The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut [1890] at sacred-texts.com

6. On account of the subject-matter.

The highest Self constitutes the subject-matter (of the entire chapter), as we see from the passage, 'Sir, what is that through which, when it is known, everything else becomes known?' (Mu. Up. I, 1, 3) in which the knowledge of everything is declared to be dependent on the knowledge of one thing. For all this (i.e. the entire world) becomes known if Brahman the Self of all is known, not if only the individual soul is known.--Another reason against the individual soul follows.