The wisdom of Dogen, excerpts from, Shobogenzo, Shin fukatoko:
The Mind that cannot be grasped is what all Buddhas are, for They personally rely upon It as supreme, fully perfected enlightenment. As the Diamond-Cutting Scripture says, “The mind of the past cannot be retained; the mind of the present cannot be held onto; the mind of the future cannot be grasped.” This expression points to the Buddha’s reliance upon the Mind that cannot be held onto, which is what all Buddhas do. It is what They have come to rely upon, saying that It is the unretainable mind of past, present, and future, and that It is the ungraspable Mind of all thoughts and things. If you do not learn from the Buddhas what They are relying upon, which is what makes this matter clear, you will not directly experience It...
There are people who, upon hearing the phrase ‘cannot be grasped’, have simply assumed that there is nothing to be attained in either case, for these people lack the living pathway of practice. Further, there are those who say that It cannot be grasped because it is said that we already possess It from the first. How does that hit the mark?
...What we call ‘Buddha Mind’ is synonymous with the three temporal worlds of past, present, and future. This Mind and the three temporal worlds are not separated from each other by so much as one single hair’s breadth. Even so, when we are discussing the two as things that are distinct and separate from each other, then they are farther apart than eighteen thousand breadths of hair.
Shobogenzo, Shin Fukatoku (written version), Hubert Nearman