Aristotle (-384 to -322) was a Greek philosopher, the founder of the Peripatetics, one of the five schools of Greek philosophy in ancient times. His teacher was Plato and he in turn taught Alexander the Great. Although Plato has been more important through most of the history of the West, Aristotle’s philosophy was on top from about 1250 to 1650, a period that saw the birth of Western science.
Aristotle was more down to earth than Plato. Unlike Plato, he trusted his senses and did not see this world as only the shadow of some higher reality. But like Plato he saw reason as the royal road to the truth.
For Aristotle a field of science starts with a set of axioms – statements whose truth is self-evident. One builds on top of this by observation and reason. This was how science was done until the time of Galileo nearly two thousand years later.
Aristotle saw the earth as a place of ceaseless change, birth and destruction. The heavens, however, were perfect, changeless and eternal.
The universe is made up of five elements: earth, water, air, fire and quintessence. Earth is the heaviest element so it sinks to the middle of the universe. That is how the earth itself came to be. Water is the next heaviest, making the seas, then comes the air. Above the air is a region of fire and above that are the heavens made of quintessence. Quintessence moves in perfect circles.
That is why the sun, moon, planets and stars all go round the earth.
Aristotle said that nothing could be physically infinite, that it was impossible for anything real to go on forever. That meant that the chain of causes that make up the universe cannot go on forever. There must be some starting point. That first uncaused cause he called the Prime Mover. Aquinas would later develop this argument into his proof of God.
Aristotle said that each physical thing or substance, like a man or a horse or a table, is made up of essence and accidents.
An essence are the parts of a thing that belong to its definition. Man, for example, is a rational animal. So his reason and animal body are part of his essence. He could not be a man without them. Accident, on the other hand includes those things that make one man different from the next, like his colour or weight, but which do not make him something other than a man.
This is only some of what he taught. He also wrote about the soul, virtue, reason, cause, motion, being, animals, the earth, government, rhetoric, theatre and much else.
Aristotle came down to the West chiefly through the Arabs. When his works appeared in Europe in the 1100s the Catholic Church at first saw him as a threat. But in the 1200s Aquinas was able to explain Christian theology, even the Eucharist, in terms of Aristotle’s philosophy. This in turn laid the groundwork for the rise of Western science.