Bhagavad Gita, Chapter 10, Verse 15

Verily, You Yourself know Yourself by Yourself, O Purusha Supreme, O Source of beings, O Lord of beings, O Deva of Devas, O Ruler of the world.

Only this source is described as capable of fully knowing itself, real self-knowledge held by no one else.

This isn't a small technicality, it points at something genuinely beyond outside comprehension. What can't be measured from outside is still shown here as fully real and fully known from within.