When we are thinking about the highest truth, we think and think and think until we come to the ultimate, then we cannot think anymore. Our thinking mind can go no further; it is at the edge of a great cliff.
It's like using a drill. As long as you can drill, it's not the end. When there is no resistance left, when you can drill no more, it goes "Ztt" and the end is reached. We say thought has penetrated through reason. When you have reached the true conclusion, there is no word. When you have destroyed the last word and arrived at chaos, it is infinite. If you come to God or Buddha or anything, it is not the end. But when you have destroyed the last word, when you cannot think of anything more, then you will see that endless gap.
Perhaps you will turn back because you are afraid. But the Zen student is not afraid; the Zen student will leap into the infinite. You must jump into it. Just jump! (Not with your body; your body still sits where it is; so you need not be afraid.) Jump into that conclusion!
While you have a word, you are not a good student. While there is some picture in your brain, you have not reached the end... You are thinking about it, but you don't SEE it. You are looking at something, so you haven't come to the end yet. As long as you are thinking something is there, and you are thinking about it, it is not the real end. But when there is nothing to think about, and your faculty of thinking has come to an end, and you all of a sudden throw yourself into chaos---into the infinite---your mind and body and soul all disappear. That is the real end.
Shakyamuni's enlightenment under the bodhi tree is an example of reaching the end. He proved Nirvana. There is nobody whose name is God, whose name is man, neither devil nor demon, who lives in Nirvana. Nirvana is infinite, boundless.
from "The Zen Eye" by Sokei-an