 The Nyāya response to sceptical objections occurs at two levels, at the first level enumerating a set of virtuous processes by which true beliefs are acquired, and the second level dealing with the ratification of those reliable or virtuous processes of belief-acquisition. Naiyāyikas admit four kinds of cognition—perceptual (pratyakṣa) inferential (anumiti), that which arises from comparison (upamiti) and verbal (śābda), and four accredited means of acquiring veridical cognition (pramāṇa), viz., perception (pratyakṣa), inference (anumāna), comparison (upamāna) and authority (śabda). They decide the number of the accredited means by empirically observing the effectiveness and reliability of the respective means in generating true beliefs. According to them, these processes generate true beliefs only when accompanied by genuine excellence or epistemic virtue (guṇa). The virtue that makes a generating process meritorious differs in each type of true belief. In case of perception the relation of the sense organ with the object characterised by the property which figures as the qualifier in the perceptual cognition is said to be the virtue. For example, when one perceives a white shell as white, our sense organ stands in appropriate relation with the object of perception, a shell in this case, and apprehends the property whiteness which characterizes the shell in question and thus gives rise to a veridical perception. In a veridical inferential cognition the mark, which is invariably concomitant with the thing to be inferred, must be known to be present in the locus of the inference. For instance, when someone correctly infers fire on a distant hill, it is known to him that smoke, which is invariably concomitant with fire, is present on the hill. In case of knowledge by comparison, knowledge of similarity is the excellence, e.g., someone rightly identifies an unknown animal as a bison on seeing its similarity with a cow which he came to know from an expert’s utterance to the effect that a bison is similar to a cow. Finally, in case of verbal knowledge, the speaker’s veridical cognition of the state of affairs described by the sentence uttered is the excellence, e.g., when an umpire declares a batsman out in a cricket match on the basis of his expertise and veridical cognition of the state of affairs. A false belief results from the presence of some defect (doṣa), and not merely from absence of the required virtue. Someone may perceive a white shell as yellow because he is suffering from jaundice or because of the yellow tinted light in the room or due to some other defective condition. These defects differ in each instance of false belief. The Naiyāyikas therefore maintain that a false belief is caused by a defect and a true belief is caused by a virtue. This principle holds in all cases and all types of belief—be it commonsensical, scientific, or philosophical.