Plato: Forms, the Cave, and the soul

Plato said no, and he spent his life arguing for that answer. Plato (about 428 to 348 BC) was an Athenian aristocrat who, as a young man, watched his teacher Socrates questioned, tried, and executed. He founded the Academy in Athens around 387 BC, a school that lasted in one form or another for centuries, and he wrote about thirty-five dialogues. In his early dialogues Socrates mostly asks questions, as Lesson 1 showed. In the middle dialogues, especially the Republic, Socrates becomes the voice for Plato's own large theories (Kraut, 2022).

Those theories shaped so much of later thought that the philosopher Alfred North Whitehead called the European philosophical tradition "a series of footnotes to Plato" (Whitehead, 1929/1978, p. 39). This lesson covers the three most famous: the Forms, the Cave, and the soul.

Lesson 1 met two thinkers who disagreed about change. Heraclitus said everything is always changing; Parmenides said real change is impossible. Socrates then spent his life asking for definitions: not this brave act or that fair deal, but courage itself and justice itself. Plato put those threads together. Every brave act we see is mixed, partial, and passing. A definition of courage, if one exists, has to be about something that does not change from case to case.

So there seem to be two kinds of thing. The objects of the senses come into being, change, and pass away; Plato called them the world of becoming. What a true definition describes is stable and the same for everyone; Plato called it being.

Because the proof is not about the chalk. Every drawn circle is a little lopsided, yet the theorem about circles is exactly true. Plato concluded that the theorem is about the circle itself, which the mind grasps and the eye never sees. He called such realities Forms (the Greek words are eidos and idea, but a Form is not an idea in anyone's head).

According to the theory of Forms, there is a Form for Circle, Equality, Justice, Beauty, and many other things. Each Form is perfect, unchanging, and non-physical. Particular things are what they are because they participate in a Form: a coin is round by sharing, imperfectly, in Circle. In the Phaedo, Socrates notes that two sticks can look equal from one angle and unequal from another, yet Equality itself never looks unequal; so knowing what equality is cannot come from the sticks alone (Plato, 2002, Phaedo 74a-c). Figure 3.1 shows the picture.

Plato thought there were levels, and in the Republic he drew them as a line cut into four unequal parts, the Divided Line (Plato, 1992, Republic 509d-511e). The lower half is the visible world, where the best we can have is opinion (doxa). The upper half is the intelligible world, where real knowledge (episteme) is possible. Figure 3.2 lays out the four parts, from the bottom up:

• Imagining: taking shadows, reflections, and images at face value.
• Belief: trusting ordinary physical things as the way the world is.
• Thought: reasoning about mathematical objects from starting assumptions, using drawings as aids.
• Understanding: grasping the Forms themselves, without images, all the way up to the Form of the Good.

The Form of the Good sits at the very top. Plato compares it to the sun: as the sun makes things visible and lets them grow, the Good makes the Forms knowable and gives them their being (Republic 507b-509b).

Plato turns the Divided Line into a story, the Allegory of the Cave, the most famous image in philosophy (Republic 514a-520a). It opens like this:

Behind the prisoners a fire burns. Between the fire and the prisoners runs a low wall, and people walk behind it carrying objects on poles, like puppeteers. The prisoners see only the shadows those objects throw on the cave wall, and they take the shadows to be the whole of reality. Figure 3.3 shows the layout.

Now one prisoner is freed and turned around. The fire hurts his eyes; at first the shadows seem more real than the objects that cast them. He is dragged up the steep way out. Outside, dazzled, he can look only at shadows and reflections, then at the things themselves, then at the night sky, and at last at the sun, which he understands is the cause of the seasons and of everything he used to see. Then he goes back down to free the others. His eyes, now used to daylight, cannot make out the shadows, and the prisoners laugh at him. Plato adds a line that every reader of the time would connect with Socrates: if anyone tried to lead them up, "let them only catch the offender, and they would put him to death" (Republic 517a, Jowett translation).

Plato tells us how to read the story: the cave is the visible world, the fire is the sun, and the journey upward is "the ascent of the soul into the intellectual world" (Republic 517b, Jowett translation). Each stage of the climb matches a segment of the Divided Line, as Figure 3.4 shows.

Plato's answer is one of the most quoted ideas in the history of education. Teachers who claim to put knowledge into a soul, he says, are like someone claiming to put sight into blind eyes. The ability to learn is already there; the work is to turn it the right way:

Two points follow. Education is a turning of the whole person, habits and desires included, not just the intellect. And the person who has seen more has a duty to go back down: Plato insists that the freed prisoner must return to help those still in the cave, even though the trip is thankless.

Plato backs the idea with an argument in the Meno. Socrates questions an enslaved boy who has never studied geometry and, by questions alone, leads him to find the square with twice the area of a given square. Socrates claims he taught nothing; the boy recollected it. Plato concludes that learning is recollection (anamnesis) of what the soul knew before birth (Plato, 2002, Meno 82b-86b).

Plato used exactly that experience to argue that the soul has parts. In Republic IV he tells the story of Leontius, who saw corpses lying by the city wall, wanted to look, felt disgust at the wish, covered his eyes, and finally ran to them, shouting at his own eyes, "Look, ye wretches, take your fill of the fair sight" (Republic 439e-440a, Jowett translation). Something in Leontius wanted, and something else in him was angry at the wanting.

Plato's tripartite soul has three parts. Reason loves truth and works out what is best for the whole person. Spirit is the source of anger, pride, and the sense of honor; it can side with reason, as when we are indignant at our own weakness. Appetite wants food, drink, sex, and money. A just person, in Plato's sense, is one in whom each part does its own work: reason rules, spirit backs reason up, and appetite is kept in proportion (Republic 441d-442b). Plato's ideal city in the Republic has the same three-part structure, shown in Figure 3.5.

Lesson 2's tools apply directly. Plato's key premise is the principle of opposites: "The same thing clearly cannot act or be acted upon in the same part or in relation to the same thing at the same time, in contrary ways" (Republic 436b, Jowett translation). His example is a thirsty person who refuses to drink, perhaps knowing the water is bad. Figure 3.6 puts the argument in standard form.

Because the theory raises hard questions, and Plato knew it. In his own dialogue the Parmenides, an elderly Parmenides presses young Socrates with what later readers call the Third Man argument: if large things are large because they share in the Form of Largeness, then the large things and Largeness itself seem to share something further, a second Form of Largeness, and so on without end (Plato, Parmenides 132a-b). If that regress cannot be stopped, the Forms explain nothing.

Aristotle, who studied at the Academy for twenty years, objected that Forms existing apart from things cannot explain how things change or what they are; "participation," he said, is only an empty word (Aristotle, Metaphysics I.9). Lesson 4 follows Aristotle's alternative, which places form inside things. Even so, Plato's conviction that the visible world is not the whole of reality, and that truth and goodness are found rather than invented, runs through the rest of this course, including Augustine in Lesson 5.

• Ask what the image is an image of. A photo, a clip, or a headline is a shadow on the wall; look for the thing that cast it.
• Expect disorientation. When a better explanation first hurts your eyes, that is often a sign of learning, not of error.
• Notice your inner conflict. When you want two opposite things, ask which part of you is speaking, and which should rule.
• Go back down. Understanding something is a reason to help others see it, patiently, without mocking anyone still in the cave.