The word ‘clue’ derives from ‘clew,’ meaning a ball of thread or yarn. It had come to mean ‘that which points the way’ because of the Greek myth in which Theseus uses a ball of yarn, given to him by Ariadne, to find his way out of the Minotaur’s labyrinth. The wirters of the mid-nineteenth century still had this image in mind when they used the word… .
The thread that led Theseus out of the maze was true to another principle of Whicher’s investigation: the progress of a detective was backwards. To find his way out of danger and confusion, Theseus had to retrace his steps, return to the origin. The solution to a crime was the beginning as well as the end of a story."
— Kate Summerscale, The Suspicions of Mr. Whicher (p. 68, 70)