In general, Aristotle thought that Plato's theory of forms with its two separate realms failed to explain what it was meant to explain. That is, it failed to explain how there could be permanence and order in this world and how we could have objective knowledge of this world. By separating the realm of forms so radically from the material realm, Plato made it impossible to explain how the realm of forms made objectivity and permanence possible in the material realm. The objectivity and permanence of the realm of forms does not help to explain the material world because the connection between the two worlds is so hard to understand. The theory of forms, therefore, is an unnecessary proposal. There is no need to split the world up into two separate realms in order to explain objectivity and permanence in our experience.

Aristotle elaborated this general criticism into two more particular objections:

1. The obscurity of the notion of participation or imitation:

According to Plato, material objects participate in or imitate the forms. It is in virtue of this relation to the realm of forms that material objects are knowable and have order. Yet, Aristotle argues, it is almost impossible to explain what exactly this participation or imitation is. The properties that the forms have (eternal, unchanging, transcendent, etc. ) are all incompatible with material objects. How, for example, can a white object be said to participate in or copy the form of whiteness? Is the form of whiteness white itself? How can there be whiteness without any thing which is white? What can a white object and the form of whiteness be said to have in common? It seems that the metaphor of imitation or participation seems to break down in these cases because of the special properties that Plato ascribes to the forms. The only link between the realm of forms and the material world, then, breaks down. The forms cannot explain anything in the material world.

This argument, like the first one, was first given by Plato himself in his later dialogues. It is related to the first objection, but is a more technical way of getting at the main problem with the theory of forms. The resemblance between any two material objects is explained by Plato in terms of their joint participation in a common form. A red book and a red flower, for example, resemble each other in virtue of being copies of the form of redness. Because they are copies of this form, they also resemble the form. But this resemblance between the red object and the form of redness must also be explained in terms of another form. What form does a red object and the form of redness both copy to account for their similarity? One can see that this will lead to an infinite regress. Whenever someone proposes another form that two similar things copy, you can always ask them to explain the similarity between the form and the objects. This will always require another form. The notion of imitation or copying used in the theory of forms, then, runs into logical difficulties. The theory of forms really explains nothing about the similarity of objects; another form is always needed beyond the one proposed. Thus to explain the similarity between a man and the form of man, one needs a third form of man, and this always requires another form. The explanation of the original similarity is never given; it is only put off to the next level.

Aristotle, then, thought that in order to explain coherence and objective knowledge in this world, form must be located in particular individual objects. Yet, he still had to explain how things could change, how they could have permanence, and how we could have knowledge. He still had to address the problem of reconciling the objective and subjective views of the world. Instead of splitting the world into two separate realms, Aristotle divides objects into two parts or aspects: form and matter. All objects are composed of a certain material arranged in a certain way. The material they are composed of is their matter. The way it is arranged is their form. Take as an example a child playing with building blocks. The child could use the same blocks to first build a wall, and then tear it down and build a house. The material or matter in each case would be the same, the blocks. Yet, the house and the wall have the matter arranged in different ways. They have different forms. The house is still just one material object; yet it has two different aspects, its form and its matter.

All objects then have matter, or the material of which they are composed, and form, the way the matter is arranged. It is the form of a thing, however, that makes a thing what it is. When the child knocked down the block wall, the blocks or matter remained. The wall no longer existed, however, because the blocks no longer had the arrangement or form characteristic of a wall. It is the form of an object that makes it the particular object that it is.

It is also the form of a thing that we know when we have knowledge of it. To know a wall or a person is to know the peculiar arrangement of matter or their form. This is what makes them what they are.

Aristotle also uses this distinction to explain how there can be both permanence and change in the world:

Explanation of Change: Change can occur because the same matter can be arranged in different ways. When the block wall was destroyed the matter, the blocks, remained. In change, therefore, it is the form that changes while the matter remains the same. Change occurs when the arrangement of the matter changes, when it moves from one form to another.

Explanation of permanence: Yet, even though the form of an object can change, it is form, not matter, that provides the order and permanence in the world. The matter of all things is ultimately the same; it could not account for the order and intelligibility that the changes of things have. There must be some part of the form of a thing, its essential form, that remains the same as the thing changes. The essential form of a thing determines what an object is and guides the changes and development of that thing. That is why we find changes intelligible or orderly. While some aspects of the form of a thing are always changing, as long as a thing remains in existence, its essential form must remain the same. For example, as a tree develops from a seed into a giant oak tree its form is constantly changing. Yet its changes are not random; it does not change into a rock or a pig. It changes in just the ways necessary to make it an oak tree. This is because some part of the tree stays the same from the time it is a seed until it is a mature oak. The essential form of a thing makes it what it is and guides the thing through its changes to its final goal. This is how there can be permanent objects in a world that is always changing.

