The main contribution of the Nyaya School is in the area of epistemology. Aksapada Gautama rejects the philosophy of scepticism and proposes that human beings have the capacity for acquiring valid knowledge. He preaches that the information on objects and processes that human beings get through their senses is correct and can be used for developing knowledge.
While refuting the philosophy of scepticism, the Gautama offers the argument that if all the means of gaining knowledge are invalid, then the demonstration of the invalidity of these means of knowledge cannot itself claim validity.
The first sutra (aphorism) in the Nyaya-Sutras states that salvation can only be achieved through knowledge, and therefore every human being must make an effort to gain knowledge. The treatise holds that there are four ways of acquiring knowledge:
1. Pratyaksha (Direct Perception)
2. Anumana (Inference)
3. Upamana (Comparison)
4. Shabda (Verbal Testimony)
Pratyaksha is the knowledge that people gain through direct contact between the senses and the object that is being perceived. The Nyaya School rejects the idea of consciousness having an effect on realty—it states that perception is not dependant on inference.
Anumana is the knowledge that precedes perception and is of three kinds: purvavat, or a priori; sesavat, or a posteriori; and samanyatodrsta, or common sense. Inference can lead to valid knowledge, and if inference leads to false knowledge then it is the person who is to be blamed and not the method itself.
Upamana is the knowledge that comes through an analysis of the similarities and differences in objects or processes.
Shabda is the knowledge that we get by listening to people who can be relied upon to speak the truth. The knowledge derived from verbal testimony is non-perceptual and non-inferential.
The Nyaya School preaches that human beings can attain salvation by attaining sixteen categories of knowledge:
1. Pramana: Knowledge achieved directly through the senses.
2. Prameya: Ontology
3. Samsaya: Doubt
4. Prayojana: Purpose
5. Drstanta: Paradigm cases that can establish a rule
6. Siddhanta: Established doctrine, tenet or conclusion
7. Avayava: Premise of a syllogism
8. Tarka: Indirect reasoning. It can also refer to a reductio ad absurdum.
9. Nirnaya: Beliefs gained through valid means.
10. Vada: An appropriate discussion
11. Jalpa: Arcane debates for defeating an opponent, rather than establishing the truth.
12. Vitanda: Cavil
13. Hetvabhasa: Persuasive but fallacious arguments
14. Chala: Quibble unfairly to contradict a statement
15. Jati: Use false analogy to post an unfair reply in an argument
16. Nigrahasthana: The point of defeat in a debate
Out of the sixteen categories of knowledge (except for Pramana and Prameya), fourteen are the derivatives of the art of disputation. Pramana and Prameya are by themselves broad enough to cover all the concepts, but the Nyaya School has proposed 16 categories to ensure clarity of the concepts and to distinguish its ideas from other schools of thought.
Three elements are involved whenever an individual is acquiring knowledge:
1. Jnatr: The knower, or the person who seeks the knowledge
2. Jneya: The metaphysical aspect of reality (an object or process) that is knowable
3. Jnana: What is finally known (the knowledge)
The Nyaya-Sutras grants great importance to the science of definitions and says that it is important to accurately define every concept to ensure that the philosophical arguments that are being proposed do not get mired in obscurity.
The relation of the words to their meanings is conventional and not natural. Particular words can have specific meanings, but the meanings can be affected by the context in which they are being spoken and the way in which they are being used in the sentence. To enable the hearer to understand the exact meaning of the verbal-proposition, the words must be clearly defined.