Aristotle's (384-322 B.C.) Physics was one of the most influential pieces of writing in science. It defined the field of physics for centuries after it was collated from Aristotle's notes by one of his students. It became the cornerstone of Western science as allowed by the Church. While many of the claims within the Physics are incorrect, it represents one of the first attempts to offer a coherent, logical, and natural explanation of motion and change within the physical world.
Greek philosophy and speculation on the world has a long history before Aristotle. His writings build from, or criticize, earlier philosophical ideas about the world. One of the main directions in early Greek philosophy was to speculate on whether the world was made from many substances or simply one in many forms. For example, Anaximenes of Miletus (c. 545 B.C.) suggested air was the base of everything, with water and earth being condensed air, and fire rarefied air.
Indeed, the concept of "The One" versus "The Many" was a common theme in Greek speculation. Parmenides (515?-445? B.C.) said that while things in the world seem to change and move, in reality they are unchanging and at rest. For Parmenides, a being cannot be created or grow, it simply is, and has no past or future. While this may seem ridiculous to us, Parmenides's pupil, Zeno of Elea (c. 495?-425? B.C.) showed that the ideas of motion and change we take for granted from our senses lead to some puzzling paradoxes. The most famous of these paradoxes is Achilles and the tortoise, where Zeno proves that even though Achilles is 100 times faster than the tortoise, he can never catch it. By the time Achilles reaches the point the tortoise started from, it will have moved a hundredth of that distance. By the time he then reaches that new starting point, the tortoise will have again moved on one hundredth of that distance, and so on to infinity. Such ideas concerned Greek thinkers greatly, and indeed, Zeno's paradoxes provoke argument and discussion even today.
Another popular philosophy was that of atomists such as Democritus (460?-370? B.C.) and Leucippus (fifth century B.C.). Atomists believed that everything was made from tiny components, which they called atoms. Some atomists believed there were an infinite amount of atoms. The notion of infinity also concerned many Greek philosophers.
Socrates (470-399 B.C.) tried to steer philosophy away from worrying about paradoxes and from esoteric speculation about the composition of the universe. He tried to answer questions that related to the human condition, such as "What is justice?" and "What is courage?" After Socrates's death, one of his pupils, Plato (427-384 B.C.), established the Academy at Athens to promote the learning of philosophical values. Aristotle arrived as a student at the Academy in 367. He was impressed by the mathematical study there, and in particular by the method of logical deductions from a small set of assumed truths (axioms). Plato suggested that the whole of science might be derived from axioms, an idea that inspired Aristotle. In Aristotle's Prior Analytics, he gave this example of the axiomatic method--if every Greek is a person, and every person is mortal, then every Greek is mortal.
Aristotle was profoundly influenced by Plato's philosophy but often disagreed with his teacher. Aristotle specifically rejected Plato's idea of another realm of ideal forms and his concept of the soul as a unifying principle. Aristotle wanted to deal only with the real world, not with abstract concepts, although he was not completely successful in this aim.
Aristotle's Physics is a work in eight sections, each divided into a number of subsections. The work is not just about the subject known today as physics, it also attempts to define all natural sciences of the physical world. However, the focus of Physics is on motion and change, and most of the examples refer to bodies in motion, so it came to define the scope of physics for centuries to come.
Aristotle thought that the terrestrial world was characterized by change and decay, whereas everything above the moon was eternal, perfect, and changeless, so the main concern of Physics is with everything below the orbit of the moon. This division of the heavens and the sublunar realm was to last until the sixteenth century.
In Physics, Aristotle made a distinction between natural objects and created artifacts. Natural objects have an intrinsic principle of motion and rest, as well as growth and propagation. For instance, if you were to plant a wooden bed, suggested Aristotle, you would not expect it to grow baby beds. If anything a tree would sprout from the wood, for a tree is wood's natural form.
Physics then went on to define luck and chance. Aristotle recognized that some events occur due to luck or accident, and he argued for a purpose in Nature, but he did not go as far as other Greek philosophers who argued for a world consciousness.
