(_f_) The poet “may not indeed destroy the framework of the received legends—the fact, for instance, that Clytæmnestra was slain by Orestes and Eriphyle by Alcmæon—but he ought to show invention of his own, and skilfully handle the traditional material”.[138] This injunction was obeyed beforehand by all the three Athenian masters; it is especially important to remember it when studying Euripides. (_g_) “The unravelling of the plot ... must arise out of the plot itself; it must not be brought about by the _Deus ex Machina_, as in the Medea.... The _Deus ex Machina_ should be employed only for events external to the drama—for antecedent or subsequent events, which lie beyond the range of human knowledge.”[139] This vital criticism will be considered later, when we discuss the _Philoctetes_[140] of Sophocles and the Euripidean drama.[141] (_h_) “Within the action there must be nothing irrational. If the irrational cannot be excluded, it should be outside the scope of the tragedy. Such is the irrational element in the Œdipus of Sophocles.”[142] Aristotle means certain strange _data_ in the _Œdipus Tyrannus_—the fact that neither Œdipus nor Jocasta has learnt earlier about the past, and so forth. § II. On the purely literary line he tells us the parts:—[143] (_a_) “The Prologos is that entire part of a tragedy which precedes the Parodos of the chorus” (see below for the Parodos). Thus a drama may have no “prologos” at all, for example the _Persæ_. The implications of our word “prologue” are derived from the practice of Euripides, who is fond of giving in his “prologos” an account of events which have led up to the action about to be displayed. (_b_) “The Episode is that entire part of a tragedy which is between complete choric songs.” “Episodes” then are what we call “acts”: the name has already been explained.[144] (_c_) “The Exodos is that entire part of a tragedy which has no choric song after it.” The few anapæsts which close most tragedies are not “choric songs”—they were performed in recitative. Thus the Exodos is simply the last act. (_d_) “Of the choric part the Parodos is the first undivided utterance of the chorus: the Stasimon is a choric ode without anapæsts or trochees: the Commos is a joint lamentation of chorus and actors.” It will be seen later[145] that by excluding trochees he probably means the trochaic tetrameter as seen in dialogue; lyric trochees are very common. § III. On the strictly dramatic line he tells us the _stages of structural development_. (_a_) “Every tragedy falls into two parts—Complication and Unravelling (or _Dénouement_).... By the Complication I mean all that comes between the beginning of the action and the part which marks the turning-point to good or bad fortune. The Unravelling is that which comes between the beginning of the change and the end.”[146]