The Mīmāṃsā school has several subschools defined by epistemology. The Prābhākara subschool of Mīmāṃsā considered five epistemically reliable means to gaining knowledge: pratyakṣa (perception), anumāṇa (inference), upamāṇa (comparison and analogy), arthāpatti (postulation, derivation from circumstances), and śabda (word, testimony of past or present reliable experts). The Kumārila Bhaṭṭa sub-school of Mīmāṃsā added sixth to its canon of reliable epistemology - anupalabdi (non-perception, negative/cognitive proof).