त्वमादिदेवः पुरुषः पुराण स्त्वमस्य विश्वस्य परं निधानम्।वेत्तासि वेद्यं च परं च धाम त्वया ततं विश्वमनन्तरूप॥
tvam ādi-devaḥ puruṣaḥ purāṇastvam asya viśvasya paraṃ nidhānamvettāsi vedyaṃ ca paraṃ ca dhāmatvayā tataṃ viśvam ananta-rūpa
Word by word
The verse says
You are the first god, the ancient person. You are the final resting place of all this. You are the knower and the thing to be known, and the highest home. The whole universe is spread out by you, you of endless form.
In plain words
Vettāsi vedyaṃ ca — you are the knower and the known.
Both ends of knowing, claimed at once.
Which means that whatever Arjuna understands here, the understanding is not happening from outside the thing understood.
Where you've felt this · in creative work
You try to describe your own handwriting and find there is nothing to say. You can describe somebody else's — cramped, loopy, forward-leaning. Your own is just what writing looks like. The thing you would be assessing it with is the thing being assessed.
Going deeper
Being both the knower and the known collapses the distance that ordinary understanding requires.
Knowing anything normally involves two positions: something doing the knowing and something being known. Arjuna's line puts both at the same address. That is not a riddle for its own sake — it is the natural consequence of everything the vision has shown him, in which there is no outside.
It also explains why he keeps producing paradoxes rather than descriptions. He is not confused. He is reporting accurately on a situation in which the usual arrangement does not hold.
And it answers, in passing, the question of why this could never have been reached by investigation. Investigation needs a gap between the investigator and the subject. Chapter 7 said nobody knows him and this is the same claim from the other side: not because the information is hidden, but because there is nowhere to stand.