Theory of Forms

Plato's thesis that the truly real beings are Forms (eidē) — self-identical, invisible, unchanging realities such as "the Equal itself," "the Beautiful itself," "the Just itself" — of which sensible particulars are deficient, transient copies that "strive to be like" them and "fall short." A Form is the "X-itself" (auto kath' hauto), "marked with the seal of 'what it is'" (Phaedo 75d). The theory is the corpus's central metaphysical thesis and the single most consequential inheritance Plato leaves to the Western tradition — but it is not a fixed doctrine: it does different work in different dialogues, and Plato himself subjects it to revision (Sophist) and criticism (Parmenides).

Genealogical guard-rail. The separated Forms are not yet present in the Meno, whose eidos is only the single common feature sought by "what is X?"; the Phaedo is where the Forms enter as a developed theory (Cooper, intro notes). The Euthyphro's "form itself … used as a model" (6d–e) is a second pre-Forms anchor — the same *ti esti* demand for a single eidos-as-paradeigma, still with no separation (auto kath' hauto), no recollection, no two-world ontology. The Hippias Major supplies a third — "the fine itself … when that form (eidos) is added" (289d) is proto-Forms language for the very kalon the Symposium later reaches as a separated "Beauty itself" — though its disputed authenticity means it bears correspondingly less genealogical weight. A fourth and even thinner case: the Alcibiades I names "what 'itself' is, in itself" (auto to auto, 130d) but explicitly skips it (130c–d), settling for the functional self (the soul as ruler) — its disputed authorship and deliberate bracketing of the metaphysical question make it the least load-bearing eidos-flirtation in the corpus. Reading the Meno's, Euthyphro's, Hippias Major's, or Alcibiades's eidos-language as the transcendent Form is an anachronism the wiki must avoid.

Key Points

The "X-itself" (auto kath' hauto): Forms exist separately, are not abstractions from particulars, and ground a particular's being-F. "It is through Beauty that beautiful things are made beautiful" (Phaedo 100e) — the Form is the explanatory cause (aitia), via "sharing in" it (methexis), with the participation relation left explicitly undefined (Phaedo 100d).
Forms as objects of recollection: we recognize sensibles as deficiently F only because the soul already knew F-itself before birth (Phaedo 74a–75c; Phaedrus 249b–c).
The localization of the Forms (Phaedrus 247c–e): the soul glimpsed the Forms — "without color and without shape and without solidity… visible only to intelligence" — in "the place beyond heaven" (huperouranios topos); this is the most explicit Platonic placing of the Forms.
The Forms as model (Timaeus): the demiurge fashions the cosmos by looking to the eternal "Living Thing," making the world a copy.
The Forms revised (Sophist): the late "communion of kinds" (koinōnia tōn genōn) makes Forms blend selectively — "the weaving together of forms is what makes speech possible" (259e) — a relational picture replacing simple separateness; and the "friends of the Forms" who make being wholly changeless are themselves criticized (248a–249d).
The Forms in the Republic (the epistemology + the Good): the Forms are the objects of knowledge against the "lovers of sights and sounds" (476–480); the Divided Line and Cave map the ascent to them; and the Form of the Good stands above the Forms as the source of their being and intelligibility, "beyond being" (509b). Learning is periagōgē (turning the soul), a model distinct from recollection.
Beauty itself in the Symposium (211a–b): the canonical Form-formula — "it always is and neither comes to be nor passes away … itself by itself with itself, always one in form" — reached by erotic ascent, not recollection or division, and the only Form thematized.
The Forms criticized in the Parmenides (131a–134e): the Third Man, the whole-or-part dilemma of participation, and the "Greatest Difficulty" (separated Forms unknowable to us) — the theory's sharpest internal critique, which Plato nonetheless retains ("without Forms the power of dialectic is destroyed," 135c).
A hedge on the vision: in the Phaedrus Socrates frames the resplendent-Forms metaphysics as "resemblance only," rhetorically tailored (per Cooper) — a confidence-calibration caveat against treating the myth as settled doctrine.
The Forms approached through five things (Letter VII 342a–344b): to know any real being requires the name, the definition, the image, knowledge (in the soul), and the object itself — the fifth, the "knowable and truly real being" (auto kath' hauto), is the Form. The first four are necessary instruments, but each is "by nature defective," so language cannot deliver the Form even as it is the indispensable means — an epistemology of access to (and the in-principle inexpressibility of) the Forms.

