While the number of pramanas varies widely from system to system, many ancient and medieval Indian texts identify six "pramanas" as correct means of accurate knowledge and to truths: Three central pramanas which are almost universally accepted, which are perception (Sanskrit "pratyakṣa)," inference ("anumāna"), and "word", meaning the testimony of past or present reliable experts ("Śabda"); and more contentious ones, which are comparison and analogy ("upamāna"), postulation, derivation from circumstances ("arthāpatti"), and non-perception, negative/cognitive proof ("anupalabdhi"). Each of these are further categorized in terms of conditionality, completeness, confidence and possibility of error, by each school of Indian philosophies.