Buddha nature is inside, outside, everywhere. The physical body is not something which has a little Buddha nature inside of it. Your whole body is Buddha nature, and yet, to have this particular form, at this particular time, in this particular place, that is a great matter.
The surprising thing is, we don’t know how wonderful a world we are. The best way is to go out and ask everything which comes to you, “Tell me your thing.” They will begin to tell you everything. And finally you will begin to feel, as they are talking, that it is their thing, but it is your thing. This is how the sutra is. At first you feel it is something totally different from you but the more you listen, you begin to feel, “This is me. The whole thing is me talking about myself.” In this sense Dogen said, “To study and to practice Buddha’s way is to practice yourself, to learn yourself. And to carry yourself among things and think that you understand things is illusion, whatever you understand, all are illusions. To let all things come to you and be awakened by them is enlightenment.” So I have a difficult problem: How do I let things completely happen in me, exist in me? Constant practice is the only way. When I keep my practice pure and honest, at that time, I begin to hear a new sound - everything is talking to me!
There are no different kinds of Buddha nature. Buddha nature is just one. But when Buddha nature takes form, all things appear. All existence appears. And without it there is no such Buddha nature which stays as Buddha nature. So we train ourselves to see the sameness, or oneness, of everything, without differentiation, without each individual existence. Maybe a good example is music. There are strings. If no one touches them, you may imagine there is rich music going on, or complete music is going on, and you are just picking out the sounds of music when you touch the strings. But what is happening is that, by themselves, the strings don’t have any music, the instrument doesn’t have any music. Only when the musician touches the strings does music appear. In the same way, complete communication among everything is happening when Buddha nature takes form. This is why Buddhism is also called a religion.