The Theory of Forms, the Republic & the Ascent to the Good
"Wonder is the feeling of a philosopher, and philosophy begins in wonder."
— Theaetetus 155d428/7–348/7 BCE · Athens
Plato (428/7–348/7 BCE) was born into one of Athens' most aristocratic families. His father Ariston traced his lineage to the early kings of Athens; his mother Perictione was related to Solon, the lawgiver. His original name may have been Aristocles — "Plato" (meaning "broad") was apparently a wrestling nickname. This aristocratic origin shaped his political philosophy: he never fully trusted Athenian democracy.
The defining event of Plato's life was the encounter with Socrates, whom he met as a young man and followed until Socrates' execution in 399 BCE. The death of his teacher by democratic vote radicalised his distrust of popular government and gave his entire philosophical project a particular urgency: how can the city be organised so that it is governed by knowledge rather than by opinion?
After Socrates' death, Plato travelled widely — to Megara with Euclid, to Egypt, possibly to Cyrene — before his first visit to Syracuse in Sicily around 388 BCE, where he encountered the Pythagorean mathematician Archytas and the court of the tyrant Dionysus I. His relationship with Syracuse — three visits across decades — ended disastrously each time, including, allegedly, a period of enslavement following his first visit.
Socrates — philosophical model and martyred teacher. Dionysius I & II of Syracuse — the failed attempt to educate a philosopher-king, repeated three times. Archytas of Tarentum — Pythagorean mathematician who may have saved Plato from slavery and whose mathematics deeply influenced the Timaeus and the Academy's curriculum.
Founded in a grove sacred to the hero Academus outside Athens' walls — the first institution of higher learning in the Western world. Incorporated mathematical training, philosophical discussion, and practical political advice. Attracted students including Aristotle (who remained for twenty years), Eudoxus, Theaetetus, and Philip of Opus. Lasted nearly 900 years until Justinian closed it in 529 CE.
Plato is the only ancient philosopher whose complete works survive. Approximately 35 dialogues and 13 letters (some disputed) across all the phases of his career. The extraordinary survival of the corpus — through Byzantine manuscript tradition — has given Platonic philosophy a canonical authority in Western thought that even Aristotle, better preserved in Arabic, only partially rivals.
Plato's most distinctive and influential metaphysical doctrine: in addition to the changing, imperfect world of sensory experience, there exists a realm of eternal, unchanging, perfect Forms (Greek: eide or ideai). A beautiful flower participates in Beauty Itself; a just action participates in Justice Itself; a triangular figure participates in the Form of Triangle. The Form is the real thing; the sensible particular is a copy, an image, a shadow.
The Third Man Argument (Aristotle's objection): if two things share a Form, and we need a Form to explain what a thing and its Form share, we generate an infinite regress. Plato anticipates this objection himself in the Parmenides — a sign of the self-critical rigour of his later philosophy.
The Divided Line (Republic VI, 509d)
The Allegory of the Cave (Republic VII, 514a)
Prisoners chained in a cave see only shadows of objects cast on the wall by a fire. If a prisoner is freed and compelled to turn toward the fire, then dragged out into daylight, he is initially blinded by the sun. Over time his eyes adjust: first he sees reflections in water, then the stars, the moon, and finally the sun itself — the Form of the Good, which illuminates all other Forms.
1. Shadows on the wall — images/shadows (eikasia). 2. Objects by the fire — sensible things (pistis). 3. Objects in daylight — mathematical forms (dianoia). 4. The sun itself — the Form of the Good (noesis). The philosopher must return to the cave to govern, even at the cost of appearing ridiculous to those who never left.
The freed prisoner who returns is at risk of death — an unmistakable echo of Socrates. Those who have seen the Good are best fitted to govern, yet governance deprives them of the contemplative life they value most. Plato's philosopher-king must rule out of duty, not desire — a qualification that distinguishes legitimate from tyrannical rule.
