Most of the work [in Zen practice] takes place while sitting zazen, because in reality there's nothing anyone can give us. There's nothing that we lack. Each one of us is perfect and complete. That's why it is said that there are no Zen teachers and nothing to teach. But this truth must be realized by each one of us. Great faith, great doubt, and great determination are three essentials for that realization. It is a boundless faith in oneself and in the ability to realize oneself and make oneself free, and a deep and penetrating doubt which ask: Who am I? What is life? What is truth? What is God? What is reality? This great faith and great doubt are in dynamic tension with each other, and work to provide the real cutting edge of koan practice*. When great faith and great doubt are also accompanied by great determination (the determination of "Seven times knocked down, eight times up"), we have at our disposal the power necessary to break through our delusive way of thinking and realize the full potential of our lives.
*An apparently paradoxical statement of question used in Zen training to induce in the student an intense level of doubt, allowing them to cut through conventional and conditioned descriptions of reality and see directly into their true nature.