Upamana is the knowledge of the relation between a word and the object denoted by that word. It is produced by the knowledge of resemblance or similarity. A man who has never seen a gavaya but is informed that a cow like animal is there in the forest comes across a wild cow in a forest and recognizes it as gavaya. The knowledge of the relation between a name and the object denoted by that name by the knowledge of similarity because the man recongizes a wild cow as a 'gavaya' when he perceives its similarity to the cow and rembmers the description that 'a gavaya is an animal like a cow'.
4. Shabda Pramana:
Shabda Pramana is divided into two:  Vaidika,  Loukika. Both are only personal authority, the former is the words of God, whereas the latter is the words of trustworthy human beings.
A word is a potent symbol which signifies an object and a sentence is a collection of words. In order to be intelligible a sentence must conform to the following conditions:
[a] mutual expectancy [the word 'eat' can have only living beings as its subject of the sentence], [b] mutual fitness [no category mistake], [c] close proximity of the words to one another in quick succession [temporal proximity], [d] intention of the speaker especially when the words used are ambiguous [Saindhava].
Other sources of Pramana are not accepted by Nyayayikas. Arthapatti is reduced to inference.
All fat persons who do not eat during the day, eat during the night.
Devadatta is a fat person who does not eat during the day.
Therefore, Devadatta is a fat person who eats during the night.
The Nyaya Theory of Causation:
A cause is defined as an unconditional and invariable antecedent of an effect and an effect as an unconditional and invariable consequent of a cause. The same cause produces, the same effect and the same effect is produced by the same cause.