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In many corners of the information sciences, including work on knowledge representation, description logics, and some object-oriented programming languages, the so-called is_a relation plays a prominent role. In this chapter, it will be argued that there are both material and formal reasons to distinguish between four kinds of is_a relation, which we will call (1) subsumption under a genus, (2) subsumption under a determinable, (3) specification, and (4) specialization. Genus-subsumption is the traditional way of creating classificatory trees of natural kinds; in particular, of creating the famous hierarchies of plants and animals in biology. However, it is also used in more practically oriented classifications of kinds such as citizens, patients, furniture, clothes, vehicles, and so forth. On the linguistic side, this kind of subsumption is usually mirrored, linguistically, by relations between nouns. Determinable-subsumption, on the other hand, is not concerned with natural kinds but with qualities (properties) of different generality. For instance, as a determinate scarlet is subsumed under the determinable red. In such cases, we find relations between adjectives on the linguistic side. Even though today, in the information and computer sciences, the two expressions ‘a is a specification of b’ and ‘a is a specialization of b’ are quite often used as synonyms of ‘a is_a b’, the terms ‘specification’ and ‘specialization’ will here be given more restricted meanings. In fact, these restricted meanings come close to the pre-computer world meanings of these terms. Whereas subsumptions are typically concerned with natural kinds and qualities, specifications and specializations are typically concerned with activities and processes. Prototypical specifications come out on the conceptual level primarily as relations between a verb-plusadverb expression and a verb: ‘painting carefully’ is a specification of ‘painting’. Analogously, specializations come out primarily as relations between a verb plus a whole adverbial adjunct clause and a (usually

Chapter 11: Four Kinds of Is_a Relation