"Śabda" (शब्द) means relying on word, testimony of past or present reliable experts, specifically the "shruti", Vedas. Hiriyanna explains "Sabda-pramana" as a concept which means reliable expert testimony. The schools of Hinduism which consider it epistemically valid suggest that a human being needs to know numerous facts, and with the limited time and energy available, he can learn only a fraction of those facts and truths directly. He must rely on others, his parent, family, friends, teachers, ancestors and kindred members of society to rapidly acquire and share knowledge and thereby enrich each other's lives. This means of gaining proper knowledge is either spoken or written, but through "Sabda" (words). The reliability of the source is important, and legitimate knowledge can only come from the "Sabda" of reliable sources. The disagreement between the schools of Hinduism has been on how to establish reliability. Some schools, such as Carvaka, state that this is never possible, and therefore "Sabda" is not a proper pramana. Other schools debate means to establish reliability.