Later Advaita Vedanta grouped this sentence with three others from different Upanishads into a set it calls the four mahavakyas, great sayings, treated as the tradition's most concentrated statements of its central claim and used, according to the Encyclopedia of Indian Philosophies volume on Advaita, as formal objects of meditation given to a renunciate at initiation into sannyasa. Sankara's commentary reads tat tvam asi as a strict identity statement, the individual self, thou, and the absolute, that, naming the same single reality under two descriptions, and the appearance of a separate individual self is a case of mistaken cognition, ignorance, to be dissolved by the right kind of knowledge rather than corrected by any action.
John Carman's study of Ramanuja's rival theology shows the qualified non-dualist school reading the identical sentence to a different conclusion. Ramanuja denies that thou and that can mean bare identity, since a self and the absolute the self depends on are not interchangeable in the way pure identity requires; he reads the sentence instead as asserting an inseparable relation, the self as an eternal mode or attribute of Brahman, distinct enough to remain a real individual, dependent enough that its own being cannot be described without reference to what it belongs to. The two readings do not disagree about which words appear in the sentence, only about what identity itself can mean, which is why the same four words became the ground on which centuries of Vedanta philosophy argued past each other.