When knowing and seeing are without seeing, this is nirvāṇa

There is a passage in the Śūraṅgama Sūtra that says: 知见立知，即无明本；知见无见，斯即涅槃。 "To establish knowing upon knowing is the root of ignorance" means that all sentient beings possess a mind of seeing, hearing, awareness, and knowing—they can see, hear, feel, and know. But if we take these faculties of knowing as real, establishing the knowing that perceives the six sense objects as true, as a permanent and imperishable self that can go on to future lives, then such a view is a wrong view (mithyā-dṛṣṭi), the root of ignorance. With that view, one will transmigrate through the six realms of birth and death, unable to attain liberation.

"When knowing is without knowing, that is nirvāṇa." If, through contemplative practice (vipaśyanā-caryā), we come to understand that these faculties of seeing, hearing, awareness, and knowing are all deluded minds subject to arising, ceasing, and change—not permanent, not eternally existing, and not the true self—then we will no longer recognize the faculties of seeing, hearing, awareness, and knowing as the self. Once such a view is firmly established, the self-view (ātma-dṛṣṭi) is eliminated, and in the future one can eliminate self-grasping and realize nirvāṇa without remainder (anupadhiśeṣa-nirvāṇa). Furthermore, if one can, upon the faculties of seeing, hearing, awareness, and knowing, directly realize the intrinsically pure mind that itself has no nature of seeing, hearing, awareness, or knowing, then one has realized the originally pure and self-natured nirvāṇa (prakṛti-pariśuddha-nirvāṇa).

To explain this passage more precisely: "when knowing is without knowing" means that, alongside the various perceptions and views of the six consciousnesses, there is simultaneously a fundamental mind—the tathāgatagarbha—that has no perceptions or views whatsoever, and He has been operating behind the scenes all along. "That is nirvāṇa" means that He is the unborn and unceasing fundamental mind, the tathāgatagarbha, within the state of nirvāṇa—that is, the mind of nirvāṇa.