It fails to uncover the pattern of logical relations which are the ground of the validity of inferences. The Nyaya pints to some of the ways in which we reason, and it asserts they are valid, but it fails to show why they are valid. Testimony , shabda, the testimony of a relevantly expert person is accepted by Nyaya as a sound source of knowledge. This certainly corresponds to how we actually come to know and understand things. Indeed, it seems improbable that anyone in history has ever come to knowledge of the world by use of perception, inference and comparison without any reliance on the testimony of others, parents, teachers, friends, associates and so forth.
Some viewpoints accept only scriptural statements as testimony.
Nyaya, however, distinguishes between knowledge of the visible and knowledge of the invisible, and sees expertise as sufficient to establish the reliability of testimony concerning the visible. Accepting the four pramanas as sound means of knowledge does not, for Nyaya, imply that they are infallible. What Nyaya claims about the pramanas is that in general the four pramanas can be trusted and that as a matter of fact they are the four sources from which our knowledge is derived. Nyaya accepts the existence of error. Most discussion of error in Indian philosophy centres on the possibility and the nature and explanation of perceptual error .