Let them think of a lecture hall that is bright while the sun is shining but is dark after the sun goes down.
They can think of the light departing with the sun and the dark coming with the night, but they cannot so think of the mind that perceives lightness and darkness. The mind that is susceptible to lightness and darkness can not be given back to anybody; it can only revert to a truer nature which is its fundamental nature.
It is only a "temporary" mind that momentarily notes changes of lightness and darkness as the sun rises and sets.
It is only a "temporary" mind that has different feelings from moment to moment with the changing circumstances of life; it is not the real and true mind. The fundamental and true mind which realizes the lightness and the darkness is the true nature of man.
The temporary feelings of good and evil, love and hatred, that have been aroused by surroundings and changing external conditions, are only momentary reactions that have their cause in the defilement accumulated by the human mind.
Behind the desires and worldly passions which the mind entertains, there abides, clear and undefiled, the fundamental and true essence of mind.
Water is round in a round receptacle and square in a square one, but water itself has no particular shape. People often forget this fact.
People see this good and that bad, they like this and dislike that, and they discriminate existence from non-existence; and then, being caught in these entanglements and becoming attached to them, they suffer.
If people would only give up their attachments to these imaginary and false discriminations, and restore the purity of their original minds, then both their mind and their body would be free from defilement and suffering; they would know the peacefulness that comes with that freedom.