Gordian Knot

The Gordian Knot is a legend of Phrygian Gordium associated with Alexander the Great. It is often used as a metaphor for an intractable problem (disentangling an "impossible" knot) solved easily by cheating or "thinking outside the box" ("cutting the Gordian knot"):

"Turn him to any cause of policy,

The Gordian Knot of it he will unloose,
Familiar as his garter" (Shakespeare, Henry V, Act 1 Scene 1. 45–47)