Hello everyone:

Each Vedantic Acharya has to provide a meaningful interpretation to "Tat Tvam Asi" - "Thou art that" that is consistent with the overall philosophy expounded by him.

Advaita gets around this problem by taking recourse to what is technically called Lakshana or indirect meaning of words.

As an illustration consider "This is that Devadatta" with regard to a person seen in Madras in a small room in an unhealthy condition first, and afterwards in Bombay in a nice flat in a healthy condition. Here the "This" and "That" refer to an identical person under different adventitious conditions. Bereft of these adventitious conditions, it can only imply an identity of the person.

This is the Advaitic position that establishes the identical essence of the Jiva and the Atman/Brahman.

Ramanuja [propounder of Visishtadvaita] objects to this on two grounds. First, when a direct meaning of an expression is sufficient, it is illegitimate to seek a Lakshana meaning.

Next, it is circular and tautological and would be a waste of words to repeat "Thou art That" when the essence of Brahman and Jiva is identical. Ramanuja holds that like the expression "Blue Lotus", blueness and lotus nature inhere in a common substratum without losing their individualities/attributes.

PS:

Reference - Introduction section of "Ramanuja Gita Bhashya".

I am not taking sides here but just want to explain the different POVs and how different streams of philosophy derive their philosophy.