We know that the Carvakas admitted perception (pratyaksha) alone as the valid source of knowledge.

The Buddhists and the Vaisheshika admitted two sources, pratyaksha and inference (anumana); Samkhya added shabda (testimony) as the third source; Nyaya adds a fourth, upamana (analogy). The principle on which the four-fold division of pramanas depends is that the causal collocation which generates the knowledge as well as the nature or characteristic kind of knowledge in each of the four cases is different.

The same thing which appears to us as the object of our perception, may become the object of inference or shabda (testimony), but the manner or mode of manifestation of knowledge being different in each case, and the manner or conditions producing knowledge being different in each case, it is to be admitted that inference and shabda are different pramanas, though they point to the same object indicated by the perception. Nyaya thus objects to the incorporation of shabda (testimony) or upamana within inference, on the ground that since the mode of production of knowledge is different, these are to be held as different pramanas (1).

1.
Samagribhedai phalabhedacca pramanabhedah
Anye eva hi samagriphale pratyakshalingayoh
Anye eva ca samagriphale shabdopamanayoh.
Nyayamanjari, p. 33.

analogy, incorporation, inference, manifestation, perception, principle, testimony