According to Badarayana the Veda is eternal and the sastra is the great authority. No amount of reflection logical argumentation can lead to the discovery of metaphysical truth.
Perception and inference
Sutra admits two sources of knowledge: pratyaksham (perception) and anumanam (inference). The revealed sruti is self-evident and is called pratyaksham. By Sruti, Badarayana means the Upanishads, and by smriti he means the Bhagavadgita, the Mahabharata and the Code of Manu. In any theory of knowledge, inference is based on perception; so also smriti is based on Sruti.
Thinkable and Unthinkable
Badarayana makes a distinction between two spheres of existence: the thinkable and the unthinkable. The thinkable consists of the region of prakrti with the elements, the mind, intellect, and egoity, whereas the unthinkable is Brahman. With regard to the knowledge of the latter the only means is the sastras.
Any reasoning which is not in conformity with the Veda is useless for Badarayana. Reasoning proceeds from characteristic marks. But of Brahman we cannot say that it is characterized by this or that to the exclusion of other attributes. Reasoning, therefore is subordinate to intuitional knowledge, which can be obtained by devotion and meditation.