At the apex of Plato's metaphysical hierarchy stands the Form of the Good — a principle so fundamental that it cannot be directly described, only approached through analogy. Socrates proposes the Sun Analogy (Republic VI, 508–509): just as the sun gives both visibility to visible things and the power of sight to eyes, the Good gives both intelligibility to the Forms and the power of knowledge to the mind. The Good is "beyond being" (epekeina tēs ousias) — not merely a Form among Forms but the source of both the being and the knowability of all Forms.
The Sun makes visible things visible and gives living things their capacity to grow. Analogously, the Form of the Good makes the Forms knowable and gives them their reality and truth. Without the sun, we are blind; without the Good, the mind cannot function philosophically. This analogy places the Good at the structural centre of Plato's entire philosophical system, as both metaphysical and epistemic first principle.
When Plato writes that the Good is "beyond being in dignity and power," he is making a claim that will reverberate through Neo-Platonic theology, Christian mysticism, and modern ethics. It means the Good cannot be captured by any predicate, not even existence. The Neoplatonist Plotinus took this to its logical conclusion, identifying the Good with the One — the simple, ineffable first principle from which all reality emanates. Levinas revived it in the twentieth century as "otherwise than being."
Ancient sources (Aristotle, Aristoxenus) report that Plato gave lectures "On the Good" whose content diverged significantly from the dialogues — more mathematical, identifying the Good with Unity and the dyadic principle of the One and the Indefinite Dyad. Plato reportedly told surprised listeners that they had expected to hear about human goods and were instead given arithmetic. The "esoteric" Plato of the Tübingen school interpretation remains controversial but attests to the depth of his engagement with the Form of the Good.
"The Good not only makes the objects of knowledge known, but also provides them with being and reality; yet it is itself not reality, but transcends reality in dignity and power."
— Republic 509b (Shorey trans., modified)The Phaedo presents Socrates arguing for the immortality of the soul on the day of his execution. The soul's immortality is tied to its kinship with the eternal Forms: if the soul knows the Forms — not through the body's senses but through pure reasoning — then it must share their imperishable nature. The four arguments form a cumulative case, each addressing a different aspect of the soul's relationship to life and death.
All opposites generate their opposites: sleeping generates waking, waking generates sleeping; larger things come from smaller and vice versa. Therefore, living things come from dead things and dead things from living things. If the process ran only one way, all things would eventually reach one terminus — the universe would have run down. The cycling of opposites guarantees that souls must persist between lives to be reborn. The argument is the weakest but the most ancient in structure, drawing on archaic Greek cyclical cosmology.
We have knowledge of perfect equality, perfect beauty, and other Forms that we have never encountered through the senses — all sensible equal things fall short of perfect equality. This knowledge must have been acquired before birth, in a disembodied state. Learning is therefore not acquisition of new knowledge but anamnesis (recollection) of what the soul already knew. This argument is also found in the Meno, where the slave boy "recollects" a geometrical theorem without instruction — demonstrating the doctrine through dramatic enactment.
Two kinds of things exist: what is composite, changeable, and visible; and what is simple, unchanging, and invisible. The body belongs to the first class; the soul, by virtue of its capacity to apprehend the unchanging Forms, belongs to the second. Composite things dissolve when decomposed; simple, incomposite things do not dissolve. Since the soul is more akin to what is simple and divine, it is most likely to persist after death — though this argument proves affinity rather than strict immortality, and Simmias' harmony-of-the-lyre objection targets precisely this gap.
The final and most metaphysically sophisticated argument. The soul is what gives a body life — life is one of the soul's essential properties, as heat is an essential property of fire. The Form of Life, which the soul participates in, cannot admit of death any more than fire can admit of cold. When death approaches, the soul does not become dead (as fire does not become cold); it withdraws, carrying its essential life-principle with it. Cebes' objection — that the soul might survive many incarnations yet eventually wear out — is met by introducing the Forms as guarantors of indestructibility.
