अव्यक्तोऽयमचिन्त्योऽयमविकार्योऽयमुच्यते। तस्मादेवं विदित्वैनं नानुशोचितुमर्हसि ॥

avyakto ’yam achintyo ’yam avikāryo ’yam uchyate tasmādevaṁ viditvainaṁ nānuśhochitum arhasi

Unmanifest, unthinkable, immutable, he is called; therefore knowing him as such, thou shouldst not grieve.

In plain words: The soul cannot be seen, cannot be fully imagined, and never changes. Knowing this, Arjuna should not grieve.