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A Psychiatrist and Philosopher Explains Plato’s Theory of Forms and Why It Lies at the Heart of Plato’s Philosophy

‘The eye of the soul must be turned around.’

By Dr Neel Burton — Psychiatrist, philosopher, and wine writer; Fellow of Green Templeton College, University of Oxford; author of over twenty books.

Plato’s Forms have profoundly shaped Western philosophy, psychology, and theology.

In On the Genealogy of Morality (1887), Nietzsche calls Plato ‘the sincerest advocate of the beyond, the great slanderer of life’. For Nietzsche, Plato inaugurates a metaphysical split between truth and appearance that systematically devalues the world we live in.

Yet the dichotomy between truth and appearance has deeper roots in pre-Socratic philosophy. Just as Plato leant upon Heraclitus’ doctrine of flux for his conception of the sensible world (the world as we see it), so he leant upon Parmenides’ unity for his conception of the intelligible world (the world as we think it), which he rendered as the immutable realm of the Forms.

The Genesis of the Forms

Plato’s authorship spanned some fifty years, from the death of Socrates in 399 BCE to his own death in c. 348 BCE. He is traditionally credited with 35 dialogues, although around ten of these are spurious or of doubtful authenticity. Today, the dialogues are often classified into early, middle, and late periods, based on their presumed order of composition.

This matters because the Theory of Forms does not appear fully formed, but gradually emerges from Plato’s development of Socratic questioning into a more systematic metaphysics.

The early dialogues are relatively short and accessible. They are sometimes referred to as the Socratic dialogues because they present a more historically faithful Socrates, typically engaged in ethical investigations of temperance, courage, or friendship with youths, friends, or supposed experts.

From these beginnings, Plato gradually develops his own philosophical system, including the Theory of Forms, which becomes central in middle dialogues such as Phaedo, Symposium, and Republic. In these longer and more literary works, Socrates is less the historical figure and more a mouthpiece for Plato’s developing doctrines. He is accordingly more didactic, putting forward positive theories rather than simply questioning and refuting. Other middle-period doctrines that are unlikely to be Socratic include the Theory of Recollection, the Theory of Reincarnation, and the Theory of the Tripartite Soul—each closely connected to the Forms.

The Forms in the Phaedo

The Phaedo is named after Phaedo of Elis, a follower of Socrates who was present at his death. It is the dialogue in which Plato most explicitly develops the Theory of Forms, although without yet offering a systematic exposition.

It also assumes familiarity with the Meno and the Theory of Recollection, which the Phaedo builds upon.

In the Phaedo, once titled On the Soul, Socrates offers four arguments for the immortality of the soul, among them arguments that draw on recollection and the Theory of Forms. The apparent immortality of the soul enables Socrates to remain calm in the face of his impending execution and provides a metaphysical foundation for the life of philosophy. The dialogue ends with a myth of the afterlife and, of course, the dramatic drinking of hemlock.

Socrates argues that what is compounded is dissoluble, whereas what is uncompounded is indissoluble and therefore unchanging. The Forms (for instance, Beauty itself), which are unchanging, are uncompounded. Their particulars (for instance, a beautiful horse), which are in a constant state of composition and decay, are compounded.

Particulars are apprehended by the senses, but the Forms can only be apprehended by the mind. Since the soul is not apprehended by the senses, Socrates argues that it is akin to the Forms and therefore not subject to dissolution in the same way as bodily things.

The embodied soul uses the body as an instrument of perception, but what the body perceives is in perpetual flux, so that the soul is often drawn into confusion. When the soul turns away from the body and contemplates itself, it moves toward the realm of the unchanging and approaches wisdom.

Upon death, not all souls undergo the same fate. The soul of the philosopher, being least attached to the body, is able to dwell in the realm of the unchanging, where it ‘lives in bliss and is released from the error and folly of men, their fears and wild passions and all other human ills’. Souls that are more attached to bodily desire remain bound to the cycle of embodiment.

