The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
6. And because scripture enjoins (works) for such (only as understand the purport of the Veda).
'He who has learnt (lit. "read") the Veda from a family of teachers, according to the sacred injunction, in the leisure time left from the duties to be performed for the Guru; who after having received his discharge has settled in his own house, studying his sacred texts in some sacred spot' (Kh. Up. VIII, 15); such passages also show that those who know the purport of the whole Veda are qualified for sacrificial action, and that hence knowledge does not independently bring about a result.--But the expression 'who has read' directly states only that the Veda is read, not that its purport is understood!--Not so, we reply. The reading of the Veda extends up to the comprehension of its purport, as thus the reading has a visible purpose 2.
289:2 According to the Mîmâmsâ principle that, wherever possible, actions enjoined must be understood to have a visible purpose (a supersensuous result being admitted only where no visible result can be made out).