One must be able to see the pain in pleasure and the pleasure in pain; the gain in the loss and the loss in the gain. What generally happens is that one is blunted to one thing and that one's eyes are open to another thing; that one does not see the loss or that one does not see the gain; if one recognizes the right, one does not recognize the wrong. Mental purification means that impressions such as good and bad, wrong and right, gain and loss, and pleasure and pain, these opposites which block the mind, must be cleared out by seeing the opposite of these things. Then one can see the enemy in the friend and the friend in the enemy. When one can recognize poison in nectar and nectar in the poison, that is the time when death and life become one too. Opposites no more remain opposites before one. That is called mental purification. And those who come to this stage are the living sages.