Metabasis, desis, and lusis
Aristotle says in the Poetics that the plot of a tragic drama must concern a change in the protagonist's fortunes, either from good to bad fortune or from bad to good fortune (ch. 7, 1451a11-15; ch. 13, 1452b28-1453a17) . (His conception of tragedy clearly differs from contemporary conceptions in not requiring that events turn out badly for the protagonist, though he does argue (ch. 13, 1453a7-17) that in the best plots the change is from good to bad fortune. For Aristotle the crucial difference between tragedy and comedy lies in the kinds of characters with which each is concerned, tragedy dealing with 'superior' characters, i.e., those of high social status with ability and ambition, and comedy dealing with 'inferior' characters, i.e., those of lower social status with faults which make them ridiculous (ch. 5, 1449a32-37).) Since a tragic plot involves a change in the protagonist's fortunes, there must be a point in the plot at which his or her fortunes change, the outcome of events becomes certain, and the action of the drama moves to its inevitable conclusion. Aristotle uses the Greek words μετάβασις (metabasis) and μεταβολή (metabole), both of which mean 'change', as technical terms for this point of transition in the plot. (See also peripeteia.)
In Poetics ch. 18 Aristotle divides the plot of a tragic drama into two parts: the desis (δέσις) or 'complication' and the lusis (λύσις) or 'resolution'. The desis is defined as the part of the plot which comes before the metabasis, and the lusis as the part which comes after the metabasis (1455b26-29). Aristotle recognises that some or perhaps all of the events which form part of the desis may be external to the action of the drama itself: they may, for example, have taken place before the action begins and merely be referred to in the drama. The lusis, he implies, lies entirely within the drama (1455b24-26). It follows that the metabasis, the point of transition, may come earlier or later in the drama: if the desis lies wholly outside the action of the drama, it may come at the very beginning and the entire drama will consist of lusis or, at the opposite extreme, it may come late in the drama, if, for example, the change is brought about by an intervention at the last minute by a god or goddess.
The distinction between desis and lusis is not discussed at length in the Poetics, though there is a reference to it later in ch. 18 (1456a7-10), when it is suggested that tragedies are particularly susceptible of comparison in respect of their plots and the way they are handled, and in this connection the distinction between desis and lusis is relevant, since many playwrights handle the desis well but the lusis badly.
The terms desis and lusis are still occasionally used in literary theory and literary criticism. Their use is not confined to the discussion of tragic drama and so is not quite the same as Aristotle's use of them in the Poetics. They may be used, e.g., in the discussion of a novel to distinguish what precedes the critical point in the plot from what follows it.
The Greek word δέσις (desis) - it is pronounced with a short 'e' as in 'deck' or 'den' - literally means 'tying together in bundles', and is a noun from the verb δέω (deo, I bind or tie). Later in Poetics ch.18 (1456a9) the word πλοκή (ploke), which literally means 'twisting or twining', is used as an alternative to desis. The word λύσις (lusis), which literally means 'loosing or releasing', is a noun from the verb λύω (luo, I loose, loosen, or untie).