Do you know why it is called tathagata ? The meaning of tathagata is that one neither comes nor goes anywhere. Why is this so? The reason is that the tathagata comes and goes very, very quickly; but, it seems that he never comes and goes. Therefore, he is able to go many places simultaneously, and also to come back at the same time. Although there is no way to see where he goes, he can be seen in every corner of the universe at any time. So, he is called a tathagata.
When we were young, we played a game where we put a small ball into a bamboo hat, and kept the ball rolling around in the hat. When it rolled around slowly, we could see the ball's movement; but if the ball rolled very quickly, it seemed as if there were no ball. This is a matter of the speed of the ball. When a person has already become a Buddha, his vibration is different; he comes and goes very quickly. But, it seems that he neither comes nor goes. He is faster than light, faster than anything, even faster than our thoughts.
We all think that the speed of light is the fastest thing in this world. If there is thunder somewhere, we can see the flash within a second, because the speed of light is very fast. For example, a bicycle is very slow, and an airplane is much faster. Everyone knows that the speed of light is very fast, but Buddhas and Bodhisattvas are faster than light. However, they never need to move physically.
Sometimes, you may watch Kung-fu movies shown on TV or Kung-fu shows where they often have intentionally slow actions, so we are able to see the techniques used in the graceful Kung-fu movements. But, if the actions are too fast, we are unable to see them clearly. Some people's Kung-fu actions are very slow, and other's are very fast. For example, Bruce Lee's movements are very quick. Usually, before we even see him begin his attack, he has finished it. Some other people can perform a sword dance or do something else as quick as that.
A Buddha is even faster, so that ordinary people cannot see their actions when they come and go. Thus, we call them tathagatas. Therefore it is said, Zen is within walking, living, sitting, and sleeping.
Those people who are just starting to learn meditation, or those who are just a little higher than beginners, know when they get into and stay in samadhi and when they come out of it. Even other people can notice this, too. But, with a very high level person who has already achieved the tathagata state, we can never tell when he is in or out of samadhi; because he goes in and comes out at every moment. Even if he is in samadhi at a certain moment, we cannot tell, because he is really too fast. So we have no way to determine whether he has gone into samadhi at any given moment and then has come out of it. In and out, we cannot determine. He goes into and out of samadhi at the same time because his level is so high and his speed is incredible.
We say, Zen is within walking, living, sitting, and sleeping. But, if we have not achieved the level where Zen is within walking, living, sitting, and sleeping, there is no way for us to have countless transformation bodies, and no way for us to be omnipresent. With a tathagata, although he is everywhere, he just stays in one place. He is supposed to be at the level where Zen is within walking, living, sitting, and sleeping, where he is both in and out of samadhi at every moment.
Similar to the game of rolling the little ball around in a bamboo hat when we were young, the ball rolled around too quickly so that we could not see it. Whether it was here or there, we could not tell. It rolled around very, very quickly; and just a while later, it seemed that the ball had disappeared and was not in the bamboo hat because the speed of the rolling was too quick.
Furthermore, the tathagata state is not the same as astral projection. Some people know how to get into a kind of samadhi very well. They can even remain in this state for several months or years. But, this is nothing to be surprised about; because they are still at the level where one can distinguish that they are sometimes in and sometimes out of samadhi.
At this level, there is still a door for getting into and out of samadhi. However, Zen has no door . Thus, its level is really high. That is why you often see an inscription no two doors when you go to Buddhist temples. This means the state where there is no inside and outside, no good and evil, even no world and no Buddha. But, you have to achieve this level of no two doors .
You should achieve the tathagata state so that you can go anywhere at any time, but also remain in one place. Everyone writes, No two doors, or Zen is within walking, living, sitting, and sleeping. But, no one understands the meaning of these expressions. This is
not something that you can use the brain to understand. Instead, you need to achieve this state. However, after you achieve this level, you will still be a normal person. You will not think that you are a Buddha.