purity of soul

Came across the term purity of soul all of a sudden.It seems so simple,so uncomplicated.Yet very difficult to adhere to.
Can a molecule of pure water see impurities.There lies the answer to purity of soul.A soul that cannot regard anything as impure and can see only the bright side of everything.Its almost impossible by a human,hence even our scriptures speak more on purity of heart rather than purity of soul.