Plato's Republic develops a parallel between the structure of the soul and the structure of the just city. Both contain three parts, and justice in both consists of each part performing its proper function under the governance of reason. The argument from the city to the soul is methodological: it is easier to read justice writ large in an imaginary city than to discern it in the individual soul.
Justice in the soul = each part doing its proper work; justice in the city = each class doing its proper work. Plato explicitly argues that the just person is happier than the unjust — the Republic is ultimately an extended argument for the value of justice to the just person, not merely to society.
Plato's most provocative political claim: cities will not flourish until philosophers become kings, or kings become philosophers (Republic 473d). This is not a call for intellectual elitism for its own sake but an epistemic argument: governing well requires knowledge of what is genuinely good for human beings, and only those trained in philosophy — who have ascended to knowledge of the Form of the Good — possess this knowledge. Democracy, on this account, is rule by opinion: the blind leading the blind.
The education of philosopher-kings is carefully described in the Republic. It proceeds through gymnastics and musical education (Books II–III), then mathematics — arithmetic, geometry, stereometry, astronomy, and harmonics — as preparation for dialectic, which trains the mind to grasp the Forms directly. The future philosopher-king must complete this programme by age 35, then spend fifteen years in practical public service before ascending to rule at 50. Crucially, they rule reluctantly: compelled by duty, not by ambition.
The auxiliaries and philosopher-kings are subject to radical social reforms: the Republic proposes the abolition of the family for the guardian class (children raised communally), equality of women in guardianship (Books IV–V), and a system of eugenic breeding described with queasy irony as "noble falsehood." These proposals have been read as both utopian idealism and as deliberate thought experiments designed to expose the tensions in any political programme based on pure reason.
Plato charts the decline of the best constitution (aristocracy/philosopher-kingship) through four degenerate forms: timocracy (rule by honour-loving spirited types); oligarchy (rule by the wealthy); democracy (rule by desire for freedom, culminating in licence); tyranny (the worst: the most enslaved soul, governed entirely by appetite, masquerading as freedom). The sequence maps political to psychological corruption.
The Republic notoriously proposes expelling most poets from the ideal city. Poetry, for Plato, imitates appearances (which are themselves copies of Forms), is therefore thrice removed from reality, and appeals to the irrational parts of the soul — stimulating grief, passion, and laughter at the expense of rational self-governance. This critique connects epistemology, metaphysics, and politics: what we consume imaginatively shapes who we become politically.
Karl Popper's The Open Society and Its Enemies (1945) launched the most sustained modern attack, reading Plato as the father of totalitarianism. Hannah Arendt, Simone Weil, and Leo Strauss offered more nuanced counter-readings. The philosopher-king doctrine continues to animate debates about technocracy, meritocracy, and the role of expertise in democratic governance.
The Symposium is Plato's most celebrated dialogue and one of the greatest works of world literature. Seven speeches on love (eros) build toward Socrates' report of Diotima's teaching: that eros properly understood is a philosophical force, driving the lover upward from the love of a beautiful body to the vision of Beauty Itself. Eros is the intermediary between human and divine — a daimon, neither mortal nor immortal, who carries prayers upward and oracles downward.
The most famous speech: human beings were originally double-bodied creatures — spherical, with four arms, four legs, two faces. Zeus split them in half as punishment for hubris. Eros is the longing of each half to be reunited with its original other half. This myth explains the diversity of sexual orientations and the overwhelming power of love as a drive toward wholeness. Though presented as comic myth, it captures something philosophically serious about the experience of erotic incompleteness.
The correct approach to love begins with one beautiful body, ascends to the recognition that beauty in all bodies is one, then to beautiful souls, activities, and knowledge, and finally to the direct vision of Beauty Itself — absolute, eternal, pure, unmixed with flesh, colours, or mortality. One who has ascended to this vision gives birth to true virtue, not its image. The Form of Beauty is approached erotically, not merely intellectually — eros is the engine of philosophical ascent.
The most psychologically vivid section: the beautiful general Alcibiades arrives drunk and delivers a biographical portrait of Socrates. He describes attempting to seduce Socrates, spending the night next to him, and waking up without anything happening — a profound humiliation. Socrates' armoured interiority, his physical ugliness masking inner beauty, his silenic nature: all suggest that Plato's Socrates is himself the highest erotic object — a man who has ascended and who inspires ascent in others.
"This is what it is to go aright, or be led by another, into the mystery of Love: one goes always upwards for the sake of this Beauty, starting out from beautiful things and using them like rising stairs."
— Symposium 211c (Hamilton trans.)The Meno — Can Virtue Be Taught?
The Meno begins with the question of whether virtue is teachable and ends with the doctrine that it cannot be — since there are no true teachers of it, only successful practitioners who act by divine inspiration (a provocative implication for Socratic pedagogy). The dialogue introduces the theory of recollection through the famous slave-boy passage: Socrates questions an uneducated slave about geometry, drawing from him, through questioning alone, the correct construction for doubling the area of a square. The slave "knows" without having been taught — therefore, Plato concludes, the soul must have possessed this knowledge prior to birth.
"How will you enquire, Socrates, into that which you do not know? What will you put forward as the subject of enquiry? And if you find what you want, how will you ever know that this is the thing you did not know?" The paradox of inquiry: you cannot seek what you entirely lack, since you would not recognise it if found; but you cannot seek what you already know. Recollection dissolves the paradox — inquiry is not discovery but remembering.
The Theaetetus — What Is Knowledge?
The Theaetetus is Plato's most sustained epistemological investigation — and one of the most rigorously aporetic of his dialogues. Three definitions are proposed and refuted: (1) knowledge is perception (Protagorean: each person's perception is their truth) — refuted because perception is always changing while knowledge must be stable; (2) knowledge is true belief — refuted because one can have true belief by lucky guessing; (3) knowledge is true belief with an account (logos) — the Gettier problem, ante litteram: what counts as an adequate account? The dialogue ends in aporia but not without progress — the concept of a justified true belief was bequeathed to modern epistemology.
"Man is the measure of all things" — Protagoras' relativism is Plato's primary target. If all perceptions are equally true for the percipient, then the doctrine of relativism is itself only true for those who believe it, not for those who don't — Plato uses this self-refutation argument against Protagoras with elegant economy. Objective knowledge requires objects that are mind-independent: the Forms.
Plato's Timaeus is his account of the physical universe — the most influential ancient cosmological text in the Western tradition. A craftsman-god (the Demiurge, from Greek dēmiourgos, "craftsman" or "public worker") creates the visible world by imposing the pattern of the Forms upon a pre-existing, chaotic material receptacle. The cosmos is not created from nothing but from the rational ordering of a recalcitrant medium — the khora (space/receptacle) — by a divine intellect aiming at the Good.
Not omnipotent — constrained by both the Forms (which he uses as models) and the recalcitrant nature of matter (the Khora). He is perfectly good and not jealous, so he wants the cosmos to be as good as possible. The cosmos is a living, rational animal with a World Soul woven from mathematical ratios. The Demiurge creates time as "a moving image of eternity" — time is an artefact of creation, not a pre-existing container.
The four elements are constructed from two types of right triangles combined into regular solids: the tetrahedron (fire), octahedron (air), icosahedron (water), and cube (earth). The fifth regular solid — the dodecahedron — the Demiurge "used for the whole" (a tantalising phrase, perhaps meaning the zodiac or the cosmos itself). This mathematical atomism influenced Kepler's Mysterium Cosmographicum and remains a symbol of the conviction that nature's deep structure is mathematical.
The Demiurge constructs the World Soul from Sameness, Difference, and Existence, blended in mathematical proportions derived from Pythagorean music theory. Individual human souls are made from the same ingredients, though diluted with a lower grade of mixture. The soul is placed in the body as a chariot in a vehicle — the body disturbs the soul's circular motions, causing perceptual confusion. Education and philosophy restore the soul's proper circular movement, aligning it with the World Soul and the cosmos.
The third principle alongside the Forms and the Demiurge: a "space" or "receptacle" that receives the images of the Forms without having any qualities of its own — apprehended by a "bastard reasoning," not by genuine knowledge or perception. Derrida found in the khora a paradigm of undecidability: it is neither sensible nor intelligible, neither inside nor outside the Platonic system, a non-place that makes place possible. Its influence on Neoplatonic matter, Christian negative theology, and deconstruction runs through the entire Western tradition.
Established c. 387 BCE in the grounds of a gymnasium dedicated to the hero Academus, about a kilometre outside Athens' walls near the Ceramicus cemetery. The curriculum emphasised mathematics — arithmetic, geometry, stereometry, harmonics, and astronomy — as preparation for dialectic. Plato believed geometry trained the mind to think about unchanging objects, preparing it for the Forms. The requirement (legendary or real) that entrants know geometry became a founding symbol of academic rigour.
Aristotle — enrolled c. 367 BCE, stayed 20 years, became the Academy's most brilliant student before founding the rival Lyceum after Plato's death. Eudoxus of Cnidus — mathematician who developed the method of exhaustion (forerunner of integral calculus) and an important model of planetary motion. Theaetetus — mathematician who contributed to the theory of irrational numbers; the dialogue named for him dramatises his death in battle. Speusippus — Plato's nephew, successor as head of the Academy.
The Academy was more than a school — it was a philosophical community and a centre for political consultation. Rulers from across the Greek world sought advice from its members. Several alumni were invited to write laws for Greek cities — an imperfect but real approximation to the Republic's philosopher-statesman ideal, and a direct expression of Plato's conviction that philosophical knowledge should inform political practice.
The internal philosophical culture was one of rigorous debate rather than doctrinal conformity. The Academy moved through several phases after Plato: Speusippus emphasised mathematical metaphysics; Xenocrates systematised Platonism; the New Academy under Arcesilaus (c. 268 BCE) turned Platonic dialectic into systematic scepticism, arguing that nothing can be known with certainty. This sceptical turn, influential on Roman thought through Cicero, represents the self-critical potential latent in the Socratic-Platonic tradition.
The Academy survived — with interruptions — for nearly 900 years, until the Emperor Justinian ordered all pagan schools closed in 529 CE. The last heads fled to the Persian court of Khosrau I — a remarkable final episode in the story of philosophical exile that began with Socrates' death.
The late dialogues represent a remarkable turn in Plato's career: instead of elaborating the Theory of Forms, he subjects it to severe criticism and shifts to new methods of classification and division. The Socratic figure retreats or is replaced; the discussions become more technical, the style more austere. Whether this represents genuine self-revision or a different pedagogical mode remains debated.
The most philosophically alarming dialogue: an elderly Parmenides subjects the young Socrates' Theory of Forms to devastating objections — the Third Man Argument (regress of Forms explaining the relation between particulars and Forms), the separation problem (if Forms are separate from particulars, how can we know them?), and the knowledge problem (if God knows only the Forms, can God know us?). Plato provides no convincing refutations. Scholars debate whether the dialogue shows Plato abandoning, revising, or stress-testing the theory, or whether it is a dialectical exercise in how to refute any position.
These dialogues introduce a new philosophical method: diaeresis (division) — a hierarchical classification procedure that divides a genus into species by successive differentiation, like a binary tree, until the target concept is uniquely defined. The Sophist also confronts the problem of not-being (the Sophist trades in falsehood, i.e., not-being; but Parmenides said not-being cannot be). Plato's solution — that not-being means "otherness" rather than absolute nothing — is a major advance in philosophical logic and metaphysics.
Plato's longest and last work — left unfinished, without the figure of Socrates. The ideal city of the Republic is abandoned in favour of a "second-best" city governed by laws rather than philosopher-kings: a constitutional oligarchy with mixed elements, austere religion, censorship of art and music, and a nocturnal council of senior citizens as philosophical guardians. The Laws is in many ways more practically oriented and more frightening than the Republic — its proposals for surveillance and religious conformity anticipate the darker side of Platonic political thought.
A return to first-order ethics: what constitutes the good life? Is it pleasure (Callicles, the Cyrenaics), knowledge (a possible Socratic answer), or a mixture? Plato argues for a carefully ordered mixture in which pure pleasures (pleasures of learning, of form, of moderate sensory experience) are combined with knowledge under the governance of measure and proportion. The Philebus introduces a sophisticated classification of pleasure and knowledge that anticipates Aristotle's detailed ethics and connects to the Forms through the concept of limit and unlimited.
Plato's philosophy is one of the most internally rich and tension-generating systems in intellectual history. The tensions below are not failures but productive sites of continuous philosophical work across twenty-four centuries.
How do sensible particulars participate in Forms? Plato uses "participation" (methexis), "imitation" (mimēsis), and "presence" (parousia) interchangeably without providing a metaphysical account of the relation. The Third Man regress (formulated by Aristotle) shows that any account of participation either generates an infinite regress of mediating Forms or collapses into identity. This remains an unsolved problem in contemporary metaphysics of universals and abstract objects.
The Republic requires the philosopher who has seen the Good to return to the cave and govern — at great personal cost to their contemplative life. Yet the Theaetetus compares the philosopher to a man who knows not the way to the agora and has no interest in knowing. Which Plato is authoritative: the one who demands political engagement or the one who celebrates philosophical withdrawal? The tension maps onto competing interpretations of Plato's own biography, including the Syracuse adventures.
Plato chose to write dialogues rather than treatises — and the Phaedrus contains a famous critique of writing itself. No character in any dialogue is straightforwardly "Plato." Does this mean the dialogues are genuinely open-ended philosophical investigations without doctrinal conclusions? Or is the Socratic figure a vehicle for Platonic positions? The tension between esoteric and exoteric readings of Plato (Tübingen school vs. Anglo-American analytic readings) has never been resolved and may be irresolvable.
Diotima's ladder requires the lover to use particular beautiful persons as steps toward transcendent Beauty — and thus to leave them behind. But the richest human relationships in the dialogues are intensely personal: Socrates and Alcibiades, Phaedo and Socrates, Plato and Socrates. Does philosophical love require the abandonment of personal love? Gregory Vlastos' classic essay "The Individual as Object of Love in Plato" identified this as Platonism's central moral failure, reviving a debate that continues in feminist philosophy and ethics of care.
If Aristotle's reports are accurate, Plato's later metaphysics reduced the Forms to numbers generated from the One and the Indefinite Dyad — a more mathematical, less dialogue-accessible system. This esoteric teaching either represents Plato's final position or a misreport by Aristotle. The gap between the dialogues and the unwritten doctrines raises fundamental questions about the relationship between Plato's published philosophy and his private teaching — an irreducible uncertainty at the core of Platonic scholarship.
The Republic's city involves censorship of art and music, eugenic breeding, the suppression of private life for guardians, the "noble lie" told to citizens about their metallic nature, and the banishment of democracy. Is this a genuine political programme or a reductio ad absurdum of applying pure reason to politics? Leo Strauss argued the Republic is ironic throughout; Karl Popper took it at face value as proto-totalitarianism. Both readings find textual support — the question is unresolvable without a theory of Platonic interpretation as a whole.
Primary Texts (in recommended order)
The best entry-point into Platonic epistemology: short, dramatically engaging, and introduces recollection, virtue, and the slave-boy demonstration. The paradox of inquiry is posed with maximum clarity. G.M.A. Grube's translation is recommended for first-time readers.
