गोविन्द भाष्य (श्रील बलदेव विद्याभूषण - अचिन्त्यभेदाभेद वेदान्त ब्रह्मसूत्र भाष्य सटीक)

saṁskāra-parāmarśāt tad-abhāvābhilāpāc ca saṁskāra-of the purificatory rituals; parāmarśāt-because of the reference; tad-of them; abhāva-of the non-existence; abhilāpāt-because of the explanation; ca-also. This is also so because the scriptures state both the necessity of undergoing the saṁskāras (rituals of purification) and the exclusion of the śūdras from these rituals. Purport by Śrīla Baladeva Vidyābhūṣaṇa In the śruti-śāstra is the passage aṣṭa-varṣaṁ brāhmaṇaṁ upanīyata tam adhyāpayed ekādeśe kṣatriyaṁ dvādaśe vaiśyam (One should perform the saṁskāra and teach a brāhmaṇa boy when he is eight years old a kṣatriya boy when he is eleven years old, and a vaiśya boy when he is twelve years old). This shows that brāhmaṇas are eligible to study the Vedas because they are also eligible for the saṁskāras. The scriptures also say nāgnir na yajño na kriyā na saṁskāro na vratāni śūdrasya (A śudra is not allowed to light the sacred fire, perform a fire-sacrifice, perform religious rituals, undergo the saṁskāras, or follow vows of penance). In this way it is established that because a śūdra is not allowed to undergo the saṁskāras he is also not allowed to study the Vedas. Now he confirms the view that the śūdras are not eligible for the saṁskāras. Sūtra 37 tad-abhāva-nirdhāraṇe ca pravṛtteḥ tat-of that; abhāva-of the non-existence; nirdhāraṇe-in ascertaining; ca-also; pravṛtteḥ-because of endeavor. (This is so) also because care is taken to determine that (a student) is not (a śūdra). Purport by Śrīla Baladeva Vidyābhūṣaṇa