What is the essential Buddhist teaching? Is it the Four Noble Truth of Suffering or the Doctrine of Emptiness? Is it the practice or the philosophy? To examine the question, we need to first interrogate the meaning of “essential” and “Buddhist teaching”. Does Buddhist teaching refer to the teaching of enlightenment or the teaching of practice, and are they to be taught? If there is such a thing as Buddhist teaching, is there an essence or anything essential to it?
Buddha's teaching is everywhere. Today it is raining. This is Buddha's teaching. People think their own way or their own religious understanding is Buddha's way, without knowing what they are hearing, or what they are doing, or where they are. Religion is not any particular teaching. Religion is everywhere. We have to understand our teaching in this way. We should forget all about some particular teaching; we should not ask which is good or bad. There should not be any particular teaching. Teaching is in each moment, in every existence. That is the true teaching. (Zen Mind, Beginner’s Mind, p.127)
It is true that in an ultimate sense there is nothing to teach or learn, nothing to know or do. Yet one is not entitled to say that unless one has actually realized down to one’s bones the truth of those statements. For truly to know that there is nothing to know is to know a great deal. Spiritual traditions are full of such glittering truth as: “You cannot enter a place you never left”; “The Absolute is a sphere whose center is everywhere and whose circumference is nowhere”; “Who sees not God everywhere sees him truly nowhere”; or “Refrain from seeking Buddhahood, since any search is condemned to fail.” (Zen Tradition and Transition, p. 67-68)
Both passages agreed that in an ultimate sense there is no particular Buddhist teaching, and yet at the same time, it is everywhere. The paradox is fundamentally related to the Buddha-nature, which is both empty and full, which implies utter transiency and instability but harmonious and steady as a background. “The Absolute is a sphere whose center is everywhere and whose circumference is nowhere.” The origin of Buddha-nature is omnipresent and thus the teaching is everywhere – it is in the rain, the dogs, and all human beings. In other words, the nature of all thing is Buddha-nature, including the nature of human beings. If we have Buddha nature, or that we are Buddha, then how is it to be taught?
Nonetheless, as the second passage asserted, though “it is true that in an ultimate sense there is nothing to teach or learn, nothing to know or do. Yet one is not entitled to say that unless one has actually realized down to one’s bones the truth of those statements. For truly to know that there is nothing to know is to know a great deal.” The enlightenment that there is nothing to know is not only any sudden attainment of its literal meaning or philosophy, which according to Buddha-nature is already within. Rather, it requires repetitive practice to foster and shall express itself in practice. Hence, I would refer Buddhist teaching to the teaching of practice – even a meditation on a philosophy is a practice. In this sense, everything is ready to become the agent for Buddhist teaching as it is constantly reminding us the its Buddha-nature and sheer emptiness. At the same time, we shall be ready to learn from everything with the beginner’s mind. That is the meaning of “There should not be any particular teaching. Teaching is in each moment, in every existence.”
Throughout history, different schools of Buddhism displayed a verity of practices in Buddhist teaching. From India to China to Japan, or even within Japan itself. At the beginning, Buddhist schools essentially served as political ideologies. During the Heian period, in attempts to amalgamate various Buddhist teaching, One Vehicle schools were formed. To Tendai school, the highest expression of the Buddha’s teachings is in Lotus Sutra, while to shingon, it in Yoga Tantra. During the Kamakura period, Zen Buddhism was introduced. To Zen, the essential teaching of practice is to dethinking thinking, the tools of which include zazen and koan. With the belief in mappo, emerges the Pure Land schools and Nichiren School. While the Pure Land school both see nembutsu as essential to Buddhist teaching, they still differ on the idea of what is essential to nembutsu.
Buddhist teaching is every-changing, which in accordance with the law of impermanence and change.
Things are “suffering,” that is, not finally satisfying, because they are impermanent: they do not last forever, or even for a moment, but are in a constant process of change; and partly because of this, there can be no Self, that is, no “abiding ego”, no “unchanging me,” and consequently no “mine”.
As demonstrated above, relatively, each school has some different essential element to the Buddha’s teaching. That is to say, there is something more important than the others in each school. However, ultimately, there is no essence to the Buddha’s teaching since like everything else, it has no self. Not even the Four Noble Truth of Suffering since in the karmic world secession shall never be achieved, nor the Doctrine of Emptiness because since itself is a construct. On the other hand, everything is essential to the Buddhist teaching, since everything is empty and thus includes everything, and everything is interdependent-originated. “When you forgot all your dualistic ideas, everything becomes your teacher, and everything can be the object of worship.” Therefore, to say one is essential is to say the others are essential. The essence of Buddha’s teaching both exists and doesn’t exist, and it neither exist nor doesn’t exist. It is the Middle Way the Buddha’s Way, to avoid the two extremes: existentialism and nihilism. (778 words)
 Zen Tradition and Transition, p. 68
 Zen Mind, Beginner’s Mind, p. 127
 The other One Vehicle Buddhist
 The Experience of Buddhism, p. 99-100