the elements, (generation and corruption).
4. Rectilinear motion: Ideal terrestrial motion would proceed straight up or straight down at constant speed. Celestial motion is always ideal, it is circular and its speed is constant.
5. Speed, weight and resistance: The ideal speed of a terrestrial object is directly proportional to its weight. In nature, however, the matter obstructing an object's path is a limiting factor that's inversely proportional to the viscosity of the medium.
6. Vacuum isn't possible: Vacuum doesn't occur, but hypothetically, terrestrial motion in a vacuum would be indefinitely fast.
7. Continuum: Aristotle argues against the indivisibles of Democritus (which differ considerably from the historical and the modern use of the term atom).
8. Aether: The "greater and lesser lights of heaven", (the sun, moon, planets and stars), are embedded in perfectly concentric crystal spheres that rotate eternally at fixed rates. Because the spheres never change and (meteorites notwithstanding) don't fall down or rise up from the ground, they cannot be composed of the four terrestrial elements. Much as Homer's æthere (αἰθήρ), the "pure air" of Mount Olympus was the divine counterpart of the air (άήρ, aer) breathed by mortals, the celestial spheres are composed of a special element, eternal and unchanging, with circular natural motion.
9. Terrestrial change: Unlike the eternal and unchanging celestial aether, each of the four terrestrial elements are capable of changing into either of the two elements they share a property with: e.g. the cold and wet (water) can transform into the hot and wet (air) or the cold and dry (earth) and any apparent change into the hot and dry (fire) is actually a two step process. These properties are predicated of an actual substance relative to the work it's able to do; that of heating or chilling and of desiccating or moistening. The four elements exist only with regard to this capacity and relative to some potential work. The celestial element is eternal and unchanging, so only the four terrestrial elements account for coming to be and passing away; also called "generation and corruption" after the Latin title of Aristotle's De Generatione et Corruptione (Περὶ γενέσεως καὶ φθορᾶ).
10. Celestial motion: The crystal spheres carrying the sun, moon and stars move eternally with unchanging circular motion. They're composed of solid aether and no gaps exist between the spheres. Spheres are embedded within spheres to account for the wandering stars, (i.e. the modern planets, which appear to move erratically in comparison to the sun, moon and stars). Later, the belief that all spheres are concentric was forsaken in favor of Ptolemy's deferent and epicycle. Aristotle submits to the calculations of astronomers regarding the total number of spheres and various accounts give a number in the neighborhood of 50 spheres. An unmoved mover is assumed for each sphere, including a prime mover for the sphere of fixed stars. The unmoved movers do not push the spheres (nor could they, they're insubstantial and dimensionless); rather, they're the final cause of the motion, meaning they explain it in a way that's similar to the explanation "the soul is moved by beauty". They simply "think about thinking", eternally without change, which is the idea of "being qua being" in Aristotle reformulation of Plato's theory.

While consistent with common human experience, Aristotle's principles were not based on controlled, quantitative experiments, so, while they account for many broad features of nature, they do not describe our universe in the precise, quantitative way we have more recently come to expect from science. Contemporaries of Aristotle like Aristarchus rejected these principles in favor of heliocentrism, but their ideas were not widely accepted. Aristotle's principles were difficult to disprove merely through casual everyday observation, but later development of the scientific method challenged his views with experiments, careful measurement, and more advanced technology such as the telescope and vacuum pump.