3rd Sutra of Brahma Sutra

शास्त्र योनित्वात्
Sastra Yonitwat.
Sastras (Scriptures) are the means of correct Knowledge.
This sutra is a continuation, support and confirmation of the earlier Sutra that Brahman being the primal cause of the world is known only through the scriptures. Thus Brahman being the cause is not concluded by means of any analogy, estimation or inference, but, is supported by the Scriptures which are but the first hand realization of the Seers of truth. This also documents that the Srutis alone are the evidence of the existence of Brahman.
Now, the objection that has been raised was- "Brahman is an already existing entity like the pot etc and thus it can be known by other means of right knowledge also other than the scriptures."
Answer: There are enough scriptural texts that testifies that Brahman is beyond comprehension and is not the object of knowledge perceptible unlike the pot etc. Any object which has a form, name, taste, smell etc can be defined by other means apart from the scriptures. For example, in the first chapter of keno Upanishads it says thus-
अन्यदेव तद्विदितादथो अविदितादधि ।
इतिशुस्रुम पूर्वेषां ये नस्तद्व्यचचक्सिरे।।
(That Brahman is definitely unlike the one that is Known, and again, its beyond the unknown. Such are the utterances that we had heard of the ancient sages who explained the same to us).
Keno Upanishad keeps on repeating the same characteristic of Brahman being non perceivable to the different sense organs, the mind and the Prana( the five vital forces). Similar facts has been echoed in the Brhadaranyaka as- "He who controls the organ of speech from within" Br. III. vii 17. Or, in the Chandogya as, " One only; without a second", VI.ii.1 and "Where one doesn't see anything else", VII,xxiv.1. " Hence, the finite is that where one sees something else" Ch. VII.xxiv.1. Thus, an object which is completely non perceptible through direct experiences of the senses and mind, intellect etc, cannot be established by inference and analogy. Therefore it can be known only through the scriptures. Earlier we had discussed, "One who is ignorant of the Scriptures cannot have right knowledge of the Brahman." The other means of knowledge does have a scope but only when Brahman is established. They might be supplementary, not independent.