A fish swims in the ocean, and no matter how far it swims there is no end to the water. A bird flies in the sky, and no matter how far it flies there is no end to the air. However, the fish and the bird have never left their elements. When their activity is large their field is large. When their need is small their field is small. Thus, each of them totally covers its full range, and each of them totally experiences its realm. If the bird leaves the air it will die at once. If the fish leaves the water it will die at once.
Know that water is life and air is life. The bird is life and the fish is life. Life must be the bird and life must be the fish. You can go further. There is practice-enlightenment which encompasses limited and unlimited life.
There is a story of an old fish swimming past two young ones who says “The water’s nice today isn’t it?”. One of the younger fish turns to the other and says “What’s water?”.
We may go through much of our lives with our attention so preoccupied by our various goals that we may not notice, but one day we may suddenly wake up to the most fundamental of things - the reality of this moment-to-moment existence. We can call it mind, or being, or life, or Tao, or Buddha, or God, or emptiness, or the present moment, or reality - it doesn't matter much - none of these concepts really captures it.
It is easy to fall into treating this experienced reality as if it was a distinct thing and becoming attached to various metaphysical beliefs about it. At the time of Buddha, Bramins claimed the whole experience of our lives was experienced by an absolute, unchanging atman or metaphysical soul which was at the same time identical with Brahma, or God. Buddha denied this of course and so does Dogen.
This 'essence of being', Dogen calls 'life' - the fish is life and the water is life, the bird is life and the air is life. The fish is not separate from the water - wherever the fish goes, water is there - wherever the bird goes, air is there. The bird and fish are in harmony with the universe. This is their practice-enlightenment.
Beings are totally surrounded by and at one with this emptiness - breathing it in and breathing it our moment after moment, totally dependent on it and inseparable from it. Buddha is like this - it is always present and yet we may see it clearly or be blind to it.