These various forms appear different in shape and size, yet they are of a single essence. . . . 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. . . .
Source: The Eye never sleeps: Striking to the heart of zen, Boston: Shambhala Publications, Inc, 1991, p 93
Contributed by: Zaady