Attempts to determine an absolute chronology for the composition of Seneca's tragedies have been frustrated by our lack of both external and internal evidence for dating. Even a relative chronology moreover will be open to dispute as long as we must depend almost solely on metrical, stylistic and linguistic considerations. Recently, for example, Calder has questioned the opinion of Leo, and other scholars, that Agamemnon was written before Thyestes. Calder has examined stylistic features of both plays, and he suggests, quite correctly, that the opening scene of Agamemnon is similar to that of Thyestes in structure and imagery but is less skilfully connected to the subsequent action. He concludes, however, that Thyestes is the earlier play, and he offers the following conjectures as proof: Seneca borrowed the opening scene of Thyestes from a play of Sophocles; then, when composing Agamemnon, instead of using the watchman's prologue of Aeschylus, he attempted to create a new opening scene after the pattern of his Thyestes. Not explicitly stated, though perhaps implied, is the unfortunate conclusion that the Thyestes opening scene is successful because Seneca followed Sophocles, that Agamemnon is flawed because Seneca later tried to innovate, to deviate from his Greek ‘models’.
Such a conclusion fails, I think, to take into proper consideration Seneca's creativity in his use of sources, and the reasons why he used an opening scene by a superhuman figure, rather than a watchman. Three of Seneca's plays begin with a scene in which appears a superhuman figure: Hercules Furens, Agamemnon and Thyestes. If we compare the first two plays with the Herakles of Euripides and the Agamemnon of Aeschylus, which both open with human figures, we find that in both Seneca has preferred not to imitate the prologues of the Greek dramatists. (We have, unfortunately, no complete Greek Thyestes with which we can compare Seneca's.) In addition, his opening scenes differ in style from those Greek plays in which a superhuman figure does appear in the prologue. Obviously, the choice of a human or non-human prologist, as Heldmann says, depends on something other than a Greek source.