維摩經玄疏 1.157

1.157

五者、別教不可說，即據佛性，大涅槃不可說也。如為盲人，說白色相，智人雖引諸相開曉，而彼生盲，終不見白相。此譬佛性不可說也。

Fifth is the ineffable of the distinctive teaching: this is based on buddha-nature, where great nirvāṇa is ineffable. It is like explaining the appearance of the color white to a blind person: though a wise person adduces various signs to instruct and enlighten, that person born blind ultimately does not see the white appearance. This is an analogy for buddha-nature being ineffable.