The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
34. Grief of him (i.e. of Gânasruti) (arose) on account of his hearing a disrespectful speech about himself; on account of the rushing on of that (grief) (Raikva called him Sûdra); for it (the grief) is pointed at (by Raikva).
(In the preceding adhikarana) the exclusiveness of the claim of men to knowledge has been refuted, and it has been declared that the gods, &c. also possess such a claim. The present adhikarana is entered on for the purpose of removing the doubt whether, as the exclusiveness of the
claim of twice-born men is capable of refutation, the Sûdras also possess such a claim.
The pûrvapakshin maintains that the Sûdras also have such a claim, because they may be in the position of desiring that knowledge, and because they are capable of it; and because there is no scriptural prohibition (excluding them from knowledge) analogous to the text, 'Therefore 1 the Sûdra is unfit for sacrificing' (Taitt. Samh. VII, 1, 1, 6). The reason, moreover, which disqualifies the Sûdras for sacrificial works, viz. their being without the sacred fires, does not invalidate their qualification for knowledge, as knowledge can be apprehended by those also who are without the fires. There is besides an inferential mark supporting the claim of the Sûdras; for in the so-called samvarga-knowledge he (Raikva) refers to Gânasruti Pautrâyana, who wishes to learn from him, by the name of Sûdra 'Fie, necklace and carnage be thine, O Sûdra, together with the cows' (Kh. Up. IV, 2, 3). Smriti moreover speaks of Vidûra and others who were born from Sûdra mothers as possessing eminent knowledge.--Hence the Sûdra has a claim to the knowledge of Brahman.
To this we reply that the Sûdras have no such claim, on account of their not studying the Veda. A person who has studied the Veda and understood its sense is indeed qualified for Vedic matters; but a Sûdra does not study the Veda, for such study demands as its antecedent the upanayana-ceremony, and that ceremony belongs to the three (higher) castes only. The mere circumstance of being in a condition of desire does not furnish a reason for qualification, if capability is absent. Mere temporal capability again does not constitute a reason for qualification, spiritual capability being required in spiritual matters. And spiritual capability is (in the case of the Sûdras) excluded by their being excluded from the study of the Veda.--The Vedic statement, moreover, that the Sûdra is unfit for sacrifices intimates, because
founded on reasoning, that he is unfit for knowledge also; for the argumentation is the same in both cases 1--With reference to the pûrvapakshin's opinion that the fact of the word 'Sûdra' being enounced in the samvarga-knowledge constitutes an inferential mark (of the Sûdra's qualification for knowledge), we remark that that inferential mark has no force, on account of the absence of arguments. For the statement of an inferential mark possesses the power of intimation only in consequence of arguments being adduced; but no such arguments are brought forward in the passage quoted. 2 Besides, the word 'Sûdra' which occurs in the samvarga-vidyâ would establish a claim on the part of the Sûdras to that one vidyâ only, not to all vidyâs. In reality, however, it is powerless, because occurring in an arthavâda, to establish the Sûdras' claim to anything.--The word 'Sûdra' can moreover be made to agree with the context in which it occurs in the following manner. When Gânasruti Pautrâyana heard himself spoken of with disrespect by the flamingo ('How can you speak of him, being what he is, as if he were like Raikva with the car?' IV, i, 3), grief (suk) arose in his mind, and to that grief the rishi Raikva alludes with the word Sûdra, in order to show thereby his knowledge of what is remote. This explanation must be accepted because a (real) born Sûdra is not qualified (for the samvarga-vidyâ. If it be asked how the grief (suk) which had arisen in Gânasruti's mind can be referred to by means of the word Sûdra, we reply: On account of the rushing on (âdravana) of the grief. For we may etymologise the word Sûdra by dividing it into its parts, either as 'he rushed into grief (Sukam abhidudrâva) or as 'grief rushed on
him,' or as 'he in his grief rushed to Raikva;' while on the other hand it is impossible to accept the word in its ordinary conventional sense. The circumstance (of the king actually being grieved) is moreover expressly touched upon in the legend 1.
224:1 The commentaries explain 'therefore' by 'on account of his being devoid of the three sacred fires.' This explanation does not, however, agree with the context of the Taitt. Samh.
225:1 The Sûdra not having acquired a knowledge of Vedic matters in the legitimate way, i. e. through the study of the Veda under the guidance of a guru, is unfit for sacrifices as well as for vidyâ.
225:2 The linga contained in the word 'Sûdra' has no proving power as it occurs in an arthavâda-passage which has no authority if not connected with a corresponding injunctive passage. In our case the linga in the arthavâda-passage is even directly contradicted by those injunctions which militate against the Sûdras' qualification for Vedic matters.
226:1 Hamsavâkyâd âtmano#nâdaram srutvâ gânasruteh sug utpannety etad eva katham gamyate yenâsau sûdrasabdena sûkyate tatrâha sprisyate keti. Ânanda Giri.