"Anupalabdi" (अनुपलब्धि) means non-perception, negative/cognitive proof. "Anupalabdhi pramana" suggests that knowing a negative, such as "there is no jug in this room" is a form of valid knowledge. If something can be observed or inferred or proven as non-existent or impossible, then one knows more than what one did without such means. In the two schools of Hinduism that consider "Anupalabdhi" as epistemically valuable, a valid conclusion is either "sadrupa" (positive) or "asadrupa" (negative) relation - both correct and valuable. Like other "pramana", Indian scholars refined "Anupalabdi" to four types: non-perception of the cause, non-perception of the effect, non-perception of object, and non-perception of contradiction. Only two schools of Hinduism accepted and developed the concept "non-perception" as a "pramana". The schools that endorsed "Anupalabdi" affirmed that it as valid and useful when the other five "pramanas" fail in one's pursuit of knowledge and truth.