Although Aristotle was the student of Plato, they differed remarkably in both the character and content of their philosophical writings. Plato was a great literary stylist, while Aristotle's works are in places barely readable. Some even suggest that they are a compilation by his students of his lecture notes. Plato's style is poetic, depending heavily on allegory (e.g. that of the cave), while Aristotle's is prosaic and literal. One might call Aristotle's writings logical in the sense that they approach the material systematically, with each alternative explored in minute detail.
Content-wise, the two philosophies are very different in their emphasis. Plato found knowledge in another world, a supernatural realm. He has even been accused of being a mystic. Aristotle largely confined knowledge to this world; he is a naturalist. The difference is reflected in the sciences which each held up as a model for correct thinking. Plato thought mathematics, with its abstract objects such as perfect circles and infinite lines (which seem not to exist in the natural world), to be the paradigm of scientific thinking. Aristotle looked to biology, which emphasizes dynamical processes. The emphasis moves away from the unchanging to the changing, as will be seen.
Plato's philosophy begins with Socrates's question: what is the form which makes things what they are? The question shapes the answer, which is that the form is separate from the thing itself. The thing is what it is by virtue of its relation to the form. Aristotle did not disagree with this question, and he too sought the forms of things. But before answering it, he came to grips with a more fundamental question: what is a thing? What is the thing which is a certain way by virtue of its form?
Aristotle's answer was that the individual substance is primary. A human being, a dog and a tree are examples of individual substances. These are biological objects, but artificial objects such as beds and watches are also clear examples. Besides being individual they are "numerically one". Aristotle says that if individual (or "primary") substances do not exist, nothing else could. This contradicts the view of Plato, who held that the form of the good is what gives reality to all else.
Once it is laid down what a thing is, the question as to what makes it what it is arises again. Here Aristotle draws a crucial distinction between ways a thing is what it is. On the one hand, there is the essence, which is given in the definition of a thing. Socrates is a human, and a human being is a rational animal. This is what he is most fundamentally. But he is also a philosopher, snub-nosed, residing in Athens, husband of Xantippe. These are accidents. Simply as a human he might or might not have been any of these things.
The definition which gives the essence of the thing has a characteristic format. There is a genus, a higher group to which the individual belongs. Humans are a species of animal, so it is of Socrates's essence is to be animal. But there are many other types of animals besides humans, so we need a "specific difference" to mark off the human animal from the others. Aristotle believed that humans are rational animals, so being rational is the difference. Socrates as human is a rational animal. The essence of Socrates, to be human, is "said of" Socrates. We may say of Socrates that he is human. But what is said of a thing is not separable from the thing, on Aristotle's view, and here he parts company with Plato, whose forms exist independently of the thing.
Accidents are "in" the subject, Socrates. Thus Socrates's snub nose is in Socrates, his height is in him, and so forth. There are nine categories used to classified ways that a subject can be: its quantity, qualification, ratio to another, place, time, position, having, doing, and being affected.
There is another way of describing the essence of things, or their nature. This way contrasts the essence as form with the material of which it is the form. The bed is made of wood, but it is the way the wood is organized which makes it the thing that it is. Thus there is the idea that the form is a way of being organized, perhaps best specified in terms of a function that it serves. Biological and artificial objects can both be thought of as having a form in this way.
The accidents of substances are always changing, and substances themselves arise and perish (Aristotle calls this generation and corruption). There are several ways of understanding the sources of change, according to Aristotle. An obvious way is in terms of the agent of change (the efficient cause). The father and mother are the cause of a substance which is generated through sexual reproduction. A falling rock kills a snake. The sun bleaches a red flower so that it becomes pink. This kind of cause is called "efficient."
Now the distinction between form and matter comes into play. The matter may be said to be a cause. What material a thing is made of is certainly a constraint on what change will take place. If I wish to make a statue, I would have to carve wood, chisel marble or melt bronze, depending on which of the three materials I chose. The form, we have seen, is a kind of organizational structure. Aristotle gives only abstract examples of how a form is a cause (the ratio 2:1 is the cause of the octave). But in the case of the statue, the form could be the shape which the material takes on, a shape which resembles that of a famous person. With machines, the form would be the function the machine takes on, and with biological organisms, it would be their physiological organization. The cause of my gaining weight, then, involves an external agent (the food I eat), the material of my body, and the functions of the systems of my body.
