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The Knowledge level contains a traditional KRR and/or planning system, using a relatively course-grained representation of objects, events (including actions), and states of affairs. For instance, objects are represented at this level as unique identifiers, typically without further detail about their physical characteristics or precise locations. It is possible to represent such detail explicitly at this level, but not required. Only if the detail becomes important to will it be represented, though not necessarily in the same way as at a lower level. For example, knowledge about the physical size and weight of an object might become available at the Knowledge Level through the agent's actively using measuring devices like a ruler or a scale, but this knowledge is not the same as the embodied knowledge about dimensions and weight represented at the Perceptuo-Motor level for the particular object or its object class. As a rule of thumb, representations at this level are limited to objects, events, and states of affairs that the agent needs to be consciously aware of in order to reason and plan, and in order to communicate with other agents at the grain size of natural language. The Knowledge level can be implemented using different KRR and/or planning systems.
Traditional use of the concept of world modeling refers to building
models of interactions between the agent and its environment at the
conscious level. These models maintain internal states for the agent.
The difference in our use of the term ``world model'' is that we do
not intend to have a precise model of all objects in the environment.
Instead, we want to model only the entities relevant to the agent's
interaction with its world. This requires filtering out some details
accessible at the Perceptuo-motor level as the entities are aligned
with their counterparts on the Knowledge level. This is known as
``perceptual reduction''. Physical details of interaction with
entities are handled at the Perceptuo-motor level. Representations at
the Knowledge level are needed only for explicit reasoning about
entities, and contain only the information necessary for doing so.
That might include details about physical characteristics in some
cases, but it need not. In other cases, it may be limited to a
nondescript intensional representation of an object. Conversely, some entities may be represented at the
Knowledge level but not at the Perceptuo-Motor level (abstract
concepts, for instance). Knowledge level representations are needed
for reasoning about entities; Perceptuo-Motor level representations
are needed for physically interacting with entities.