It's Not a Thing
When Zen Master Huai Jang first visited Hui Neng, the Sixth Patriarch, he was already an experienced practitioner. After he bowed, Hui Neng asked him, "Where do you come from?" Huai Jang replied, "From Sung Shan." The Patriarch then asked, "What thing is it that you have brought here?" Huai Jang said, "If you call it a 'thing' you have already missed the mark." If it's not a "thing," as Huai Jang says, then the question for each of us is... what is this thing that we have brought here?
When you are thinking your mind and everything in the universe are separate. When you cut off all thinking, your mind and this whole universe become one. Subject and object fall away and you return to your original nature, our substance. For example, if you raise the big question, "What am I?" and you look deep inside, always "don't know" appears. This "don't know" cuts off all thinking. That is your substance. But, before thinking there aren't any names or words because it's before thinking. That means that "don't know" is not "don't know"; it is just a name for substance.
Over the centuries, humans have given "substance" many names. Eskimos have given eleven different names to snow, but that is nothing compared to our ability to give names to substance. The morning bell chant says that Buddha alone has been given 360,000,119,500 names! Some people have called substance Buddha, some have called it enlightenment, God, the Absolute, Truth, Tao, the Void, Energy, Primary Point, don't know and even substance. On and on... But true substance is before thinking, so if you open your mouth it's already a mistake. The wisdom of the ancient Jews was right on the mark with respect to this; they refused to even utter the name of God. This same wisdom resides in Buddhism, too. The last of the Four Great Vows says: The Buddha way is inconceivable, we vow to attain it. In fact, we are infinite in time and space, so how could words possibly capture it? And yet we are very attached to the words we use to define and understand ourselves.
Dong Sahn Yonsa said, "Shakyamuni Buddha, the Buddha of the past and Maitraya, the Buddha of the future, are servants of another." Do you know who that is?