  The heart of this method lies in the use it makes of definitions, conceived of as differentiating marks of the thing defined. The Manual of Reason (TSD 3d) refines the idea: it defines a ‘definition’ of a class of things as any characteristic which is co-extensive with that class. A defining characteristic of the class ‘cow’ is the property ‘having dewlap’. Note that this does not tell us what the essence of the class is—it merely supplies us with a syndrome or trait by means of which we can identify the thing in question. The Naiyāyikas, we might say, have a ‘diagnostic’, rather than an ‘essentialist’, conception of definition. The purpose of the ‘examination’ now becomes clear: it is to see whether the alleged defining trait really is co-extensive with the class to be defined, or whether it is faulty, either by ‘over-covering’ (cf. ativyāpti; applying to things outside the definiendum) or by ‘under-covering’ (cf. avyāpti; not applying to everything within the definiendum), or both. A properly defining characteristic has to be, to use modern terms, both a necessary and a sufficient property of the thing to be defined. We see the pattern of enumeration, definition, and examination repeated again and again in Navya-Nyāya texts like The Manual of Reason.