The Zen monk Bassui wrote a letter to one of his disciples who was about to die, and in it he said “Your end which is endless is as a snowflake dissolving in the pure air.” The snowflake, which was once very much a discernible subsystem of the universe, now dissolves into the larger subsystem which once held it. Though it is no longer present as a distinct subsystem, its essence is somehow still present, and will remain so. It floats in Tumbolia, along with hiccups that are not being hiccuped and characters in stories that are not being read… That is how I understand Bassui’s message.
Zen recognizes its own limitations, just as mathematicians have learned to recognize the limitations of the axiomatic method as a method for attaining truth. This does not mean that Zen has an answer to what lives beyond Zen any more than mathematicians have a clear of the forms of valid reasoning which lie outside of formalization. One of the clearest Zen statements about the borderlines of Zen is given in the following strange kōan, very much in the spirit of Nansen:
Tōzan said to his monks, “You monks should know there is an even higher understanding in Buddhism.” A Monk stepped forward and asked, “What is the higher Buddhism?” Tōzan answered, “It is not Buddha.”
There is always further to go; enlightenment is not the end-all of Zen. And there is no recipe which tells how to transcend Zen; the only thing one can rely on for sure is that Buddha is not the way. Zen is a system and cannot be its own metasystem; there is always something outside of Zen, which cannot be fully understood or described within Zen.
― Douglas R. Hofstadter, GEB p.255