Plato's method of definition
Diaeresis is a method of definition based on partitition which occurs in the Platonic dialogues Phaedrus, Sophist, Statesman and Philebus. It is a means of reaching a definition by which a large group is continually divided into relatively equal parts until the definition has been found.
A complementary term is merismos (cf. English merism: parsing or the distinguishing of parts, as opposed to diairesis, which is the division of a genus into its parts).
For example, in the Sophist (§235B), the Eleatic Stranger is examining illusions, which consist of words and "visual objects." By using diaeresis, he divides visual objects, by which it becomes clear he means works of art, into two categories: eikastikē technē, the art of making likenesses or eikones; and phantastikē technē, the art of creating illusionary appearances. The Stranger is much more fond of the former; the latter are only created to produce an appearance of beauty.
The method of diairesis in the history of philosophy
Forerunners of Plato
Opinions about possible forerunners of Platonic diairesis are varying; they even reach back until Homer. Also an adoption from the fields of mathematics has been considered, like one from musicology, one from pre-scientific and everyday divisions and one from medicine. About forerunners in the field of philosophy there are as well different opinions. Under consideration are Prodicus of Ceos, Democritus and Leucippus and the sophists. It was even suggested that Plato wants to say - in Sophistes 267d - that he himself found the new method, which shows that it is possible that Platon had no forerunners at all.
Later exponents of the method of diairesis
The platonic method of division is found to be applicated at the first steps of classifying biology, namely in the zoology of Aristotle and in the botany of Theophrastus. Diaeresis is central to Galen's therapeutics; see for example 'Therapeutics to Glaucon' 1 (XI, 4 K), where Galen, attributing the method to Plato, asserts that 'the errors of the [medical] sects and whatever mistakes the majority of physicians make in the care of the sick have incompetent division as their principal and major cause' (tr. Dickson.) Philosophically relevant methodical divisions or statements about the method of diairesis can be found at exponents of the Platonic Academy (especially Speusippus and Xenocrates), exponents of the Peripatetic school (especially Aristotle, Aristoxenus, Theophrastus), at the Stoicism (especially Chrysippus), at the Middle Platonism (especially Alcinous, Maximus of Tyre, Philo) and at the Neoplatonism (especially Plotinus, Porphyry). In medieval times the so called method of divisio was a common method. Over unreconstructable interpositions division plays as well a role in the modern age.
- (german) Hans Leisegang: Denkformen, Walter de Gruyter, Berlin 1951, p. 220
- (german) Hermann Koller: Die dihäretische Methode, in: Glotta Vol. 39, Vandenhoeck & Ruprecht, Göttingen 1961, p. 23
- (german) Hans Herter: Platons Naturkunde, in: Rheinisches Museum für Philologie Vol. 121, 1978, p. 111, online: 
- (german) Hans Herter: Platons Naturkunde, in: Rheinisches Museum für Philologie Vol. 121, 1978, p. 116
- (german) Christian Schäfer (editor): Platon-Lexikon, Wissenschaftliche Buchgesellschaft Darmstadt 2007, p. 92
- John Lloyd Ackrill: In Defense of Platonic Division, in: Essays on Plato and Aristotle, Clarendon Press, Oxford 1997, p. 105
- (german) Julius Stenzel: Studien zur Entwicklung der platonischen Dialektik von Sokrates zu Aristoteles, 2. edition 1931, Nachdruck: Teubner, Stuttgart 1961, p. 112
- (german) Artur v. Fragstein: Die Diairesis bei Aristoteles, Adolf M. Hakkert, Amsterdam 1967, p. 80
- (german) Margot Fleischer: Hermeneutische Anthropologie - Platon, Aristoteles, Walter de Gruyter, Berlin 1976, p. 143
- Richard Robinson: Plato's Earlier Dialectic, Oxford University Press, Oxford 1953, p. 89
- (german) Hans Herter: Platons Naturkunde, in: Rheinisches Museum für Philologie Vol. 121, 1978, p. 123
- Hans Herter: Platons Naturkunde, in: Rheinisches Museum für Philologie Vol. 121, 1978, p. 115
- Keith Dickson: 'Stephanus the Philosopher and Physician, Commentary on Galen's Therapeutics to Glaucon', Leiden:Brill, 1998
- See John Dillon (1998). "Speusippus." In E. Craig (Ed.), Routledge Encyclopedia of Philosophy. London: Routledge. Retrieved October 10, 2007, from http://www.rep.routledge.com/article/A111SECT3
- (german) Hans Leisegang: Denkformen, Walter de Gruyter, Berlin 1951, p. 252
- Method and Metaphysics in Plato's Sophist and Statesman, by Mary Louise Gill, (Stanford Encyclopedia of Philosophy)