Touchstone

"In this world the bitter sea and the sweet sea (are divided)— between them is a barrier which they do not seek to cross. 

Know that both these flow from one origin. Pass on from them both, go (all the way) to their origin!
Without the touchstone you will never know in the assay adulterated gold and fine gold by (using your own) judgement.
Any one in whose soul God shall put the touchstone, he will distinguish certainty from doubt." The Masnawi I, 297-300