What the Concept Does

Grounds knowledge against flux. If everything flowed there could be no knowledge and nothing to know (Cratylus 439c–440e); stable Forms are what knowledge of something requires. The Forms are introduced as the condition of the possibility of epistēmē.
Replaces physical with logico-ontological explanation. The Phaedo's "second sailing" abandons efficient/material causes ("taller by a head") for the "safe answer" that things are F by the Form F — relocating explanation from mechanism to ground.
Underwrites naming and method. The rule-setter looks to "the form of the name" (Cratylus 389a); dialectic ("collection and division") carves reality "at its natural joints" because there are joints — kinds with real boundaries (Phaedrus 265d–266c; Sophist).

What It Rejects

Protagorean relativism ("man the measure") and Heraclitean flux — both deny the stable natures the Forms secure (Cratylus 386, 439–440).
The reduction of being to body — against the "giants" (materialists) of the Sophist's gigantomachia (246a).
Knowledge through names/images — one must reach "the things themselves," not their linguistic deposits (Cratylus 438e).

Stakes

If the Forms are accepted, knowledge, value, and intelligible order have a non-sensible ground, and philosophy is the ascent to that ground. The theory installs the supersensible/sensible "two-world" schema that defines Western metaphysics — and thereby becomes the target of its great reversals: Nietzsche's *Umdrehung des Platonismus* (the overturning, and ultimately the Herausdrehung / twisting-free, of the two-world structure) and, in a different key, MP's experimental Platonism, which inverts the Form-precedes-instance relation into "essence follows fact." On the Heideggerian periodization, Plato's installation of the idea (εἶδος, "look"/aspect) as the measure of being is the beginning of metaphysics (cf. parmenides as the pre-metaphysical Anfang).

Positions

Phaedo: Forms as objects of recollection and as causes; the "X-itself"; participation left undefined.
Phaedrus: the resplendent Forms beyond heaven, recollected via beauty — but framed as "resemblance only."
Timaeus: the Forms as the demiurge's eternal model (and the khôra as the "third kind" that is emphatically not a Form — neither model nor copy).
Critias: the ideal city as a paradigm realized in narrative — primeval Athens instances the Republic's kallipolis the way the Timaeus's cosmos copies its eternal model, completing the Republic→Timaeus→Critias frame; but only the institutional layer is enacted, not the philosopher-rulers or the Good. See claims#plato-republic-timaeus-critias-enacted-paradigm (live claim).
Sophist: the relational revision — Forms blend (communion of kinds); the "friends of the Forms" position criticized.
Cratylus: "the form of the name" and "the beautiful itself" anchor language and knowledge.
Republic: the Forms as the objects of knowledge against the lovers of sights and sounds (476–480); the Line, Cave, and the Good "beyond being" above the Forms; learning as periagōgē, not recollection.
Symposium: Beauty itself reached by erotic ascent (211a–b) — the canonical Form-formula and the sole Form thematized.
Parmenides: the Forms put through six objections (the Third Man, the whole-or-part dilemma, the Greatest Difficulty) and retained as the condition of dialectic — "refurbishing," not refutation.
Philebus: the "serious one-and-many" problem (15a–c) — how a monad is "one and the same in many things" — treated as a genuine difficulty requiring the divine method.
Letter VII: the Forms reaffirmed and given an epistemology of access — the five things (name/definition/image/knowledge/object), where the fifth is the Form and language's "weakness" means writing can never secure it. The most explicit Platonic statement that the Form is reached only by a kindred soul, "like a leaping spark," never by formula.
Unresolved within the corpus: the participation relation (how particulars "share in" Forms) is undefined in the Phaedo (100d) and is the target of the Parmenides's self-criticism. The Parmenides neither refutes nor repairs it but demands "refurbishing" — a tension the wiki leaves standing. See claims#parmenides-self-critique-retains-forms (candidate).

