Recently, I wrote an article highlighting a snippet of the Rig Veda where it observes that the enlightened act by means of that which remains to be united. That part that is not “enlightened”.
A similar passage comes up in the Isha Upanishad.
Into blinding darkness enter those who worship ignorance.
Into as if still greater darkness enter those who delight in knowledge.
“It [Atman} is other than knowledge,” they say. “It is other than ignorance,” they say.
Thus we have heard from the wise who have explained it to us.
Knowledge and ignorance – he who knows both together,
crosses beyond mortality through ignorance and attains immortality through knowledge.
Into blinding darkness enter those who worship the unmanifest.
Into as if still greater darkness enter those who delight in the manifest.
“It is other than existence,” they say. “It is other than nonexistence,” they say.
Thus have we heard from the wise who have explained it to us.
Manifest and unmanifest – he who knows both together,
crosses beyond death through the unmanifest and attains immortality through the manifest.
— Isha Upanishad 9-14 (from the Katz-Egenes translation)
Knowledge and ignorance, manifest and unmanifest, Shakti and Shiva; it takes both for immortality. It takes both to fully realize the Self which is immortal.
While the subject is Atman, it is used somewhat interchangeably with Brahman here. As existence is a quality of Atman, the second part is about Brahman.
It’s also worth noting that “he who knows both together” is not a reference to knowing the duality of existence and non-existence but rather the wholeness where they are united.