August 30, 2002

Introduction to Plato’s Theory of Forms

In the following paper, I provide a brief overview of Plato’s theory of Forms. As we will see, the theory of Forms is the backbone of Plato’s entire philosophy. For this reason, although this paper is an introduction to Plato’s theory of Forms, it can also be used as an introduction to Plato’s philosophy as a whole.

The Existence and Nature of Forms

Plato’s philosophy often defies common sense; it often abides “in the heavens.” Nevertheless, his philosophy begins “on earth” with two assumptions that virtually no one would deny.

Plato’s first assumption is that universals exist. That is, qualities exist that are shared by different things in this world. For example, the quality of redness is shared by such things as tomatoes, lobsters, and sunsets; the quality of beauty is shared by various people and objects; the quality of justice is shared by various people, institutions, and actions. Plato refers to these qualities as Forms or Ideas.

Plato’s second assumption is that Forms are non-physical entities. No matter how far one searches in the universe and no matter how many rocks one looks under, he will never see, hear, touch, taste, or smell redness, beauty, justice, or any other Form.

Plato never offers an argument for these assumptions, for he believes that virtually everyone holds them. Everyone, he believes, would agree that tomatoes, lobsters, and sunsets are red; and to admit that these things are red is essentially to admit that there exists a non-physical quality called redness and that these things possess this quality. More than just possessing the attributes of universality and immateriality, Plato believes that Forms are (1) independent of physical things, (2) eternal, (3) unchanging, (4) the source of existence for physical things, and (5) more real than physical things. Let’s briefly examine why Plato believes these things about Forms.

First, Forms are independent of physical things; or, to put it another way, Forms would exist even if physical things did not exist. Plato believes that proof for this is found in the fact that we have the ideas of Forms even when they are not fully manifested in physical things. For example, we have the idea of (perfect) equality even though we’ve never seen perfectly equal physical things, and we have the idea of (perfect) justice even though we’ve never known a perfectly just individual. Therefore, Plato concludes that the existence of equality and justice (and other Forms) is independent of the existence of physical things.[i]

Second, Forms are eternal. Plato believes the fact that Forms are independent of physical things proves that Forms must “always exist,” or, in other words, be eternal.[ii] Third, Forms are unchanging. Plato notes that physical objects can change. Red tomatoes, if not eaten, turn into brown tomatoes; beautiful people sometimes become ugly; just people often become unjust. But redness, beauty, and justice cannot change. Even if all the red objects in the world ceased to be red, redness would not become different than it is.

Fourth, Forms make physical things the things that they are. For example, red things are red by virtue of their relationship with the Form of redness, beautiful things are beautiful by virtue of their relationship with the Form of beauty, and so on. Plato never clearly tells us how this works; he only speaks metaphorically, telling us that physical things both participate in Forms and imitate or strive to be like Forms.

Fifth, Forms are more real than physical things. In his famous analogy of the Divided Line, Plato claims that, just as physical objects are more real than their shadows and reflections, Forms are more real than physical objects.[iii] Forms are more real than physical things for at least three reasons: first, their existence is independent of physical things, while the existence of physical things is dependent upon them; second, unlike physical things, they are unchanging and eternal; and, third, physical things imitate, or strive to be like them.[iv]

Given the nature of Forms, Plato concludes that they are far greater and much more worthy of our thought and study than the physical things we encounter in this physical world. Therefore, he concludes that the greatest and highest life one can live is a life spent learning about and contemplating Forms.

How People Gain Knowledge of Forms

Before discussing what philosophies Plato builds on his theory of Forms, we should look at how people gain knowledge of Forms. The belief that people can gain knowledge of Forms is called into question by Meno, the namesake of one of Plato’s dialogues. According to Meno, people cannot learn anything. First, one cannot learn what he already knows, ‘for since he knows it there is no need of the inquiry.’ Second, one cannot learn what he does not know, ‘for in that case he does not even know what he is to look for,’ and even if he came ‘right up against it,’ he would have no way of knowing that he had found the thing he didn’t know.[v]

Although Meno’s argument appears to be obviously false, Plato seems convinced by it and, as a result, concedes that we cannot gain knowledge of Forms. However, as we’ve seen, Plato holds that people possess knowledge of Forms; this, to him, seems undeniable. How, then, does Plato reconcile these beliefs? He reconciles them by holding that, although we cannot gain knowledge of Forms in this life, we gained knowledge of Forms in a previous life. During this previous life, our souls existed in disembodied states and had some sort of intimate relationship with Forms that enabled them to gain knowledge of Forms in a way that is not possible in this life. Upon being born into this life, we forgot what we had learned about Forms. Therefore, when we appear to gain knowledge of Forms, we are actually just recollecting what we already know but have forgotten.

Plato lists two ways that people recollect Forms. First, they are sometimes reminded of Forms by experiencing physical objects. When I see a chair, for example, I am reminded of chairness; when I see two equal objects, I am reminded of equality.

Second, people are reminded of Forms when they develop adequate definitions of them through the process of dialectic. An adequate definition of any given thing lists the essential characteristics of that thing and excludes any non-essential characteristics that thing may have. For example, it seems that there are three characteristics that are essential to triangles: first, they are geometrical figures, second, they are three-sided, and, third, the sum total of their interior angles equals 180°. An adequate definition of triangles will only list these three qualities and not other qualities that some triangles may have but that are not essential to triangularity. For instance, an adequate definition of triangles will not list redness as an essential quality of triangles since, even though many red triangles may exist, color is a non-essential characteristic of triangularity.

It is only through this second way of recollection that one can come to a full knowledge of Forms. The man who sees a picture of a right triangle and, as a result, is reminded of triangularity, might be unable to provide an adequate definition of triangularity. Sure, seeing a picture of a triangle provides him with enough recollection of triangularity to correctly point out that the picture he sees is a triangle, but it is quite possible that his recollection is only partial and that he may, for instance, believe that having a right angle is essential to triangularity. The same man may later see a picture of an obtuse triangle, and since it does not have a right angle, he may wrongly claim that it is not a triangle. On the other hand, the man who has been successful in the dialectic process and has come to the correct definition of triangle has a full recollection of triangularity. Therefore, unlike the first man, he will always judge correctly which figures are triangles—be they right, isosceles, equilateral, obtuse, acute, or scalene.

At this point, you might be thinking, So what? So what if one man has a true knowledge of triangularity, while another merely has partial knowledge—such knowledge is totally impractical. And Plato would probably agree that it really doesn’t matter whether one has full or partial knowledge of triangularity. But, he would continue, it does matter whether one has full or partial knowledge about other Forms. For instance, it is crucial that our political leaders have a full knowledge of the Form of justice. Yes, the man with partial knowledge of justice may generally know which actions are just and which are not, but there may come times in which, because he only has a partial knowledge of justice, he does not know whether a certain action—say, going to war—is just. When such cases arise, it is crucial that our leaders have full knowledge of the Form of Justice—for if they do, they will always know which actions are just.


[i] Hilary Stanliand, Universals, (Garden City, NY: Double Day & Company, Inc., 1972),4.
[ii] Stanliand 5.
[iii] Republic 509-511.
[iv] Stanliand 5.
[v] Republic 80.

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