The Gateless Gate/Three Gates Of Tosotsu
Tosotsu built three barriers and made the monks pass through them. The first barrier is studying Zen. In studying Zen the aim is to see one's own true nature. Now where is your true nature?

Secondly, when one realizes his own true nature he will be free from birth and death. Now when you shut the light from your eyes and become a corpse, how can you free yourself?

Thirdly, if you free yourself from birth and death, you should know where you are. Now your body separates into the four elements. Where are you?

 Mumon’s comment: Whoever can pass these three barriers will be a master wherever he stands. Whatever happens about him he will turn into Zen.
Otherwise he will be living on poor food and not even enough of that to satisfy himself.
 An instant realization sees endless time.
 Endless time is as one moment.
 When one comprehends the endless moment
 He realizes the person who is seeing it.