Nyaya, one of the six orthodox schools of Hindu philosophy, places great emphasis on logic and epistemology in its pursuit of truth. In order to understand how Nyaya defines valid and invalid knowledge, it is important to first understand its concept of pramana, or valid means of knowledge.

According to Nyaya, there are four pramanas - perception, inference, comparison, and testimony. Perception refers to direct sensory experience, while inference is the process of arriving at a conclusion based on observation and logical reasoning. Comparison involves the use of analogies and similarities to gain knowledge, and testimony refers to the knowledge gained through the words of reliable sources.

Based on these pramanas, Nyaya defines valid knowledge as that which is obtained through a reliable and trustworthy means of knowledge. This means that the knowledge must be based on a valid pramana and must be free from any errors or distortions. In other words, the source of knowledge must be reliable and the information obtained must be accurate.

On the other hand, invalid knowledge is that which is obtained through an unreliable or false means of knowledge. This could be due to errors in perception, faulty reasoning, or unreliable sources of information. Nyaya also considers knowledge gained through imagination, hallucinations, and dreams as invalid, as they do not correspond to reality.

In order to determine the validity of knowledge, Nyaya has laid down certain criteria. Firstly, the source of knowledge must be reliable. For example, if one is seeking knowledge about a particular subject, they must turn to a reliable and knowledgeable source rather than relying on hearsay or rumors.

Secondly, the knowledge obtained must be free from any errors or contradictions. This is where the role of logic and reasoning comes into play. Nyaya emphasizes the use of logical reasoning to arrive at valid conclusions and to eliminate any inconsistencies in knowledge.

Thirdly, the knowledge must be verifiable. This means that it must be possible to confirm the accuracy of the knowledge through direct experience or through the use of other valid means of knowledge.

In conclusion, Nyaya defines valid knowledge as that which is obtained through a reliable and trustworthy means of knowledge and is free from any errors or contradictions. It places great importance on the use of logic and reasoning in determining the validity of knowledge and emphasizes the need for verification. By following these principles, Nyaya aims to attain a clear and accurate understanding of reality.