There are three kinds of evidence: pratyakṣa, anumāna and śabda. Pratyakṣa means "direct evidence." Direct evidence is not very good because our senses are not perfect. We are seeing the sun daily, and it appears to us just like a small disc, but it is actually far, far larger than many planets. Of what value is this seeing? Therefore we have to read books; then we can understand about the sun. So direct experience is not perfect. Then there is anumāna, inductive knowledge: "It may be like this"—hypothesis. For instance, Darwin's theory says it may be like this, it may be like that. But that is not science. That is a suggestion, and it is also not perfect. But if you receive the knowledge from the authoritative sources, that is perfect. If you receive a program guide from the radio station authorities, you accept it. You don't deny it; you don't have to make an experiment, because it is received from the authoritative sources.
Vedic knowledge is called śabda-pramāṇa. Another name is śruti. Śruti means that this knowledge has to be received simply by aural reception. The Vedas instruct that in order to understand transcendental knowledge, we have to hear from the authority. Transcendental knowledge is knowledge from beyond this universe. Within this universe is material knowledge, and beyond this universe is transcendental knowledge. We cannot even go to the end of the universe, so how can we go to the spiritual world? Thus to acquire full knowledge is impossible.
There is a spiritual sky. There is another nature, which is beyond manifestation and nonmanifestation. But how will you know that there is a sky where the planets and inhabitants are eternal? All this knowledge is there, but how will you make experiments? It is not possible. Therefore you have to take the assistance of the Vedas. This is called Vedic knowledge. In our Kṛṣṇa consciousness movement we are accepting knowledge from the highest authority, Kṛṣṇa. Kṛṣṇa is accepted as the highest authority by all classes of men.