Medea by Euripides (-431)

"I would rather stand three times in the front line of battle," Medea tells the women of Corinth, "than bear one child." She has borne two of Jason's sons, and by the play's end she will have killed them both. What makes this unbearable is not madness but clarity. Jason has left her for a Corinthian princess and come to explain, reasonably, that the match benefits them all; he expects thanks. Medea, who once killed her own brother and abandoned her homeland to follow him, plans her answer in daylight, reasoning every step aloud. She sends the bride a poisoned gown, then turns to the children, because she knows nothing will wound him more. The speech where she holds the boys, wavers, and chooses anyway gives us the first mind in Western drama to argue itself toward the unforgivable rather than stumble there through frenzy. Twenty-five centuries on, every tragedy that sets a lucid mind at the edge of the worst thing it could do is working in this play's shadow.