Gordian Knot

When Zoe told the Second Doctor that "you just can't cut through hyperoxide", the Second Doctor remarked that "the Gordian Knot couldn't be untied, either". Zoe was immediately perplexed, and Jamie, trying to impress the newly-met girl genius, stumbled around for a definition, whereupon the Doctor revealed that it meant "there's always a solution". (TV: The Wheel in Space)

Behind the scenes
Unremarked in the story is any sort of context for the phrase, which is based on a real knot of Alexander the Great's that was thought to be so complicated as to be impossible to untie. In truth, however, it was easy to untie as long as one figured out its secrets.