The last element in Aristotle's description of causes is the "end" or "final cause." In most cases, this is closely related to, if not identical with, the form. Thus in the case of eating (sensibly), the final cause is my health. But health can be understood as the optional functioning of the organism. Similarly, the final cause of the statue is a shape resembling a famous person, which, in turn, is the form the statue takes on. So we should not see the four causes as four separate things, but rather as four related elements in a single process of change.
I began with a review of the doctrine of the "four causes," which are ways of explaining change in things. There is the material of which the thing is made: bronze may be melted but wood only chiseled to make a statue. The agent is that which causes a change in something else, as the sculptor causes the statue to be made. The form is the functional structure of a thing, as when the sculptor has in mind a plan for the shape of the statue. Finally, there is the end or that-for-the-sake of which or telos (Greek). This is generally closely connected with the form. A statueCs end might be to be a likeness of Socrates, and this is the form had in mind by the sculptor.
If we are fully to explain the universe by appeal to causes, it might seem that every event that occurs does so of necessity. That is, if a cause is sufficient to bring about the effect it does, then the presence of the cause means that the result must turn out the way it actually does. Aristotle (and subsequent philosophers) were troubled by this possibility, and Aristotle himself proposed an answer. An event may occur by "luck" or "chance" if its occurrence is an accidental, in the sense that it is not that for the sake of which the agent acted. I go to the supermarket to buy some groceries and find a $20 bill along the way. This finding is by chance, since finding money on the ground was not the final cause of my activity. This is a clever answer, but is it satisfying? There were still other causes which brought about my finding the money, so it might still be necessary that I found it, though not in the sense of necessity which contrasts with what is an accident.
It is not at all clear that everything in the world has an end or that it has a functional organization. A stone seems to be less an organized structure than a mere aggregate of particles stuck together. If it is not a substance, then in a sense it is less real than are substances, a conclusion with which one might easily disagree.
We can say something about the material of rocks, however. All things in the part of the universe between the moon and the center of the earth are composed of one or more of the four elements, earth, water, air and fire. So the material of the rock would be almost entirely earth. A ball of mud would be a mixture of earth and water, etc. The elements are constantly being mixed up with and separated out from one another.
Aristotle inquired into the principle of the movement of bodies. Other than animals which move themselves, bodies in the neighborhood of the earth are moved by other bodies, and these in turn by still others. Aristotle believed that if there were no constant source of motion, the elements would settle out into their natural places: earth in a sphere immediately surrounding the center of the earth (which is also the center of the universe), water in a shell around sphere of earth, air in a shell encompassing that of water, and fire in the outermost shell. It is the tendency to settle into their natural place which explains the natural motion of bodies.
Unnatural or violent motion, away from the natural place, is explained as mentioned above, by the pushing of body against body. The ultimate source of this pushing motion is the movement of the heavens. There are shells above that of fire, each having fixed to it either the moon, the sun, one of the planets or the stars. These shells are always in motion (as, Aristotle thought, can be observed by anyone) and thus are a constant external source of motion in the world below the moon (the CsublunaryC sphere).
Of course, we can then ask why the heavenly (or celestial) spheres move eternally. The answer is that there is a mover outside the heavens which moves the heavens. But this mover is not itself moved by anything. It is the stopping point in the process. Aristotle believed there must be such an unmoved mover, lest there never be an end to the series of the moved and their movers. The unmoved (or prime) mover is not material, not sensible. In fact, it is pure thought. (How pure thought can move anything Aristotle could hardly explain.) The object of the thought of the unmoved mover is its own thinking. Such an immaterial being superior to the heavens could only be God.
We may summarize the positions of Plato and Aristotle on two topics: reality and knowledge. Concerning reality, Plato held that there are degrees of reality, with the forms constituting the highest reality, sensible objects a middle level, and images of things the lowest level. Using the analogy with the sun, Plato held that the forms, especially the form of the good, are the source of the reality of all things. Aristotle, on the other hand, did not distinguish between degrees of reality between sensible objects and the non-sensible prime mover. The role of the prime mover is that of a cause of motion, rather than the origin of reality itself.
Concerning knowledge, Plato held that only reason can grasp the forms; the senses at best yield opinion. Humans can train themselves for the intellectual vision of the forms only through an education in mathematics. Aristotle, on the other hand, believed that all knowledge begins with our familiarity with sensible objects. Reason discovers principles and causes for what we observe. Thus the prime mover is not an object of an intellectual vision, but rather is invoked as an explanation of the motion of things we observe. In common between the two is a feature we might call "Greek rationalism," according to which the proper exercise of reason can yield knowledge of the ultimate truths.
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Philosophy 22 Lecture Notes on Aristotle's Philosophy of Science