Connections

is inverted by experimental-platonism — Ruyer/MP/Beith make the "ideal type" follow its instances (essence follows fact), reversing Form-precedes-instance.
is overturned by umdrehung-des-platonismus — Nietzsche (on Heidegger's reading) abolishes the true-world/apparent-world schema the Forms install; "Plato is not yet Platonism."
is the condition of intelligibility of anamnesis — recollection is recollection of the Forms (in the Phaedo and Phaedrus; not yet in the Meno).
is anticipated by socratic-definition — the Socratic "what is X?" demand for a single eidos-as-model (Euthyphro 6d–e, Meno) is the pre-Forms seed of the X-itself, before the separation.
contrasts with khora — the receptacle is the one thing in the Timaeus that is neither a Form nor a copy; the Forms' "other."
is crowned by form-of-the-good — the Good is the source of the Forms' being and intelligibility, itself "beyond being" (Republic 509b).
is criticized by third-man-argument — the Parmenides' regress objection; retained despite it.
is reached by ascent in eros — the Symposium's ladder culminates in Beauty itself.
is given an access-epistemology by plato-letter-7 — the five-things schema: the Form is the "fifth thing," reachable only through (and beyond) the four defective linguistic instruments. See claims#plato-critique-of-writing-spine (live claim).
is a genealogical seed of the Idea-as-ground lineage — "beautiful things are beautiful by the Beautiful" (Phaedo 100e) is an origin of the tradition in which the Idea is the explanatory ground of the particular, transformed in German idealism (cf. Hegel's Idea).

Motif Weight & Corpus Recurrence

This concept homes several Plato-side motifs tracked in motifs (each STRUCTURAL corpus-wide — HUB-weight within the Plato sub-corpus but thin cross-author, since the wiki engages the Forms chiefly as the anti-Platonic target of MP/Carbone/Deleuze):

§"the Forms / X-itself / auto kath' hauto / participation (methexis) / separation (chōrismos) (Plato)" — STRUCTURAL, 7 Plato sources (primary home).
§"being vs becoming / being vs seeming / appearing (phainesthai) (Plato)" — STRUCTURAL, 7 Plato sources (a home; motifs.md marks no single primary among theory-of-forms / non-being / mimesis).
§"recollection / anamnesis / learning-as-recovery (Plato)" — STRUCTURAL, 3 Plato sources (secondary home; the primary home is anamnesis).

See motifs.md for the current attestation lists, source-level weights, and genealogy / cross-tradition links. Update both this section and the motifs.md entries when corpus weight shifts.

Open Questions

How should the wiki weight the Sophist's relational revision against the Phaedo/Phaedrus "separate and resplendent" picture? They may be stages, or registers, or genuinely different theories.
The Parmenides's critique of participation is now on the wiki (third-man-argument + the whole-or-part dilemma): it shows the gap is self-criticism Plato declines to resolve, retaining the Forms as the condition of dialectic. How the Sophist's relational Forms actually answer it remains open.
Is the Republic's Good "beyond being" a Form at all, or a principle of another order? And how does it relate to the Philebus's good-as-measure?
Is "the form of the name" (Cratylus) a Form in the full sense, or a looser use of eidos? The dialogue's own irony complicates the answer.

Sources

plato-phaedo — Forms as recollected objects and as causes; the "X-itself" (74a–80b, 100b–101e).
plato-phaedrus — the place beyond heaven; recollection via beauty (247c–250e); the "resemblance only" hedge.
plato-timaeus — the Forms as the demiurge's model (and the khôra as their non-Form "other").
plato-sophist — the communion of kinds; the critique of the "friends of the Forms" (248a–259e).
plato-cratylus — "the form of the name"; "the beautiful itself" against flux (389a, 439c).
plato-republic — the Forms as objects of knowledge; the Sun/Line/Cave and the Good "beyond being" (476a–509c).
plato-symposium — Beauty itself, the canonical Form-formula reached by erotic ascent (211a–b).
plato-parmenides — the critique of the Forms and their retention (131a–135c).
plato-philebus — the one-and-many problem of the monads (15a–c).
plato-letter-7 — the five-things schema and the Form as the "fifth thing" (342a–344b); the inexpressibility of the Forms in writing.
plato-euthyphro — a pre-Forms anchor: the eidos-as-model "what is X?" demand (6d–e), without separation (the genealogical guard-rail).
plato-hippias-major — a third pre-Forms anchor: proto-eidos language for "the fine itself" (289d), weighted down by the dialogue's disputed authenticity.
plato-critias — the Forms-paradigm enacted in narrative: ancient Athens as the Republic's kallipolis realized (110c–112e); the paradigm/instance relation staged across Republic→Timaeus→Critias.
plato-alcibiades-1 — a guard-rail anchor: "what 'itself' is, in itself" (auto to auto, 130d) named but skipped (130c–d); the Alcibiades gives the functional ruling-self, not a Form. (Authorship disputed.)