Why is the use of a deus ex machina discouraged in storytelling?

The phrase comes from ancient Greek and Roman dramas, when a seemingly insolvable situation was miraculously solved by a god brought on stage by an elaborate crane. Thus, deus ex machina, literally “god from the machine.” While writers might not send gods to swoop in and save the day all that often anymore, there is the temptation to introduce artificial or improbable elements to resolve a difficult conflict. A character surrounded by snarling wolves, backing toward the edge of a steep cliff, who wakes up to learn it was all just a dream. Or two friends down and out in Vegas, hounded by thugs they owe money to, who hit it big on the slots.

Aristotle criticized the device as clumsy, and contemporary readers will do the same. For one thing, it can present a rather unlikely circumstance, one so improbable that it strains the reader’s ability to believe in the story. When a long lost aunt dies and leaves a poverty-stricken family with a huge inheritance, the reader is going to wonder where that aunt came from. And how convenient that her death comes just as the landlord is knocking on the door to begin the eviction process. Of course, anything can happen in fiction, but the writer’s job is to make it believable, and a sudden solution appearing out of nowhere isn’t believable.

A deus ex machina also ignores the fact that a character is an important player in the action. Characters should be active in bringing about the action, whether they intend to or not. In The Great Gatsby by F. Scott Fitzgerald, Gatsby is shot due to a moment of mistaken identity. It seems out of the blue, but Gatsby is an active participant in what brings that action about. He pursues the already-married Daisy. A group including Gatsby, Daisy, and her husband Tom travel to the city where Gatsby pressures Daisy to tell Tom that she never loved him. She and Gatsby drive home in Gatsby’s car. Daisy, frantic at the wheel, hits and kills a woman who happens to be Tom’s mistress. The woman’s husband tracks down Gatsby, thinking he’s the culprit. Gatsby didn’t orchestrate the mistaken identity, but he did put pressure on Daisy, which resulted in her distracted driving. When she hit the woman, he orchestrated the aftermath, getting Daisy home to try and protect her from what she’d done.

When the man wakes from the dream, or the two friends hit it big in Vegas, or the family gets a windfall, the character’s actions are rendered irrelevant. They have almost nothing to do with what happens. And why bother learning about a character if what he or she does doesn’t matter?