Nyāya approaches knowledge with a striking sobriety: what matters is whether a cognition truly corresponds to reality and arises from a trustworthy route. Valid knowledge, termed *pramā*, is described as *yathārtha-jñāna*—a cognition that matches its object as it really is, is not already known, and is not later overturned by a stronger, contradicting cognition. Such knowledge is not merely correct in an abstract sense; it is grounded in an appropriate causal process, free from epistemic defects. When a cognition is both true and produced by a reliable means of knowing, Nyāya regards it as a successful contact with reality.

This reliability is articulated through four recognized *pramāṇas*, or valid means of knowledge. Perception (*pratyakṣa*) is direct awareness produced by the senses in proper contact with their objects. Inference (*anumāna*) arises from logical reasoning based on observed relations, when the middle term is sound and free of fallacies. Comparison (*upamāna*) yields knowledge by recognizing similarity between something already known and something newly encountered. Verbal testimony (*śabda*), especially from trustworthy or scriptural sources, is accepted when the speaker is reliable and free from deceit or error.

By contrast, invalid or non-valid knowledge, *apramā*, includes all cognitions that fail to meet these standards of truth and reliable origin. Nyāya speaks of outright false cognition (*mithyā-jñāna* or *viparyaya*), such as mistaking a rope for a snake, where the object is misrepresented. Doubt (*saṃśaya*) is another form of non-valid cognition, marked by oscillation between alternatives, as when one hesitates between seeing a man or a post. Memory, hypothetical reasoning, and other such cognitions are also not counted as fresh, valid knowledge in this framework, because they do not present a new, decisively grounded contact with reality.

The roots of invalidity lie in defects (*doṣa*) that disturb the proper functioning of the means of knowledge. These may be flaws in the sense organs, in the conditions of observation—such as poor lighting or excessive distance—or in the structure of reasoning, as with a faulty middle term in inference. Testimony fails when the speaker is unreliable, and comparison misleads when superficial similarity masks deeper dissimilarity. From a Nyāya standpoint, spiritual and philosophical clarity emerges not merely from accumulating cognitions, but from carefully discerning which arise from sound *pramāṇas* and which are compromised by hidden defects.