The reliability of the source is important, and legitimate knowledge can only come from the "Sabda" of reliable sources. The schools of Hindu philosophy have debated if, how and when reliability of source can be objectively established. Gautama, in the Nyayasutras, offers a description for a reliable source. Some schools, such as Charvaka, state that this is never possible, and therefore "Sabda" in the Vedas or anyone else, can never be a proper "pramana". Other schools debate means to establish reliability.