There are three ways to know something — pratyaksha (perception), anumana (guessing) and sabda (Sastraic authority), elaborated M.A. Venkatakrishnan in a discourse.
Under pratyaksha would come seeing, hearing etc. In other words what we experience through the indriyas would all come under pratyaksha. Some things are guessed. For example, someone who has seen a fire being lit knows that fire gives out smoke. So if he sees smoke emanating from the other side of a hill, but can’t actually see a fire, he guesses that there must be a fire there. Under sabda would come the Sastras, especially Vedas, Upanishads, Itihasas, Puranas, Agamas, Divya Prabandha verses of the Azhvars, and commentaries on sacred texts. All these textual authorities constitute sabda.
Sage Vyasa in his Brahma Sutra said that the Supreme One can be known only through sabda, not through pratyaksha or anumana. Pratyaksha cannot reveal Him to us. Even though the Lord showed His Viswarupa form to Arjuna, the latter could see it only because the Lord gave him divine sight. With normal sight Arjuna could never have seen His Viswarupa form. Anumana can be misleading. For example, if we look at huge buildings, we will tell ourselves that for something so huge to have been constructed, several people must have put in their efforts. But can we say the same of the Universe, and come to the conclusion that many are responsible for it? To say so would be to think of Him in simplistic terms and to thereby lower His stature.
That is why Vyasa said that we can know Him only through the Sastras. The Vedas are apaurusheyam — not man made. And it is the divine Vedas and the subsequent texts inspired by them that show us the Lord.
