四分律比丘含注戒本 2.5

2.5

謂見、聞、觸、知，及不見、知八種。見者，眼識能見。乃至意識能知。不見者，除眼識，餘五識是。如是類反也。

This refers to the eight kinds: seen, heard, sensed, known, and unseen, unheard, unsensed, unknown. "Seen" means the eye-consciousness is able to see. Up to "known" means the mind-consciousness is able to know. "Unseen" refers to the other five consciousnesses apart from the eye-consciousness. The remaining types are reversed accordingly.