The central work. Books I–II introduce the question of justice; Books V–VII contain the philosopher-king, the Sun, the Divided Line, and the Cave; Book IX the comparative happiness of just and unjust man; Book X the critique of poetry and the myth of Er. The complete text rewards sustained reading, but these selections capture the philosophical core.
The most beautiful and accessible of the dialogues. Read in the Nehamas-Woodruff translation for its combination of accuracy and literary elegance. The speeches of Aristophanes and Diotima/Socrates followed by Alcibiades' confession constitute the most dramatic philosophical sequence in the entire corpus.
The Phaedo for the soul's immortality and philosophy as practice of dying; the Timaeus for Plato's cosmology, the Demiurge, and the construction of the universe from mathematical Forms — the dialogue that transmitted Platonism to the medieval Latin West via Calcidius' partial translation.
Secondary Literature
The best philosophical commentary on Plato's central work: takes the arguments seriously without antiquarianism, engages with contemporary ethics and political philosophy, and is honest about the Republic's frightening implications. Indispensable for anyone teaching or studying the dialogue.
The standard scholarly translation in English: accurate, readable, with helpful notes. The Hackett Plato Complete Works (Cooper, ed.) is the recommended one-volume scholarly edition of the entire corpus — all dialogues in modern translations with introductions.
Authoritative essays on all major aspects of Plato's thought by leading scholars: epistemology, metaphysics, psychology, ethics, political philosophy, and the later dialogues. The best single secondary reference for a systematic overview of contemporary Platonic scholarship.
"The safest general characterisation of the European philosophical tradition is that it consists of a series of footnotes to Plato." Whitehead's process philosophy is the most ambitious modern attempt to revise Platonic metaphysics while retaining its basic impulse — the conviction that abstractions are more fundamental than concrete particulars.
Plato bequeathed to Western civilisation not a set of solutions but a set of problems so precisely formulated that two and a half millennia of philosophy have been unable to dissolve them. The problem of universals, the mind-body problem, the nature of knowledge versus opinion, the legitimacy of political authority, the relationship between eros and the good — all were given their canonical shape in the Platonic corpus.
Plotinus (205–270 CE) built Neoplatonism on the foundation of the Platonic One, Intellect, and Soul. Augustine of Hippo encountered Platonic philosophy through Porphyry and Plotinus and declared that the Platonists had come closer to Christian truth than any other pagans. The Platonic Form of the Good became the Christian God; the ascent of the soul to Beauty Itself became the soul's return to its creator. Medieval scholasticism, Islamic philosophy (al-Farabi, Avicenna), and Renaissance Platonism all flow from this stream.
The Platonic conviction that the deep structure of reality is mathematical recurs throughout the history of physics: in Kepler's geometric models of the solar system, in Galileo's declaration that the book of nature is written in mathematics, in Dirac's principle that beautiful mathematics must describe physical reality, in the extraordinary unreasonable effectiveness of mathematics in the natural sciences. Mathematical Platonism — the view that mathematical objects exist independently of minds — remains the majority view among working mathematicians.
Aristotle demythologised the Forms by placing universals in things rather than apart from them. Kant transformed Platonic Ideas into regulative principles of reason. Hegel converted the ascent to the Good into the dialectical movement of Spirit through history. Heidegger read the allegory of the Cave as the moment Western metaphysics forgot Being in favour of beings. In each case, Plato is both the starting point and the problem — the thinker whose questions cannot be abandoned because they are the questions that make us philosophical beings.
"Until philosophers are kings, or the kings and princes of this world have the spirit and power of philosophy, and political greatness and wisdom meet in one, cities will never have rest from their evils — nor the human race, as I believe — and then only will this our State have a possibility of life and behold the light of day."
— Republic 473d (Jowett trans.)428/7–348/7 BCE · The ascent toward the Good has not been completed.