Much of Physics deals with problems raised by earlier philosohers, such as Zeno's paradoxes, and the ideas of the atomists. It contains an extended discussion of terms that perplexed earlier philosophers, such as infinite, void, and time. Aristotle rejected the notion of infinity as suggested by some of the atomists. He argued that even mathematics does not need infinity. This rejection helped him overcome Zeno's "Achilles and the tortoise" paradox, although many later commentators have noted that his argument is not convincing. However, Aristotle's rejection of infinity was only partial, as, for example, he also tried to say that the world has always existed and will not end. His arguments against infinity led later atomists to either abandon the notion or try to disprove Aristotle's arguments.
Another reason why Aristotle rejected infinity was directly related to his concept of motion. He believed that all things were made from four elements (air, water, earth, and fire), and they all tried to move to their natural places in the universe. The heavy elements want to move to the center of Earth, and the light to the edge of the heavens. Because he believed this, Aristotle could not accept the notion of an infinite universe--it must have an edge.
Aristotle believed that all objects have weight and lightness, which are two separate qualities, and that weight and lightness determine an object's natural motion. He stated that heavy objects fall faster than light objects, a claim that caused problems for later scientists whose experiments showed otherwise.
Aristotle tried to keep his analysis of motion simple, allowing only straight or curved motion. Curved motion, such as that of the planets, can be eternal. However, straight motion stops and starts. This simplistic analysis caused some complications for Aristotle's view, so he devoted a large section to dealing with the end points of motion.
Aristotle related the speed of a moving object to its weight and the density of the medium it is moving through. For this reason, Aristotle denied that there could ever be a vacuum in nature, as that implied a zero density, and so an object moving in a vacuum would have an infinite speed, which would be impossible.
Aristotle claimed that everything that moves is moved by something. This led him to conclude that there must be some original source of motion, a "changeless source of change." This "Prime Mover" was taken by some readers to mean a god, and was later interpreted by Christian writers to mean the Creator God.
Aristotle's writings dominated subsequent Greek philosophy and the studies of Byzantine and Arabic scholars. Physics became the key, and for many the only, text on the subject of motion and change. However, the writings of Aristotle were reinterpreted in a variety of ways by later copyists. Christian scholars in Europe, such as Thomas Aquinas (1225-1274), were particularly adept at rewriting Aristotle's words to support their religious beliefs. Christians rejected Aristotle's statement that the world was eternal, as the Bible stated it had both a beginning and an end. They blended Aristotle's ideas with the works of other Greek thinkers to form a coherent philosophy of the world that agreed with the Bible. Later commentators even attempted to reconcile the ideas of Aristotle with those of his teacher Plato, in spite of the fact that Aristotle had specifically opposed many of Plato's doctrines.
After some initial difficulty, Aristotle's works were accepted as the basis of science as taught in European universities from the thirteenth to the seventeenth centuries. Aristotle's Physics defined the field. Students committed his works to memory and considered every word he had written to be true. Indeed, when Nicolaus Copernicus (1473-1543), Galileo Galilei (1564-1642), and others began to challenge the concepts of Aristotle's Physics through observation and experiment, they found themselves accused of challenging the theological basis of Christianity, the two concepts had become so intertwined. Only when Isaac Newton (1642-1727) published his Principia Mathematica, was Aristotle's Physics supplanted as the most widely read and influential natural philosophy text.
Aristotle wrote a number of other works on science that developed his overall worldview of change and cause, covering such diverse subject areas as zoology, astronomy, chemistry, geography, meteorology, and psychology. He also wrote on law, constitutional history, ethics, the arts, logic, language, and other topics.
Many of his logical steps are questionable, and his conclusions often incorrect, but in his Physics, Aristotle defined the subject and offered a coherent explanation for the observed motion of objects in the real world without resorting to supernatural or abstract explanations. The success of Physics was also its failure, for it was so influential that later thinkers treated it as gospel and did not even consider testing or challenging Aristotle's conclusions for many centuries.