Deus ex machina

Definition and stakes

Roy C. Flickinger The Greek theater and its drama (1918)

It is plain that in all or practically all of these the god is raised above the other performers, as would be only appropriate for an effective close; but whether the deity merely came forward upon the logium or was brought into view by means of a machine is not always an easy matter to determine. By a natural extension of meaning, however, such an apparition at the close of a play came to be called a “god from the machine” (θεὸς ἀπὸ μηχανῆς; deus ex machina) regardless of the method used for his appearance.
Source: Gutenberg

Gilbert Norwood Greek tragedy (1920)

But the deus ex machina needs a few words. “The god out of the machine” is a phrase of two applications. It may mean a deity brought in to round off the play by giving information about the future history of the personages. Or the god may be introduced when the plot, owing to the human limitations of the characters, has become knotted and progress is impossible; then a being who miraculously knows all the facts appears and “cuts” the knot. In the first case the epiphany is practically outside the drama; in the second it is only too vital to it.
Source: Gutenberg

Alexander H. Japp Robert Louis Stevenson: A Record…

As for Miss Barbara Grant, of whom so much has been made by many admirers, she is decidedly clever, indeed too clever by half, and yet her doom is to be a mere deus ex machinâ, and never do more than just pay a little tribute to Stevenson’s own power of persiflage, or, if you like, to pay a penalty, poor lass, for the too perfect doing of hat, and really, really, I could not help saying this much, though, I do believe that she deserved just a wee bit better fate than that.
Source: Gutenberg

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