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Different indexers create different indexes; different kinds of knowledge require different ontologies. There is a boundless variety of opinions about what's important to know about various subjects, what the important subjects are, the terms and/or names used to identify subjects. If I want to exploit multiple indexes or knowledge bases, I have to learn a lot about each of them -- the terms they use, the relationships they bother to reflect, etc. What if I could use several indexes simultaneously, and use everything they both have to offer? How can their contents be integrated into what appears to be a seamless resource, with value greater than the sum of its ingredients?

The Topic Maps Reference Model provides an answer. The answer, first of all, is to *view* each of the knowledge bases, or each of the indexes, as a set of subject reifiers. In this slide, we see three reifiers of the subject (which is the Statue of Liberty itself). (By the way, the lightbulb is shown in the upper right-hand corner of the slide just to remind us that that's the subject that's presumably being reified by all three reifiers. Remember, only human imagination can grasp subjects. There is no connection between reifiers and their subjects except for the connections that are made by the imaginations of human beings.)

These three reifiers represent information from three different knowledge bases, each of which has its own vocabulary.

Reifiers of the Statue of Liberty, each exhibiting some knowledge about it.