What is the Essential Foundation of Mental Purity ?

The Blessed Buddha once said:Bhikkhus, there are these Four Foundations of Awareness. What four?When a Bhikkhu keenly contemplates:1: The formation within all Body...2: The sensation within all Feeling...3: The mentation within all Mind...4: The discrete states within all Phenomena..while always acutely aware & clearly comprehending, he thereby removes any urge, envy, jealousy, frustration and discontent rooted in this world...These are the Four Foundations of Awareness...