Socrates also suggests that Forms explain the deep structure of reality: things are beautiful because they participate in the Form of Beauty, two is two because it participates in the Form of Duality, and so on. A thing that participates in a Form may also participate in other related Forms. For instance, three pencils participate in the Form of Threeness and also in the Form of Oddness. However, opposite Forms such as Even and Odd cannot admit of each other in the same respect. In this way, the soul—being essentially alive—cannot admit its opposite, death, and is therefore closely bound to immortality.

The Symposium: Love as the Engine of Wisdom

In the Symposium, Socrates recounts the teaching of the priestess Diotima on the proper ascent to the love of beauty. A young person begins by loving one beautiful body. He then realises that all beautiful bodies share something in common, and that it is irrational to be attached to only one.

From there, love expands to all beautiful bodies, then to the beauty of the soul, then to beautiful practices and forms of knowledge. Eventually, the lover ascends to the vision of Beauty itself, no longer tied to any particular instance of it.

At the highest stage of this ‘ladder of love’, the soul apprehends Beauty itself rather than its appearances. In doing so, it comes as close as possible, within human life, to something stable and eternal.

The Republic: The Form of the Good

In Book VI of the Republic, in the context of educating the philosopher-kings, Plato introduces the elusive Form of the Good. It is by apprehending the Good that the philosopher becomes fit to rule, since it is the source of intelligibility and value in all things.

Because the Good cannot be directly defined, Socrates conveys it indirectly through three famous metaphors: the sun, the divided line, and the cave (discussed in a separate article).

Legacy of the Forms

In the Phaedo, the Theory of Forms is presented as the most plausible account of reality, without sustained criticism. In later dialogues, Plato becomes more self-critical; in the Parmenides, the Theory of Forms is subjected to a severe and systematic critique.

However arresting it may be, the Theory of Forms is never fully finalised. It becomes less prominent in the late dialogues and increasingly problematised from within.

Part of the pleasure of reading Plato is that he is thinking with us rather than simply instructing us. The dialogues are not dogmatic systems but staged explorations of philosophical possibility.

In any case, our interest in the Theory of Forms is less in its strict logical coherence than in its enduring influence. The Phaedo helped establish a set of deep dualities that have shaped much of Western thought: soul and body, mind and matter, reason and sense perception, reality and appearance, good and evil, heaven and earth.

In later Western traditions, these distinctions were often intensified into moral and metaphysical hierarchies. The body is frequently associated with limitation, desire, and error, while the soul or intellect is associated with truth and transcendence. Yet this inheritance is uneven: other traditions, including various strands of Eastern philosophy and practice, place less emphasis on such sharp oppositions and more on integration, discipline, or transformation within embodied life.

At the same time, Plato’s vision is not simply world-denying. The discipline of detachment in the Phaedo and the Symposium also points toward a kind of freedom from compulsion—whether read metaphysically, ethically, or psychologically.

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A Psychiatrist and Philosopher Explains the Three Great Metaphors at the Heart of Plato’s Republic

Plato’s Allegory of the Cave: prisoners mistake shadows for reality, until one escapes into the light of truth.

By Dr Neel Burton — Psychiatrist, philosopher, and wine writer; Fellow of Green Templeton College, University of Oxford; author of over twenty books.

Plato’s Sun, Line, and Cave are not three separate allegories, but one extraordinary vision of reality, knowledge, and truth.

Plato’s Republic culminates in three of the most influential metaphors in the history of philosophy: the Sun, the Divided Line, and the Cave. Together they explain the nature of reality, the possibility of knowledge, the purpose of education, and why philosophers should rule.

When Plato first appeals to the Forms (or Theory of Forms) at the end of Book V of the Republic, he assumes that the reader is already familiar with the concept, perhaps from earlier dialogues such as the Phaedo and Phaedrus. But now, in discussing the education of the guardians, he introduces the elusive Form of the Good [hē toû agathoû idea]. It is by attaining the Form of the Good that the philosopher-king is made fit to rule.

Because the Form of the Good is impossible to describe and difficult even to imagine, Socrates conveys its essence through three connected metaphors: the Sun, the Line, and the Cave.

The three metaphors are not independent. The Sun explains what the Good is. The Line explains the levels of knowledge. The Cave shows the soul’s journey through those levels.

The Metaphor of the Sun

Just as it is by the light of the sun that the visible is made apparent to the eye, so it is by the light of truth and being – in contrast to the twilight of becoming and perishing – that the nature of reality is made apprehensible to the soul.

Just as light and sight may be said to be like the sun, and yet not to be the sun, so science and truth may be said to be like the Good, and yet not to be the Good; it is by the sun that there is light and sight, and it is by the Good that there is science and truth.

Just as the sun is the author of nourishment and generation, so the Good is the author of being and essence. Thus, the Good is beyond being, and the cause of all existence. The Sun is therefore not merely a metaphor for goodness, but for the very possibility of knowledge itself.

The Metaphor of the Line

A line is cut into two unequal parts, and each of them is divided again in the same proportion. The two main divisions correspond to the intelligible world and to the visible world. One section in the visible division consists of images, that is,
shadows and reflections, and is accessed through imagination. The other, higher section in the visible division consists of sensible particulars and is accessed through belief. One section in the intelligible division consists of Forms and is accessed through thought, but via sensible particulars and hypotheses, as when geometers use a picture of a triangle to help reason about triangularity, or make appeal to axioms to prove theorems. The other, higher section in the intelligible division also consists of Forms but is accessed by understanding, a purely abstract science which requires neither sensible particulars nor hypotheses, but only an unhypothetical first principle, namely, the Form of the Good. The purpose of education is to move the philosopher through the various sections of the line until he reaches the Form of the Good.

The Metaphor or Allegory of the Cave

Human beings have spent all their lives in an underground cave or den which has a mouth open towards the light. They have their legs and their necks chained so that they cannot move, and can see only in front of them, towards the back of the cave. Above and behind them a fire is blazing, and between them and the fire there is a raised way along which there is a low wall. Men pass along the wall carrying all sorts of statues, and the fire throws the shadows of these statues onto the back of the cave. All the prisoners ever see are the shadows, and so they suppose that the shadows are the objects themselves.

If a prisoner is unshackled and turned towards the light, he suffers sharp pains, but in time he begins to see the statues and moves from the cognitive stage of imagination to that of belief. The prisoner is then dragged out of the cave, where the light is so bright that he can only look at the shadows, and then at the reflections, and then finally at the objects themselves: not statues this time, but real objects. In time, he looks up at the sun, and understands that the sun is the cause of everything that he sees around him, of light, of vision, and of the objects of vision. In so doing, he passes from the cognitive stage of thought to that of understanding.

The purpose of education is to drag the prisoner as far out of the cave as possible; not to instil knowledge into his soul, but to turn his whole soul towards the sun, which is the Form of the Good. Once out of the cave, the prisoner is reluctant to descend back into the cave and get involved in human affairs. When he does, his eyes are no longer accustomed to the darkness, and he appears ridiculous to his fellow prisoners. However, he must be made to descend back into the cave and partake of human labours and honours, whether they are worth having or not. This is because the State aims not at the happiness of a single person or single class, but at the happiness of all its citizens. In any case, the prisoner has a duty to give service to the State, since it is by the State that he was educated to see the light of the sun.

The State in which the rulers are most reluctant to govern is always the best and most quietly governed, and the State in which they are most eager, the worst… You must contrive for your future rulers another and a better life than that of a ruler, and then you may have a well-ordered State; for only in the State which offers this, will they rule who are truly rich, not in silver and gold, but in virtue and wisdom, which are the true blessings of life… And the only life which looks down upon the life of political ambition is that of true philosophy. Do you know of any other?

Taken together, the Sun, the Line, and the Cave are not three separate metaphors but one of the most ambitious philosophical visions ever conceived. They explain not only what reality is, but how we come to know it, why education transforms us, and why wisdom carries with it a responsibility to serve others.