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What is a Knowledge Representation?
Perhaps the most fundamental question about the concept of knowledge representation is, What is it? We believe that the answer is best understood in terms of the five fundamental roles that it plays.
Role I: A KR is a Surrogate
Any intelligent entity that wishes to reason about its world encounters an important, inescapable fact: reasoning is a process that goes on internally, while most things it wishes to reason about exist only externally. A program (or person) engaged in planning the assembly of a bicycle, for instance, may have to reason about entities like wheels, chains, sprockets, handle bars, etc., yet such things exist only in the external world.
This unavoidable dichotomy is a fundamental rationale and role for a representation: it functions as a surrogate inside the reasoner, a stand-in for the things that exist in the world. Operations on and with representations substitute for operations on the real thing, i.e., substitute for direct interaction with the world. In this view reasoning itself is in part a surrogate for action in the world, when we can not or do not (yet) want to take that action. (1)
Viewing representations as surrogates leads naturally to two important questions. The first question about any surrogate is its intended identity: what is it a surrogate for? There must be some form of correspondence specified between the surrogate and its intended referent in the world; the correspondence is the semantics for the representation.
The second question is fidelity: how close is the surrogate to the real thing? What attributes of the original does it capture and make explicit, and which does it omit? Perfect fidelity is in general impossible, both in practice and in principle. It is impossible in principle because any thing other than the thing itself is necessarily different from the thing itself (in location if nothing else). Put the other way around, the only completely accurate representation of an object is the object itself. All other representations are inaccurate; they inevitably contain simplifying assumptions and possibly artifacts.
Two minor elaborations extend this view of representations as surrogates. First, it appears to serve equally well for intangible objects as it does for tangible objects like gear wheels: Representations function as surrogates for abstract notions like actions, processes, beliefs, causality, categories, etc., allowing them to be described inside an entity so it can reason about them. Second, formal objects can of course exist inside the machine with perfect fidelity: Mathematical entities, for example, can be captured exactly, precisely because they are formal objects. Since almost any reasoning task will encounter the need to deal with natural objects (i.e., those encountered in the real world) as well as formal objects, imperfect surrogates are pragmatically inevitable.
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