The Vedānta school built upon the teachings of the Upanishads and Brahma Sutras from the first millennium BCE and is the most developed and well-known of the Hindu schools. The epistemology of the Vedantins included, depending on the sub-school, five or six methods as proper and reliable means of gaining any form of knowledge: 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 have been further categorized in terms of conditionality, completeness, confidence and possibility of error, by each sub-school of Vedanta.