The Nyaya school was founded by Gautama with his Nyaya-sutra, but evolved greatly after that. The Nyaya school’s chief concern was epistemology. What is correct thinking, and how can we come to know reality? Only when we know the answers to these questions can we achieve liberation.
How do we test if what we “know” corresponds to reality?
The Nyaya, Vaisheshika, Jaina, and Buddhist schools said that true knowledge led to success in practical activity, while false knowledge led to failure and disappointment.
According to Nyaya, there were four valid sources of knowledge — perception, inference, comparison, and testimony
Also four sources of invalid knowledge: memory, doubt (when there is no definite mark that would distinguish the truth from illusion), error (false certainty), and hypothetical argument (“If there was no fire, there wouldn’t be smoke, but there is smoke, so there must be fire”).
Perception was an unerring belief produced by contact between an object and the senses. When I have clear and certain sight of a table, this is perception, and valid knowledge. If I see a shape in the distance that could be a man or a post but I cannot tell which, this is doubt, and not valid knowledge. If I am certain that I see a snake in a coil of rope, but there is no snake, this is error.
Inference, then, was knowledge that followed from some other knowledge: “Gautama is mortal, because Gautama is a man, and all men are mortal.” Each case of inference has at least three propositions and, as in Aristotle’s logic, a minor, major, and middle term.
Here, Gautama’s manhood is the minor term (paksa), for it is the subject we are considering.
Gautama’s mortality is the major term (sadhya), for it is that which we want to establish by means of inference.
The mortality of all men is the middle term (linga), for it is what grounds our inference from Gautama’s manhood to his mortality.
Also as in Aristotle’s logic, each proposition in Nyaya inference was categorical.
However, most Nyaya adherents preferred to state these categorical syllogisms in five propositions, like so:
Gautama is mortal;
Because he is a man;
All men are mortal, for example Siddhartha, Brhaspati, and Kapila;
Gautama is also a man;
Therefore, Gautama is mortal.
First, the conclusion is asserted. Second, the reason for the conclusion. Third, the universal proposition is given, which connects the conclusion with the reason given, supported by known instances. Fourth, we apply the universal proposition to the present case. Fifth, the conclusion is restated.
After perception and inference, the third form of valid knowledge for Nyaya was comparison (upamana). This was the knowledge of what a word denotes. A young girl who does not know what a jackdaw is may be told it is like a crow, but bigger and of grey and black color. If she later encounters a bird like a crow but bigger and of grey and black color, she may think, “This must be a jackdaw,” and she would know this by “comparison.”
The final source of valid knowledge for Nyaya was testimony, which consisted of understanding the meaning of what is said by a trustworthy person.
Using these sources of knowledge, what did Nyaya say about the physical world? Like some ancient Greeks, they said the world is made of eternal atoms of earth, water, fire, and air. The self could not be merely the body, for the body by itself is unconscious. It could not be the senses, for these cannot account for imagination. It could not be a stream of thoughts, for this would not explain memory. But neither was the self Brahman, for each self has its own thoughts, feelings, and desires that are not shared by other selves.
When one achieved true knowledge of the world and the self, one could achieve liberation from pain. When one realized that the self is separate from the body, one could be liberated from the body, and therefore from all the pain and pleasure that comes to the self through it.
What of God? God arranged the world from the eternal atoms, space, time, ether, and souls. God was all-powerful, all-beautiful, all-knowing, perfectly moral, and perfectly free from attachment.