The heart of Aristotle's work in natural philosophy comprises four central works: Physics, On the Heavens, On Coming-to-be and Passing-away, and Meteorology. Spanning eight books, Physics, has little to do with what we know as "physics" and is more properly characterized as natural science. The first book modifies the traditional understanding of first principles. Most natural philosophers had assumed that first principles would be contraries–rare and dense, solid and void. But to this Aristotle would add a third principle, called the substratum. Two opposites cannot act upon each other; rather, they presuppose a substance. The substratum furnishes the substance to be acted on by the contraries. Thus, when a given substratum undergoes a change, there are actually three components involved. Matter consists of the substratum itself, form consists of the change undergone, and a third element called privation is presupposed by this change. That is, when the process was begun, the substratum consisted of two contrary elements, of which one would persist as the form and the other, the privation, would be replaced by its opposite.
Some of Aristotle's most celebrated work comes in the second book of Physics. He begins by attempting a definition of nature: natural objects possess an internal source of movement. The second chapter turns to physics (again, meaning natural philosophy) and mathematics, which Platonists had previously distinguished as studies of different objects. Aristotle argued that the two subjects studied the same objects in different ways. In other words, the objects studied in physics consist of the very properties–planes and solids, for example–with which mathematics deals. Thus mathematics abstracts from the limits of physical bodies so that the properties are investigated without regard to motion.
The most famous part of Physics is Aristotle's theory of the Four Causes. In this task he is searching for the kinds of causes that a natural philosopher must study: 1) the element or material from which a thing comes into being (e.g., the bronze of a statue); 2) the form or blueprint (e.g., the ratio 2:1 provides the formula for an octave); 3) the immediate origin of motion or rest (e.g., the father is the origin of the child); and 4) the end or objective (e.g., health is the cause of exercise). Aristotle goes on to distinguish between accidental and essential factors. A sculptor is an essential cause of a statue, but "a white male" would be true only incidentally.
Books 3 through 7 deal generally with motion and the process of change. By the eighth book he is prepared to make some loftier conclusions. He postulates the existence of a prime mover that accounts for all movement in the world. Moreover, he argues that the prime mover is itself unmoved, just as a builder need not be built. He goes on to argue that the mover must be eternal, since it produces eternal motion.
On the Heavens deals with questions of the universe. Aristotle's picture placed the earth at the center, surrounded by two spheres. The outer sphere contained the fixed stars, and between this region and the earth was the sphere that contained the planets. As he attempts to find first principles, he argues that circular motion is primary, since it is complete (in that it does not need a reversal to return to its starting point). And since circular motion is not natural for any of the four simple bodies (earth, air, fire, and water), Aristotle postulates the existence of a fifth element. This body he calls the aether, and it is what the heavens are formed from. The remainder of the work continues to deal with motion as well as other properties, such as weight and lightness, which are nevertheless expressions of motion. On the whole, he attempts to prove through generally convoluted arguments that the universe is eternal–it had neither a beginning and will have no end–but is spatially finite.
On Coming-to-be and Passing-away deals primarily with the four elements. He characterizes them as changeable and prefers the term "simple bodies," as they possess various distinctive qualities in different combinations. Meteorology discusses not only what the title suggests to our modern-day sensibilities, but also subjects such as astronomy, geography, and geology. The work is the least useful of the four discussed here, as it deals with theories that have long since been disproved and discarded.
Aristotle's ventures in natural science are of a different sort from that which we might be accustomed. The link to philosophy is much stronger, for although he attempts to ground his theories in observation, there simply was no possibility of testing many of them with the resources available at the time–particularly regarding cosmology. Thus what generally remains useful or interesting is often not the particular theories but the underlying principles used by Aristotle. These principles can still serve a purpose in guiding investigation. But Aristotle's achievements in physical science have never compared with the work he did in biology.