buddhi

Human evolution develops the mental principle, Manas; Super-human evolution develops the spiritual body, Buddhi.

Now, in the first place, nothing in the context warrants the explanation of the first 'tat' by buddhi.

For this can apply to the intelligent individual soul only, not to the non-intelligent buddhi.

Buddhi, though meaning intellect, is used rather in the sense of ascertaining or perception.

Hence we conclude that the buddhi and the individual soul are spoken of.