The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
5. On account of the declaration of difference.
The passage 'Know him alone as the Self' moreover implies a declaration of difference, viz. of the difference of the object of knowledge and the knower. Here the individual soul as being that which is desirous of release is the knower, and consequently Brahman, which is denoted by the word 'self' and represented as the object of knowledge, is understood to be the abode of heaven, earth, and so on.--For the following reason also the individual soul cannot be accepted as the abode of heaven, earth, &c.