The third prama@na, which is admitted by Nyaya and not by Vais'e@sika, is _upamana_, and consists in associating a thing unknown before with its name by virtue of its similarity with some other known thing. Thus a man of the city who has never seen a wild ox (_gavaya_) goes to the forest, asks a forester--"what is gavaya?" and the forester replies--"oh, you do not know it, it is just like a cow"; after hearing this from the forester he travels on, and on seeing a gavaya and finding it to be similar to a cow he forms the opinion that this is a gavaya. This knowing an hitherto unknown thing by virtue of its similarity to a known thing is called _upamana_. If some forester had pointed out a gavaya to a man of the city and had told him that it was called a gavaya, then also the man would have known the animal by the name gavaya, but then this would have been due to testimony (_s'abda-prama@na). The knowledge is said to be generated by the upamana process when the association of the unknown animal with its name is made by the observer on the strength of the experience of the similarity of the unknown animal to a known one. The naiyayikas are thorough realists, and as such they do not regard the observation of similarity as being due to any subjective process of the mind. Similarity is indeed perceived by the visual sense but yet the association of the name in accordance with the perception of similarity and the instruction received is a separate act and is called _upamana_ [Footnote ref 521].
S'abda-prama@na or testimony is the right knowledge which we derive from the utterances of infallible and absolutely truthful persons. All knowledge derived from the Vedas is valid, for the Vedas were uttered by Is'vara himself. The Vedas give us right knowledge not of itself, but because they came out as the utterances of the infallible Is'vara. The Vais'e@sikas did not admit s'abda as a separate prama@na, but they sought to establish the validity of testimony (_s'abda_) on the strength of inference (_anumiti_) on the ground of its being the utterance of an infallible person. But as I have said before, this explanation is hardly corroborated by the Vais'e@sika sutras, which tacitly admit the validity of the scriptures on its own authority. But anyhow this was how Vais'e@sika was interpreted in later times.