DIAIRESIS is ‘division’. This Ancient Greek word expresses the concept of ‘dissection’, ‘partitioning’. In logic, <diairesis> it is the form of classification to systematize notions and reach definitions.
The process starts with us dividing a broad notion into two or more sub-notions. Then, after eliminating one of the two sub-notions, we repeat the same operation. The procedure goes on as many times as we need in order to reach a definition. The founder of <diairesis> as a method is Platon (Plato).
<Diairesis> is ‘division’. For example, in the Sophist, the Eleatic Stranger analyses illusions, which consist of words and “visual objects”. By using <diairesis>, he divides visual objects (works of art), into two categories. One category is <eikastikē technē>, the art of making likenesses. The second category is <phantastikē technē>, the art of creating illusionary appearances. The Stranger appreciates the former because the latter only produce an appearance of beauty.
This modus operandi is present in the first steps of classifying biology, that is in the zoology of Aristoteles (Aristotle) and in the botany of Theophrastus. Also it is important in Galen‘s therapeutics. In fact, Galen says that many errors that the doctors make in the care of sick people have incompetent divisions as their principal cause. Mentions and references regarding the method of <diairesis> are quite widespread among exponents of the Platonic Academy (especially Speusippus and Xenocrates).
Also among exponents of the Peripatetic school (especially Aristoteles, Aristoxenus, Theophrastus). Moreover among philosophers ascribable to Stoicism (especially Chrysippus). Again among some thinkers of Middle Platonism (especially Alcinous, Maximus of Tyre, Philo). In conclusion among representatives of Neoplatonism (especially Plotinus, Porphyry). In medieval times the <divisio> was common.