C. The four causes:

In order to understand how a thing comes about there are four things that it is necessary to know: (1) What type of material it is made of. (2) What type of thing it is. (3) What caused it to come into being; and (4) What purpose or function the thing is meant to fulfill. Take, for example, a table. To understand the table fully you need to know: (1) its material, that it is made of wood; (2) the arrangement of that material, the type of table it is or its shape (this is the form of the table); (3) how it was built, the various thing that had to be done to manufacture the table; and (4) the function of the table, that it is meant to be a dinner table or a desk. The first of these is the material cause. The second is the formal cause. The third is the efficient cause. The fourth is the final cause.

It was the fourth of these causes that was Aristotle's most original contribution and which played the greatest role in both his theory of nature and his theory of form.

D. The theory of nature:

Aristotle applied his doctrine of the four causes to the study of the natural world. Although all natural objects were composed of a certain matter and certain immediate causes for all their changes, it was the formal and final causes that drew most of Aristotle's attention. All objects, both alive and inanimate, have an essential form that makes them what they are and a goal or final state that they are progressing towards. These two causes were very closely related in Aristotle's natural science. Consider, for example, an acorn. It has a particular form, particular way that its matter is arranged. This is what makes it the type of thing it is, in this case the seed of an oak tree. This form defines for the acorn a goal or final state which defines it and guides all the various changes the acorn will go through. In this case the goal or final state is to be an oak tree. The goal or final cause guides the object through the various changes of form that the object goes through on the way to the accomplishment of its goal. It provides coherence, order, and intelligibility to the change that an object undergoes. The essential form of the thing, however, determines what goal it pursues. The final state of a thing depends upon what type of thing it is. This close connection of the final and formal causes led Aristotle to combine the two in his later account of what form is.

One of the most important applications that Aristotle made of this theory was to the explanation of the motions of physical objects. Aristotle held that there were only four main types of things or elements. These were earth, air, fire, and water. Each of these had a natural place, which it strived to move towards. Earth strived to move towards the center of the planet Earth. Water's natural place was on the surface of the planet Earth. Air's natural place was next furthest from the earth, followed by fire. Finally, outside of the realm of the planet Earth, were the spheres of the stars. This was the place of a fifth, different element, aether. It was what made up the stars. The natural motions of objects and their weight depended, then, upon what type of object they were, for this determined their natural place or goal. Dirt for example moved down towards the natural place of Earth and was heavier than cotton, because it was composed mostly of earth instead of air and water. Everything moves the way it does because of the type of thing it is, because of its form. It was this that determined the natural place of the thing.

E. God in Aristotle's theory of nature and theory of form:

God, for Aristotle, plays the role of both the eternal cause of all motion and the ultimate final cause of all motion and change. Motion is eternal for Aristotle; it is impossible that there could be a first motion, for this would require another motion to get it started. Nothing moves without a cause for Aristotle. The cause of this eternal motion cannot be simply another motion in the chain, it must itself be eternal and it must be unmoved itself. If it were not eternal it could not explain eternal motion. If it were moved itself, this motion itself would require another explanation. The eternal motion of nature requires an eternal unmoved mover or cause. This unmoved eternal mover is God for Aristotle.

God is also the ultimate final cause of all things. All things tend towards God as their final state or goal, as a lover moves toward his beloved. It is the desire for this ultimate goal or fulfillment that fuels each object's development towards its own particular goal. God is seen as perfect activity or pure form; it is thought thinking itself.

F. Aristotle's theory of the soul:

For Aristotle, the soul is the form of a living creature. It is the arrangement of the matter of its body so as to allow it to carry on all of the peculiar functions of living organisms. He identifies five such distinctive powers of the soul, all of which are not found in all organisms: (1) The nutritive: This is the power living beings have to grow and take in nourishment. (2) The appetitive: This is the power of desiring. (3) The sensory: This is the power of perceiving things with the senses. (4) The locomotive: This is the ability to move. (5) The reasoning.

These are the different functions which the souls of different organisms fulfill. They are not parts of the soul, but different powers which organisms organized by a soul or form of the right type can exercise.

G. Human nature and happiness:

The essential form of human beings is their reason. Humans are rational animals. This determines what our final goal or purpose is. Reason is the ability to separate form from matter in abstract thought. For example, when I think of a table, I have the form of the table in my mind, but not the wood or matter. Thus, reason is the one way we can separate form from matter. Our final goal, then, is to separate our form from our matter and become like God, pure form or thought. We do this by developing our reason through the speculative life.