These various forms appear different in shape and size, yet they are of a single essence... To realize this One-essence is to free yourself... The Sixth Patriarch called it "essence of Mind"... Here the Third Patriarch calls it "timeless Self-essence." Bankei called it "unborn Buddha-mind." They all refer to the same thing: Buddha-nature, true self.
This essence is not born and can never die.
It exists eternally.
Some call it energy;
others call it spirit.
But what is it?
No one knows.
Any concept we have of what it is can only be an analogy...
(p.93)