Sokushin zebutsu, an important work by Zen master Dogen (1200-1253), is commonly translated as “The Mind itself is Buddha.” The word “Shin” in sokushin is literally “mind,” but since body and mind are not separate, it actually refers to the “body/mind” aggregate that constitutes an individual being. Sokushin zebutsu is analogous to sokushin jobutsu or “this very body is Buddha,” a concept, as well as the title of a work, from an earlier Japanese master, Kukai (774–835) founder of the Shingon school. Essentially, “This very body” means what it implies: this body, the one we inhabit in this lifetime, not some future body or life. This, in turn, relates to the Tendai position, as later put forth by Annen (841-899?) of sokushin jobutsu as “Buddhahood in this very lifetime.”
This line of thought breaks down the traditional notion that it take many lifetimes to attain Buddhahood. It brings awakening into the present, into the here and now. David Shaner, a professor at Furman University specializing in Japanese Buddhist philosophy, explains:
“The crucial point is that enlightenment is not some other worldly truth to be grasped via a mystical experience. Rather, it involves a keen awareness of that which is already present.” [1.]
A crucial point, indeed, and a subtle one, which can be easily misunderstood. Dogen’s view is that all things are Buddha-dharma, or inseparable from the teachings of the Buddha, and all things are Buddha-nature, or mind itself. “All things” naturally includes sentient beings, therefore all sentient beings posses this Buddha mind or nature. However, this does not mean that all sentient beings are automatically Buddhas. Dogen taught that while awakening is innate within our minds, actualizing this awakening lies in awakening to its existence, which is possible only through practice.
There must be some action taken, a first step, what we call bodhicitta, the thought of awakening:
“’This very mind is buddha’ is aspiration, practice, bodhi, and nirvana; if there is no aspiration, practice, bodhi, and nirvana, there is no ‘this very mind is buddha.’” 
Although the process of actualizing awakening in the mind begins with an initial thought, once the thought is produced, we need not continue to search for awakening. In the Genjo koan (“Actualizing the Fundamental Point”), Dogen explains that “the mind itself is Buddha” is realized without thinking. He says that
“[When] buddhas are genuinely buddhas there is no need to be conscious that they are buddhas. Yet they are realized buddhas, and they continue to realize buddha.” 
The mundane self (sans ego)that can be awakened and the path of awakening are ultimately the same. But to cease the search for awakening does not mean to abandon the practice that leads to the realization of the Buddha-mind. As I see it, Dogen repeatedly stressed the need to practice, which he saw as a process of letting the body/mind complex drop off. “The mind itself is Buddha” is really no-mind. This is close to what T’ien-t’ai master Chih-i meant when he said that once you realize the One Truth, you realize No-Truth. This is why the Heart Sutra says there is “no path, no wisdom and no attainment with nothing to attain.”
It means to transcend the limitations of mind, of thought, to go beyond the relative distinctions between subject and object. It means to see things in another way, to lose our attachment to conceptual thinking, to see the wholeness of things, to see true emptiness.
In the Sokushin Zebutsu, Dogen puts it this way:
Now you know clearly: what is called ‘mind’ is the great earth with its mountains and rivers; it is the sun, the moon, and the stars. Even so, when you take what is being expressed here one step further, something is lacking; when you draw back from what it is saying, something has gone too far. The mind that is the great earth with its mountains and rivers is simply the great earth with its mountains and rivers: there are no surging waves nor is there any wind-driven spindrift to disturb or upset it. The mind that is sun, moon, and stars is simply sun, moon, and stars: there is no fog nor is there any mist to obscure its clarity. The mind that is the coming and going of birth and death is simply the coming and going of birth and death: there is no ‘being deluded’ nor is there any ‘realizing enlightenment’ . . .
Since this is the way things are, “Your very mind is Buddha” means, pure and simply, that your very mind is Buddha; all Buddhas are, pure and simply, all Buddhas.
Thus, “Your very mind is Buddha” refers to all Buddhas, that is, to Those who have given rise to the intention to realize Buddhahood by practicing and training until They awaken to Their enlightenment and realize nirvana.” 
And, of course, when we realize nirvana, we find that essentially it is nothing more than this very world of suffering. Nirvana is right here, right now, a potentiality within the present moment, within this very body, this life, this mind itself.
1. David Shaner, The Bodymind Experience in Japanese Buddhism: A Phenomenological Perspective of Dogen and Kukai (Albany: State University of New York Press), 1985, 75.
2. A Study of Dogen:His Philosophy and Religion, Masao Abe and Steven Heine,SUNY Press, 1992, 158.
3. Ibid., 123.
4. SHOBOGENZO The Treasure House of the Eye of the True Teaching by Eihei Dogen, Translated by Reverend Master Hubert Nearman, Order of Buddhist Contemplatives, Shasta Abbey Press, Mount Shasta, California 2007.