तस्मादेवं विदित्वैनं नानुशोचितुमर्हसि।।2.25।।
It is said that This is unmanifest; This is inconceivable; This is unchangeable. Therefore, having known This thus, you ougth not to grieve.
Continuing from the last verse, Bhagvan states that this atman is beyond the realm of senses. That this atman is not the object of any of the senses (like shabda is the object of sense of hearing or rasa is the object of sense of taste etc). And due to this reason, this atman cannot be objectified. And therefore, this is unmanifest, avyakt.
Further, it is also inconceivable, achintya. Only that stuff can be conceived which is the object of senses. Since this atman is not the object of any of the senses, it is inconceivable.
Further, this atman is changeless that is it is unlike the milk which changes into curd if mixed with it. Further, it is changeless due to the fact of partlessness too. Because anything which is partless, non-composite, has never been observed to be mutable. Thus, not being subject to change, the atman is said to be immutable, avikarya.
Now, therefore, having understood this atman as explained, you should not lament thinking, ‘I am the killer, these have been killed by me’.
Now, presuming without accepting that the atman is temporal and mutable, Bhagvan states (in next shloka).