The Nyaya epistemology considers knowledge (jñāna) or cognition (buddhi) as apprehension (upalabdhi) or consciousness (anubhava). Knowledge may be valid or invalid. The Naiyayikas (the Nyaya scholars) accepted four valid means (pramaṇa) of obtaining valid knowledge (prama) - perception (pratyakṣa), inference (anumāna), comparison (upamāna) and verbal testimony (śabda). Invalid knowledge includes memory (smṛti), doubt (saṁśaya), error (viparyaya) and hypothetical reasoning (tarka).
Pratyakṣa (perception) occupies the foremost position in the Nyaya epistemology. Perception is defined by Akṣapāda Gautama in his Nyaya Sutra (I,i.4) as a 'non-erroneous cognition which is produced by the intercourse of sense-organs with the objects, which is not associated with a name and well-defined'.. Perception can be of two types, laukika (ordinary) and alaukika (extraordinary).
Ordinary (Laukika or Sadharana) perception is of six types - visual-by eyes, olfactory-by nose, auditory-by ears, tactile-by skin, gustatory-by tongue and mental-by mind.
Extraordinary (Alaukika or Asadharana) perception is of three types, viz., Samanyalakshana (perceiving generality from a particular object), Jñanalakshana (when one sense organ can also perceive qualities not attributable to it, as when seeing a chili, one knows that it would be bitter or hot), and Yogaja (when certain human beings, from the power of Yoga, can perceive past, present and future and have supernatural abilities, either complete or some).
Determinate and indeterminate perception
The Naiyayika maintains two modes or stages in perception. The first is called nirvikalpa (indeterminate), when one just perceives an object without being able to know its features, and the second savikalpa (determinate), when one is able to clearly know an object. All laukika and alaukika pratyakshas are savikalpa, but it is necessarily preceded by an earlier stage when it is indeterminate. Vātsāyana says that if an object is perceived with its name we have determinate perception but if it is perceived without a name, we have indeterminate perception. Jayanta Bhatta says that indeterminate perception apprehends substance, qualities and actions and universals as separate and indistinct something and also it does not have any association with name, while determinate perception aprrehends all these together with a name. There is yet another stage called Pratyabhijñā, when one is able to re-recognise something on the basis of memory.
Anumāna (inference) is one of the most important contributions of the Nyaya. It can be of two types: inference for oneself (Svarthanumana, where one does not need any formal procedure, and at the most the last three of their 5 steps), and inference for others (Parathanumana, which requires a systematic methodology of 5 steps). Inference can also be classified into 3 types: Purvavat (inferring an unperceived effect from a perceived cause), Sheshavat (inferring an unperceived cause from a perceived effect) and Samanyatodrishta (when inference is not based on causation but on uniformity of co-existence). A detailed anaysis of error is also given, explaining when anumana could be false.
Upamāna, which can be roughly translated as comparison is the knowledge of the relationship between a word and the object denoted by the word. It is produced by the knowledge of resemblance or similarity, given some pre-description of the new object beforehand.
Śabda or verbal testimony is defined as the statement of a trustworthy person (āptavākya), and consists in understanding its meaning. It can be of two types, Vaidika (Vedic), which are the words of the four sacred Vedas, and are described as the Word of God, having been composed by God, and Laukika, or words and writings of trustworthy human beings. Vaidika testimony is preferred as the infallible word of God, and Laukika testimony must by its nature be questioned and overruled by more trustworthy knowledge if such becomes available.
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