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 . The various schools vary on how many of these six are valid paths of knowledge. For example, the Cārvāka nāstika philosophy holds that only one (perception) is an epistemically reliable means of knowledge, the Samkhya school holds three are (perception, inference and testimony), while the Mīmāṃsā and Advaita schools hold all six are epistemically useful and reliable means to knowledge.