Doubt is a powerful and necessary tool. Wonder, mystery, unresolved and unresolvable puzzlement. Greater than doubt is Great Doubt. Great Doubt and the Great Secret.
The ancients spoke of three essential conditions for Zen practice:
First: great faith; second: great doubt; third: great determination. These are like the three legs of a tripod.
Now, what is great doubt? The type of doubt being referred to here is not intellectual doubt, such as we have when asking about the meaning of a koan. Instead, we can think of great doubt as utterly becoming one with our practice—whether we are counting the breath or practicing with the koan “Mu”—to the point that our entire body and mind are like a single mass of inquiry….
The great root of faith naturally activates this great ball of doubt. If the root of faith appears, the great ball of doubt will arise without fail. Spurred on by great doubt we continue the practice of [the koan] Mu, without seeking or expecting awakening. The quickest way to awaken when completely absorbed in Mu is to throw away all thoughts about it. Awakening has nothing to do with any kind of intellectual knowledge or discrimination.
Koun Yamada, Zen: The Authentic Gate
In the Hour of Doubt
It is told:
In the city of Satanov there was a learned man, whose thinking and brooding took him deeper and deeper into the question why what is, is, and why anything is at all. One Friday he stayed in the House of Study after prayer to go on thinking, for he was snared in his thoughts and tried to untangle them and could not. The holy Baal Shem Tov felt this from afar, got into his carriage and, by dint of his miraculous power which made the road leap to meet him, he reached the House of Study in Satanov in only an instant. There sat the learned man in his predicament. The Baal Shem said to him: “You are brooding on whether God is; I am a fool and believe.” The fact that there was a human being who knew of his secret, stirred the doubter’s heart and it opened to the Great Secret.
Martin Buber, Tales of the Hasidim