For Plato, ultimate reality is metaphysical and exists in the “world of the forms” and that things in this world are merely shadows of the forms. Most famously, Plato discussed his theory of forms through his Allegory of the Cave in Book VII of the Republic. Here Plato suggests that the forms are accessible through reason and through the work of those who have managed to escape the “cave” of sense-experience to appreciate the ultimate reality beyond. While Plato’s central point about ultimate reality being metaphysical is convincing, his lack of any real argument for the forms and the apparent inconsistency of his position on them make Plato’s so-called “theory” of the forms unconvincing.
Plato is unclear about precisely which forms exist metaphysically. As Julia Annas observes, “Plato never offers an argument for Forms that would establish them as entities suitable for a theory” Further, Plato discusses the forms is presented in several different dialogues, dating from different times in his long philosophical career. In Book X of the Republic, Plato implies that there are separate forms for tables, beds etc. Socrates says to Glaucon: “Whenever a number of individuals have a common name we assume them to also have a corresponding idea or form. Do you understand me? [I do] Let us take any common instance; there are beds and tables in the world – plenty of them – are there not? [Yes] But there are only two ideas or forms of them – one of the idea of the bed, the other of a table.” Meanwhile, in Book VI of the Republic, through his Allegory of the Sun, Plato implies that only one form ultimately exists – the Form of the Good – and that our impression that a diversity of things exist is a belief rather than actual knowledge, a result of our ideas being clouded by sense-experiences and so not being clear or distinct. Here, as Julia Annas explains, “Plato contrasted Forms, which are objects of knowledge, with particular instances of Forms (things that ‘partake in’ Forms), which are objects of belief…”  The lack of any explicit argument for the forms and the inconsistency of Plato’s position make Plato’s theory of the forms unconvincing.
In addition, it is not possible to support Plato’s theory of the forms, however it is presented, through either evidence or argument. Nothing we can observe supports the existence of “forms” whether separately of beds and tables or indeed of the good. Plato’s position – and that of modern Platonists who accept his theory of the forms – depends on reason alone. Plato – through the character of Socrates – argues that the existence of the forms is known a priori, before and even without experience, because their necessary existence is contained within our understanding of all other things. When we experience a chair, we understand what it is and how to use it because we have an idea of a chair which does not depend on having experienced that or any chair. This explains why we can see a chair that is different from every other chair we have seen and still understand that it is a chair and how to use it. We can even make judgments of whether it is a good chair or not and know how to design and make a different sort of chair. “The maker… makes a bed or makes a table for our use, in accordance with the idea…” Yet what is the difference between this sort of rational argument for ultimate reality in the world of the forms and speculation? What could count against Plato’s argument for the forms if all experience is discounted? Today, there is evidence to support Plato’s claim that ultimate reality is metaphysical. Cosmology shows that neither time nor space are absolutes and Quantum Physics that matter consists only in potentialities. The claim that “man is the measure of all things” and that the way our five human senses perceive reality is the way it really is, is incredible. Further, there are alternative evidence-based, scientific explanations for our ability to understand, judge and create variations on what we perceive through the senses which do not rely on postulating a supernatural world of the forms. For example, Noam Chomsky argues that the human brain is “hard wired” for language and that the forms exist in the structures of the brain and the parameters of human language rather than in some metaphysical world. When there is no evidence to support the metaphysical existence of the forms and when such convincing scientific explanations are available, the claim that we know the forms a priori cannot be maintained. Plato’s theory of the forms is not convincing, because it is asserted and assumed rather than systematically and consistently argued for.
On the other hand, as Julia Annas observes, “Books on Plato often refer to Plato’s “Theory of Forms”, but this has to be handled with caution. Plato not only has no word for “theory” Perhaps Plato never intended to present the forms as a “theory” and it is consequently unfair to evaluate his work as such. Further, Plato’s theory of the forms has been enormously influential through history and continues to capture the public imagination through films as varied as The Matrix and Interstellar. However, neither of these counterclaims serve to make Plato’s forms convincing. Even if Plato never intended to present a coherent theory, his work has widely been interpreted as doing this and other scholars have more than made up for Plato’s lack of argument. For just one example, Descartes developed Plato’s claim that ultimate reality is metaphysical, supporting this with his famous cogito argument in his “Meditations on First Philosophy”. Descartes pointed out that the five senses are limited and often present flawed data; the only think that I can know with certainty is that I exist as a thinking being and from this I can establish the certain metaphysical existence of “clear and distinct ideas” very much like Plato’s forms. However, Descartes’ argument has been widely criticised. While he establishes that he exists as a thinking being, his arguments for the existence of other clear and distinct ideas such as God is widely deemed to have failed. Kant said that Descartes so-called Ontological Argument for God was “so much labour and effort lost” Because of this, Descartes fails to offer Plato’s world-view much support. In addition, that Plato’s theory has been influential and remains popular suggests nothing about whether it is convincing or not on a philosophical level. A lot of unconvincing theories are popular and influence society… think carrots helping you see in the dark, spinach making you strong and magic charms getting rid of warts. None of these “old wives tales” have any scientific basis, and yet many people still believe them and behave accordingly. That Plato’s theory is influential is a poor reason to find it convincing, on a philosophical level.
In conclusion, Plato’s theory of the forms is unconvincing. While his basic point about ultimate reality being metaphysical might well be true and remains influential and popular, he fails to argue for the existence of a “world of the forms” or even to present a coherent picture of the same.
- Class Notes
- Crash Course video on Descartes
- Julia Annas “An Introduction to Plato’s Republic” Chapters 9 & 10
- Plato “The Republic”
 An Introduction to Plato’s Republic p.234
 The Republic, Book X
 An Introduction to Plato’s Republic p.210
 The Republic, Book X
 An Introduction to Plato’s Republic p217
 Critique of Pure Reason, 1781