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do not lay eggs, whereas reptiles do. (Reptiles also share this property with birds, fish, and most amphibians, but we will not elaborate that distinction here.)
Again, if you consider the Linnaeus biological taxonomy, the property that distinguishes a specific subclass at the lower level (closer to the leaves) is probably one specific property.
Similarly, what are the distinguishing properties between the three hammers shown in Figure 7.3? We know we can talk about their different functions, and so functional properties distinguish them. But we can also see that there are physical differences, which may distinguish them too. Actually, the functional properties necessarily influence the physical properties; the physical properties depend on the functional properties (i.e., pounding or retracting nails, or pounding a stake into the ground). We know that the leftmost hammer is the common claw hammer (but related to the longer and heavier framing hammer) and that it is used to drive and pull nails. The middle hammer is the ball peen hammer, which is generally used for shaping or working with metal. Finally, the rightmost hammer is the sledge hammer, which is used to pound stakes, work concrete, hit wedges to split wood, and so on. In general, we might say that in many cases, "form follows function" or "purpose proposes property"—at least for human-designed artifacts.
Kingdom: Animalia Phylum: Chordata
Subphylum: Vertebrata Class: Mammalia Subclass: Theria
Infraclass: Eutheria Order: Primates
Suborder: Anthropoidea Superfamily: Hominoidea Family: Hominidae Genus: Homo
Species: Sapiens Class: Diapsida (Reptiles, Dinosaurs, Birds) Figure 7.2 Linnaean classification of humans.
Figure 7.3 Different hammers: claw versus ball peen versus sledge.
What's important to remember from this discussion is that there usually is (and should be, especially if the taxonomy is trying to be a semantically rich and well-defined structure, a semantically stronger taxonomy) a specific distinguishing property for each subclass of a taxonomy. Furthermore, the specificity—that is, the degree of fineness or granularity—of the property increases as you go down the taxonomy.
But enough with the insects and hand tools. What does this notion of distinguished property mean to you? Well, consider: Is a manager an employee? Should a manager and an employee really be distinguished at the same level in the taxonomy, as subclasses of person, as is displayed in Figure 7.1? Isn't a manager an employee too? So, shouldn't manager and some other information entity (call it X for now) be considered as subclasses of employee? Maybe the distinction should be between manager and nonmanager. But then perhaps these distinctions are somehow incorrect. Maybe person is a legitimate class of information entity, but manager and employee are not really subclasses of person; instead, they are different roles (a different relation) that any given person may have. This latter view complicates the picture, of course, but it may be more accurate if your intent is to model the real world as semantically accurately as possible. After all, a manager is an employee too, no? He or she is an employee of an organization that also has employees who are not managers.
This concept is similar to a subdirectory in a file directory: A subdirectory is a file (at least, when you look at how it's actually implemented) that contains files. Of course, in this latter case, the subdirectory is more like an aggregation or collection of files. Files are part of a subdirectory. And yes, the part of relation itself, quite like the subclass of relation, can constitute a taxonomy. A taxonomy based on the part of relation would be an aggregation taxonomy (as opposed to a generalization/specialization taxonomy, the first kind of taxonomy we looked at). As business folks, we know all about parts trees, bills of materials, and related notions, don't we? Well, now we also know these are taxonomies.
Table 7.1 displays a portion of the better-known taxonomy used in electronic commerce, the Universal Standard Products and Services Classification (UNSPSC, http://www.eccma.org). Although this taxonomy is displayed in tabular format, we can display it in tree format, as in Figure 7.4, with the Segment node being the root (of the subtree of Live Plant and Animal Segment 10) and the Family nodes being the first branch level (beneath which would be the Class and then the Commodity branches).
Taxonomies are good for classifying your information entities. They express at least the bare minimum of the semantics necessary to distinguish among the objects in your information space. As such, they are a simple model of the distinguishable items you are interested in. They are a way of structuring and characterizing your content meta data. Because taxonomies are trees, sometimes there is redundant information in a taxonomy. Why? Because there is only one parent node for each child node, you may sometimes have to have duplicate children nodes under different parents. For example, if you had the subclasses of Manager and Employee situated under Person, as in the example discussed previously, all managers would be placed under both nodes, since they are both managers and employees, resulting in duplication. Much therefore depends on how the taxonomy is structured. As we will see in the next section, ontologies use taxonomies as their backbones. The basic taxonomic subclass of hierarchies act as the skeleton of ontologies, but ontologies add additional muscle and organs—in the form of additional relations, properties/attributes, property values. So, taxonomies provide the basic structure for the information space, and ontologies flesh it out.