Nyāya is one of the ancient philosophies founded by sage Gautama (not Buddha). This philosophy lays more emphasis on inquiring within for a valid reason (liberation). For any inquiry, there has to be a valid reasoning. The former is called practice and the latter is called knowledge. Knowledge without practice or practice without knowledge will be futile. What is knowledge and how it is derived is the basis for the formulations of different philosophies. However, at the end of knowledge and practice (attaining perfection in both), all philosophies uniformly say, there is realization. According Nyāya philosophy, the most important factor is knowledge, without which liberation is not possible (as is the case of almost all other philosophies). According to Nyāya, right kind of knowledge is derived through perception, inference, comparison and testimony. Knowledge is not right, when it is derived through memory (recollecting from past experience), uncertainty (lack of faith, mind swing, mind is full of doubts), mistakes (mistaken identities like mistaking a rope for a snake) and invalid reasoning (questioning everything and asking for evidence and proof for subtler things) every time). One cannot ask for proof for the existence of Brahman as no one can every produce the same.
Knowledge is produced only if a soul takes a form. Unless knowledge is there, the material world cannot be perceived. The correct knowledge about an object is the reality and the incorrect perception of an object arises due to lack of knowledge or inadequate knowledge. It is the mind that derives the right kind of knowledge through sensory organs with regard to objects. It is the same mind that derives knowledge about Brahman without the aid of sensory organs. Therefore, it is the basis of perception that matters when it comes to Brahman. Brahman cannot be perceived like an object, because this perception happens without having past experience. Everything cannot be related to or recollected from past experience. For example, Brahman cannot be recollected from past experience as spiritual experience differs from time to time (and also from person to person). Realization is possible only after several such spiritual experiences. Even after physically seeing an apple, if one does not believe it is as an apple, then it is lack of faith. Faith is the most important aspect in spirituality and realization, irrespective of the philosophies. Another aspect that causes lower knowledge is mistaken identity. Generally, mistaking a rope for a snake is cited as an example. Mistaken identity arises mainly due to fear. The rope is mistaken for a snake only out of fear for the snake and fear for life. If one understands that fire always produces smoke, then he is wrong. On seeing fire, one cannot argue that it is not fire as it does not produce smoke. This is invalid or illogical reasoning. These four are not the knowledge that is enough to liberate a person, as Brahman can be attained only through proper perception, right inference, logical comparison and testimony.
Brahman cannot be physically seen is the proper perception. If one thinks that Brahman too can be perceived with shape and form, it is due to his ignorance. Nyāya speaks about a number of perceptions, higher level, mediocre level and lower levels of perception. Level of perception is directly related to the level of spiritual attainment. In a lower level of perception, one sees an object and he understands an apple as an apple. In the higher level of perception one understands the apple not merely on its shape and colour, but also its taste, its variety, its origin etc. This is complete knowledge and the former is incomplete knowledge. Inference is another part of right knowledge as per Nyāya. Broadly inference is of two types, positive and negative which depends upon one’s mental attitude. One cannot wait to douse fire till he notices smoke. This is negative inference. When one sees smoke, he has to rush to douse the fire and this is positive inference. Inference is directly related to one’s attitude. Perception and inference are closely related and form the main components of knowledge. If someone reads an Upaniṣad, it can be read in two different ways. One, going through the crux of the Upaniṣad, skipping all the minute details and another way of reading is to read all the verses and understand the Upaniṣad thoroughly. There is significant difference between the two. The one who reads the crux of the teaching has to make further inquiries to know Brahman, whereas in the case of the one who reads thoroughly, he understands Brahman in that Upaniṣad itself. Because of his right perception (derived from higher level of knowledge), he could infer Brahman in the most appropriate way.
The third aspect of Nyāya is comparison. Generally comparison is the word denounced by psychology. But the comparison discussed in Nyāya is not the comparison between two individuals based on merits. Certain students have not seen an elephant and the teacher describes various parts of an elephant to make them understand what an elephant is. A student with higher level of knowledge could identify the elephant on seeing it for the first time and the one with inferior knowledge cannot perfectly infer the elephant by effectively comparing with what was taught by his teacher. Let us take another example. Upaniṣad-s say that Brahman is Self-illuminating and His illumination is often compared to the brightness of the sun. This is only a comparison and does not mean that sun is Brahman and there is no comparison between Him and the sun. Here sun is compared to Brahman to make one understand what Brahman is. Comparisons are necessary to know the truth as Upaniṣad- s elucidate Brahman through negations. The last one is testimony, which is in the form of words. When the teacher by showing an elephant says this is an elephant, it is testimony. This is verbal testimony and of mundane nature as we are able to see what is described through words. There is a higher level of testimony which is not perceptible. When Upaniṣad- s say that Brahman is in the form of Light within, it is also a testimony, but this testimony cannot be objectively verified and can only be experienced.
Nyāya also talks about cause and effect. When cause and effect theory is accepted, karmic theory has also to be accepted. Cause, as always is Brahman and effect is His manifestation of the universe. Nyāya goes with Vaiśeṣika with regard to God. During creation multitude of atoms come together and during dissolution, these conjoined atoms are withdrawn under instructions from God. Technically speaking, Nyāya accepts the theory of māyā, but does not use the word māyā, not even once. Liberation of the soul is only due to the knowledge. Due to this knowledge, one is able to make out the difference between the soul and the material world. In whatever philosophies we take, knowledge is paramount in God realization. Perfect knowledge that is required for realization is attained from śravaṇa (the act of listening), manana (meditation) and nididhyāsana (repeated and profound meditation).