Aristotle on Motion

Aristotle divided motion into two classes: natural motion and violent motion¹Sometimes translated as “forced motion” in later medieval discussions.. These ideas are not really part of modern physics anymore, but they are important because they were among the first serious attempts to explain motion logically rather than through myth or superstition.

Aristotle thought that natural motion was caused by “nature” of an object itself. He said that everything was made up of four elements²earth, water, air and fire. Every element had its proper place in the universe and objects moved naturally to that place. A lump of clay falls because it is mostly ‘earth.’ Smoke rises. Mostly air. A feather falls too, but slower than clay, for it holds more air, and is less dominated by earth. Aristotle thought that heavier objects would “strive harder” to get to their natural place and so concluded that heavier things should fall faster than lighter ones.

Natural motion could be either up or down as in earthly motion or circular as in heavenly motion. Aristotle thought that celestial motion was different from earthly motion because circular motion has no beginning or end point – it goes on forever without change. He thought the heavens were perfect and unchanging, made of a special substance called quintessence³Quintessence is the fifth essence, the other four being earth, water, air and fire.. He thought the stars and planets were perfect spheres moving in perfect circles⁴Circular motion was considered the most perfect form of motion in ancient Greek cosmology.. The Moon was the only object in the sky that people could see imperfections in, and medieval scholars said the Moon’s imperfections were there because the Moon was close enough to the world of Earth to be affected by the imperfect world below.

The second kind of motion according to Aristotle was violent motion or motion produced by pushes or pulls. Motion imposed from outside, like a person pushing a cart, throwing a stone or lifting a weight. Other examples: wind driving a ship, floodwater carrying tree trunks downstream. The basic idea was that objects did not move on their own unless it was part of their natural motion. If it moved otherwise than naturally, it must have been through some outside agency.

But it caused a problem. A bowstring only pushes an arrow while the arrow is in contact with the bow. What pushes the arrow after it leaves? Aristotle believed that the arrow pushed the air out of the way and the air rushing in behind it kept pushing it on. We know today that this explanation is wrong but the important thing is that Aristotle was trying to explain motion using reason. His ideas were to shape the way people understood the universe for almost 2000 years. Most thinkers believed that the natural state of an object was rest⁵This idea would later be overturned by Galileo and Newton’s principle of inertia., and, since the Earth appeared to be perfectly still, it was clear to them that the Earth itself could not be moving.

References and Footnotes

• 1
  Sometimes translated as “forced motion” in later medieval discussions.
• 2
  earth, water, air and fire
• 3
  Quintessence is the fifth essence, the other four being earth, water, air and fire.
• 4
  Circular motion was considered the most perfect form of motion in ancient Greek cosmology.
• 5
  This idea would later be overturned by Galileo and Newton’s principle of inertia.