Aristotle thought that Plato's theory of forms with its two separate realms failed to explain what it was meant to. 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:
First, 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 nearly impossible to explain what exactly this participation or imitation is. The properties that the forms have are all incompatible with material objects. How, for example, can a red object be said to participate in or copy the form of redness? Is the form of redness red itself? How can there be red without anything that is red? 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.
The second argument 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