The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
36. But also (persons standing) between (are qualified for knowledge); for that is seen (in scripture).
A doubt arises whether persons in want who do not possess means, &c., and therefore are not able to enter one or the other of the âsramas., standing between as it were, are qualified for knowledge or not.--They are not qualified, the pûrvapakshin maintains. For we have ascertained that the works incumbent on the âsramas are the cause of knowledge, and those persons have no opportunity to perform those works.--To this the Sûtrakâra replies. 'But also between.' Even a person who because he does not belong to an âsrama stands between, as it were, is qualified for knowledge. 'For that is seen.' For we meet with scriptural passages declaring that persons of that class--such as Raikva and the daughter of Vakaknu--possessed the knowledge of Brahman (Kh. Up. IV, 1; Bri. Up. III, 6, 8).