1.5 The Meaning of a Hierarchy 19
Figure 1.10 Result of adding a new element to an XML document. Anoteelement
has been added to ageneelement. The active element is now thenoteelement, and
its attributes appear in the window on the right side.
zations of knowledge.
Aristotle (384-322BC) was the first who understood the difficulty of cat-
egorizing living organisms into classes according to their anatomical and
physiological characteristics (Asimov 1964). Since then, this tradition of clas-
sification has been one of the major themes in science. Figure 1.11 illustrates
a hierarchy of chemicals taken from EcoCyc (EcoCyc 2003). For example,
proteinis more specific thanchemical,andenzymeis more specific thanpro-
tein. Classifications that organize concepts according to whether concepts
are more general or more specific are calledtaxonomiesby analogy with bio-
logical classifications into species, genera, families, and so on.
Hierarchies are traditionally obtained by starting with a single all-inclu-