। ॥ ४२ ॥
In plain words
Tat tvam asi—That you are—states the identity of the two words tat and tvam. Enough. Through this recognition of identity, sound reasoning makes right Jñāna—knowledge—firm.
Close to the Sanskrit
The identity of the two words tat and tvam is indeed 'tat tvam asi—That you are'; enough. In this way, through recognition of their identity, right Jñāna—knowledge—is made firm by reasoning.