Ancient and medieval Hindu texts identify six pramāṇas as correct means of accurate knowledge and truths: pratyakṣa (perception), anumāṇa (inference), upamāṇa (comparison and analogy), arthāpatti (postulation, derivation from circumstances), anupalabdi (non-perception, negative/cognitive proof) and śabda (word, testimony of past or present reliable experts) Each of these are further categorized in terms of conditionality, completeness, confidence and possibility of error, by each school . Which school believes only postulation is a valid form of knowledge