॥ १५ ॥
In plain words
Because when you said, 'I am a Brāhmaṇa's son of such a family,' you mistook the body—with its Jāti, lineage, and Saṃskāras—for Ātman, which is free from Jāti and lineage.
Close to the Sanskrit
Because, with a statement such as 'I am a Brāhmaṇa's son of such a lineage,' you identified the body—with its distinct Jāti, lineage, and Saṃskāras—as Ātman, which is free from Jāti and lineage.