Ontologies are necessary for "reasoning". An ontology licenses the conclusions that can be drawn from statements about subjects that are expressed in terms of the ontology. But we can only reason about the subjects that we have identified in terms of the ontology. What if someone identifies the same subject differently than we have? If a subject is identified in different ways, the existing information about it may not all be discovered whenever there is reason to discover it.
The problem of subject identification is important because neither humans nor computers can do any reasoning in the light of undiscovered information. Our ontologies and reasoning abilities are useless beyond the perimeter of the information we can find.
The subject identification problem resists a general solution. New ways of identifying subjects constantly appear, and they are often idiosyncratic. Existing techniques and rules for identifying subjects can mutate and/or become obsolete. An additional complication is that different individuals, and different communities of people, may reason about the same subjects differently.
And different people who all happen to participate in the same set of communities may well have different perspectives on the question of exactly when any two communities are talking, in their different ways, about the same subjects. They may express such perspectives as "subject mappings". The differences in their perspectives may be gross, or subtle, or both. Different perspectives may be useful separately and/or in combination with each other.
A subject mapping can be of significant value. If, in its own special way, each community has separately amassed information about the same subject, then, in the light of a mapping that reveals that they are talking about the same thing, both communities can gain access to a larger knowledge pool. The pooling itself may trigger the emergence of new knowledge, so that the new whole is greater than the sum of the parts. New knowledge may emerge because, in effect, we have extended the useful impact of our ontologies beyond the information that happens to be expressed solely in their own terms.
Subject mappings cure the top-down brittleness of individual ontologies. They allow cross-community semantic integrations to be maintained, even without prior constraints on the distribution/delegation of ontological authority. Where there is a marketplace of mappings, ontologies can be allowed to evolve independently, in response to local needs. The diversity of communities and their ontologies can be cherished and harnessed. The inflation of the infoverse becomes a cheery symptom of a diverse and robust knowledge economy, and a source of rich opportunities to exploit both human and automated reasoning.
The key is to amplify the exploitability of cross-community, cross-ontology human expertise. A little subject mapping can have a big impact.