While the Tathagata, in his teaching, constantly makes use of conceptions and ideas about them, disciples should keep ill mind the unreality of all such conceptions and ideas. They should recall that the Tathagata, in making use of them in explaining the Dharma always uses them in the resemblance of a raft that is of use only to cross a river. As the raft is of no further use after the river is crossed, it should be discarded. So these arbitrary conceptions of things and about things should be wholly given up as one attains enlightenment. How much more should be given up conceptions of non-existent things (and everything is non-existent).

(The Maha Prajna Paramita)