The fuel that drives the Zen adept on to realize the subtle essence of Mind is the feeling of great doubt (大疑情). But there is more to it than these simple words can express. Yes, doubt (疑) or an honest search for the right answer is integral. Adding to this, in my case, it was somewhat like realizing you’ve been poisoned, and have to find the proper antidote. What I am suggesting, there is a great sense of urgency that takes possession of a person. No more sitting in the rocking chair—it’s do or die. This kind of urgency is hard to imagine.
Those who have a cursory interest in Zen can easily intellectualize the feeling of the great doubt without entering into it; keeping it at arm’s length. But it is not the same experience. A cursory interest in Zen never goes any deeper and further than, for example, zazen practice. It hasn’t yet dawned on such people that Zen is aimed at acquiring kensho which is seeing one’s true nature, directly. Short of this, one is going no place. Zen is not a special lifestyle.
Having a Buddha statue on your home shrine, some meditation cushions, some candles and incense, is not the feeling of great doubt. One may also be assuming (and wrongly) that Zen means “fundamentally there isn't the slightest bit of anything, but when we have tea we drink tea, and when we have rice we eat rice." This is the Zen of awakened teachers—not the Zen of students seeking awakening. Before this Zen master realized his or her true nature, they didn’t just drink tea or eat rice. Their mind was focused on passing through the Mu barrier or some other barrier of Zen which guards the gateless gate so that nothing conditioned may pass through.
Without being energized by the feeling of great doubt; putting it all on the line to realize, in my example, pure Mind, one will get nowhere in Zen. This, I need to add, it not the same as following a teacher. Great doubt, pertains to our present inability to comprehend the absolute. Following a teacher around like a little calf ; calling this an example of the feeling of great doubt is being deluded. Great doubt begins only in us. It sets the pace, so to speak. It determines who we seek out. It might even create the conditions for our awakening.