Nan-in, a Japanese master during the Meiji era (1868-1912), received a university professor who came to inquire about Zen.
Nan-in served tea. He poured his visitor's cup full, and then kept on pouring.
The professor watched the overflow...
‘It is the single (nature of ) mind
which encompasses all of Samsara and Nirvana. Even though its inherent nature
has existed from the very beginning, you have not recognized it.
(Self-Liberation Through Seeing With Naked Awareness)