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 forms. Plato explains the resemblance between any two material objects 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. Since 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. Wittgenstein also criticised Plato’s theory with his language games. He argued that for meaning in language it must define the concepts. Concepts therefore do not gain meaning from the objects to which they refer but from the way we use them in language. This is governed by a series of formal and informal rules that control the games.
Wittgenstein observed from games that it is impossible to offer a simple explanation of the word game because not all the uses of it include the same concepts. The games have a family resemblance but no defining set feature. The use of the word game gains its meaning from the way in which it is used. Therefore those who understand how to use it will understand what it means. This is a problem for Plato because he has already told us that the Forms are simple, yet Wittgenstein suggests that some concepts are so complex that there cannot be a simple blueprint or pattern that ties them all together.