Collection and Division (Diairesis)

Plato's mature dialectical method, defined in the Phaedrus and deployed throughout the Sophist: collection (synagōgē) — "seeing together things that are scattered about everywhere and collecting them into one kind" so as to define — and division (diairesis) — "to cut up each kind according to its species along its natural joints… not to splinter any part, as a bad butcher might do" (Phaedrus 265d–266c). It is the philosopher's science: in the Sophist, discerning which kinds blend with which (koinōnia tōn genōn) "takes expertise in dialectic" (253d). This is dialectic in a specifically taxonomic key — distinct both from the elenctic refutation of the early dialogues and from the contradiction-driven dialectic of Hegel and Merleau-Ponty.

Key Points

Two complementary movements: collection gathers a manifold into one Form to be defined; division articulates a kind downward "along its natural joints" into its species.
"Carving at the joints" presupposes that reality has joints — that there are real kinds with real boundaries (grounded in the Forms). The "bad butcher" who splinters a part is the image of a division that cuts where nature does not.
Reflexive in the Sophist: the method is applied to itself (the angler paradigm 218e–221c; the method-reflection at 253d–e and 264b–268d), and the Socratic elenchus ("noble sophistry") is demoted to one species of division (230b–231b).
The "secret" of the Phaedrus speeches: Socrates' two speeches on love were themselves collection-and-division — dividing madness into a diseased "left-hand" and a divine "right-hand" love (265a–266b).
Distinct from the Meno's hypothetical method: the Meno's "investigate from a hypothesis" (86e) is an earlier, different methodological instrument; collection-and-division is the late method proper.
The Philebus's "divine method" (16c–18d): the most explicit statement of the method — but reframed as enumeration of the determinate number of kinds lying between the one and the unlimited (letters, musical intervals), grounded in the limit/unlimited ontology rather than bifurcation-at-joints. "Going straight from the one to the unlimited and omitting the intermediates" is its picture of bad division (17a).
The Statesman's refinements (continuing the Sophist's diairesis): cut at a real class (eidos/genos), not a mere part — "Greek + barbarian" gives a name without a class (262c–e); prefer to bisect "through the middle of things" (262b), but concede that not everything bisects, so some things must be cut "limb by limb, like a sacrificial animal" (287c) — a complement to the Phaedrus's "bad butcher"; and class ≠ part ("a class is necessarily also a part… but not that a part is a class," 263b). The method is explicitly subordinated to "becoming better dialecticians" (285d) and made measurable by the doctrine of the due measure.
Sedimented into doxography (the spurious Definitions): the pseudo-Platonic Definitions freeze the method's output into a ~185-term lexicon — multiple stacked formulae per term ("collection" without the live "division"), the sophist entry copied verbatim from Sophist 231d. This is diairesis as residue: results detached from the cutting that produced them (see claims#plato-spuria-academy-reception-stratum, candidate).

What the Concept Does

Makes definition rigorous — replacing the inconclusive "what is X?" elenchus (which ends in aporia) with a constructive procedure that locates a kind within a structured genus-species map.
Grounds true rhetoric (Phaedrus) — real psychagōgia requires dividing kinds of soul and matching kinds of speech to them; rhetoric becomes applied dialectic.
Solves the sophist (Sophist) — only by mapping which Forms blend can the Visitor define the sophist as a maker of false appearances.

What It Rejects

Handbook rhetoric — which mistakes the parts of a speech (Preamble, Statement, Refutation…) for the art (Phaedrus 266d–267d).
Division that ignores natural joints — the "bad butcher"; cutting by arbitrary similarity (the Sophist warns "be especially careful about similarities," 231a).

Stakes

Collection-and-division is the wiki's marker for what "Platonic dialectic" actually names in the late dialogues — a taxonomic articulation of fixed kinds, not a movement of self-negating contradiction. This matters because the wiki's other "dialectic" pages are German-idealist- and MP-weighted: Hegel's determinate negation drives a developmental dialectic through contradiction and Aufhebung, and MP's hyper-dialectic is "dialectic without synthesis." Platonic diairesis belongs to neither family — it presupposes a stable order to be mapped, where Hegelian and MP dialectic generate or refuse order. Keeping these distinct prevents a pervasive false friend (treating "Plato's dialectic" as a forerunner of Hegel's).

Connections

contrasts with bestimmte-negation — diairesis maps stable kinds; determinate negation generates content through self-negating contradiction. Same word "dialectic," structurally opposite operations.
contrasts with hyper-dialectic — MP's dialectic-without-synthesis refuses the fixed termini that collection-and-division presupposes.
contrasts with propaedeutic-dialectic — MP's "leading-back" / "truth of X" entry is a propaedeutic movement through inadequate positions; Platonic division is a taxonomic descent through a settled genus.
requires theory-of-forms — "natural joints" presuppose real kinds; division is reliable only because there are Forms to carve along.
is deployed by the Sophist's solution — the communion-of-kinds analysis (which Forms blend) is collection-and-division applied to the greatest kinds.
is grounded in number by peras-apeiron — the Philebus's "divine method" ties division to the determinate number of kinds between the one and the unlimited.
is refined by plato-statesman — cut at real classes (not arbitrary parts); bisect through the middle but "limb by limb" where bisection fails (287c); class ≠ part (263b); division for the sake of dialectical training (285d).
is measured by the-mean — the Statesman subordinates the length and manner of division to the due measure (to metrion), the fitting rather than the merely shorter.
is sedimented by the Definitions — the method's output frozen into a doxographic lexicon (the sophist entry = Sophist 231d verbatim); live division become reference residue (spurious). See claims#plato-spuria-academy-reception-stratum (candidate).

Open Questions

Is collection-and-division continuous with the Republic's "dialectic" (the ascent to the Good), or a distinct method? They differ in direction — diairesis descends through a genus; the Republic's noēsis ascends to the unhypothetical first principle — so the corpus may house two "dialectics" under one word. The Philebus's number-based reframing adds a third variant, suggesting the method is a family rather than one procedure.
Does the Phaedrus's pairing of collection-and-division with the resplendent-Forms myth (two registers Cooper calls almost incompatible — "patient mapping" vs "awe-inspiring vision") indicate one method or two?

Sources

plato-phaedrus — the definition of collection and division (265d–266c); the method as the soul of true rhetoric.
plato-sophist — diairesis throughout (the angler paradigm; the reflexive method-passages 253d–e, 264b–268d); the communion of kinds.
plato-statesman — the methodological refinements: cut at real classes (262a–263b); "limb by limb" non-bisection (287c); class ≠ part; division for dialectical training (285d).
plato-philebus — the "divine method" as enumeration of determinate number between one and unlimited (16c–18d).
plato-spuria — the Definitions as collection-and-division sedimented into a ~185-term lexicon (the sophist entry = Sophist 231d). Spurious.