‘I often use the concept of a fraction as a teaching aid. Usually I condemn the use of concepts in Zen, but sometimes they are a necessary evil.
We know that every fraction has a numerator over a denominator. As the denominator, I use a circle containing an eight on its side. The circle of course means zero, and is empty and void. The eight on its side is the mathematical symbol for infinity. Therefore, the encircled horizontal eight stands for the empty-infinite, and the empty-infinite is a characteristic of our essential nature. It IS our essential nature.
You may ask why our essential nature is empty. Just consider what we call our mind or consciousness. Does it have any form? No. Any color? No. Any length or breadth or width? No. Can we locate it? No, we do not know where it is. The mind has nothing. The mind IS nothing. It is void and empty, and our essential nature is nothing but the boundless extension or manifestation of this ordinary mind of ours.
While being empty, our mind has, at the same time, limitless and infinite capabilities or activities. It can see, it can hear, it can stand up, it can sit down, it can take a walk. It can feel, it can think, it can imagine, it can forget. Though void, it is limitless and infinite. Therefore I call it the empty-infinite, or the empty-limitlessness, and this is our essential nature. This constitutes the denominator of the fraction.
The phenomenal world is the numerator. Anything will do – a dog, a cat, a finger, a cart, an oak tree, or Mu, or the sound of one hand, or even the whole phenomenal universe itself. Ordinary people are at home in this numerator world and think that it alone exists. They are completely unaware of the denominator world.
A most important point to remember is that the numerator and the denominator are intrinsically one. I have set up the concept of a fraction and divided oneness into two, to try to help you understand that all phenomena have empty-limitlessness behind them, so to speak.’ (Commentary on the Gateless Gate)