There are two ways of viewing this one reality. One is to see things as a whole, the other is to see things as independent. these two ways of seeing things are really important in understanding Mahayana Buddhist philosophy. In Mahayana Buddhist philosophy the two aspects of this one reality of our life is called "the two truths," one is absolute truth and another is conventional truth.
For example, in the Heart Sutra emptiness is considered to be absolute truth, there's no eyes, no ear, no hand, no nose, no tongue, no anything because this reality is just working as one; emptiness. Yet, from the other side, each has form, eyes are eyes, nose is nose, tongue is tongue; this person, Shohaku is Shohaku; I'm not you and you are not me. Even when you eat delicious food my stomach is not filled or vice versa. So we are completely different individual people. And yet, as a whole, we are living the same life; as living beings, we are interconnected completely together with all beings. This whole universe is just one thing, as five fingers are just one hand.
In Zen this reality is called sabetsu (distinction, inequality) and byodo (equality). Everything is different and independent on the one side, and everything is equal and interconnected on the other side. To see one reality from those two sides is the basic view of Mahayana Buddhism including Zen. As a form, everything is different. Everything has different form and yet those forms are empty; empty means no discrimination and separation. And yet this emptiness is form. We see one reality as an intersection or merging of equality and uniqueness.
In Chinese Zen literature, such as the Sandokai (merging of difference and unity) composed by Zen master Sekito Kisen, it says these two sides are called difference and unity. this difference and unity should merge. In Sandokai, Sekito expresses this side of oneness or unity as dark, and the other side is light. When it's bright outside we can see things and different forms, different colors, different names and different functions; when it's completely dark all beings are there but we cannot distinguish them. As a whole, it's one darkness. These are two aspects of one reality. ...
This is the basic way we see reality in Buddhism and Zen. It's important to understand this point to understand any Zen literature or Buddhist philosophy.
In the case of Dogen, however, to see one reality from two sides is not enough. We should express both sides in one action. For example, in the Heart Sutra two sides are expressed as "form is emptiness and emptiness is form." But, Dogen Zenji said in Shobogenzo Makahannya-haramitsu, "Form is form. Emptiness is emptiness." When we say form is emptiness and emptiness is form, there is still separation of form and emptiness. If form is really emptiness and emptiness is really form, we can only say form is form and emptiness is emptiness. When we say form, emptiness is already there. And when we say emptiness, form is already there. If we understand this basic point we can understand the first three sentences (paragraphs) of Genjokoan.
When we study and practice according to Dogen Zenji's teachings, it's important not only to understand with our intellect those two aspects; ...
In the Genjo-koan, Dogen Zenji expresses individuality as " a drop of water," and universality is expressed as "moonlight," and he said that even in a small drop of water, the moonlight is reflected. This is the reality of our life. We are individual and yet universal. The vast, boundless moonlight is reflected in us like a drop of water. The point of our practice, according to Dogen's teaching in Genjo-koan, is how we can keep awakening to that reality of individuality and universality together. Through our practice, we try to actualize one reality which has two sides. We go to extremes when we cling to our thinking. Thinking comes out of our experience, that is our karma. Depending upon our past experiences, we have tendency to think that this side should be important, or the other side should be more important. And we lose sight of the reality as a whole.
In our practice of zazen and also our practice in our daily lives, we awake to reality as a whole. We are free from either side and find the middle path. Both sides should be really there. This is the most vivid and healthy way of life.