(The). The letter Y, used by Pythagoras as an emblem of the
straight narrow path of virtue, which is one, but, if once
deviated from, the farther the lines are extended the wider
becomes the breach. When reason doubtful, like the Samian
letter, Points him two ways, the narrower the better. Source:
Dictionary of Phrase and Fable, E. Cobham Brewer, 1894 More on
Samian Letter from Infoplease: