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Belief Revision: The Resubsumption Theory 355

concept or theory that is changing but the relation between a knowledge struc-
ture and its domain of application. A knowledge structure that was thought
to have extension A is discovered to have the wider extension A+B. This is a
change in how events, objects and phenomena are grouped rather than in the
content of the relevant belief system. Paradoxically, the Resubsumption Theory
thus claims that changes in content occur primarily in the course of mono-
tonic belief formation, not during conversion. Conversion is a choice between
already formed beliefs.
In this respect, the Resubsumption Theory generalizes the ontologi-
30
cal category shift theory proposed by Michelene Chi. She has argued that
conceptual change requires that the relevant event, object or phenomenon
be re-classified under a different ontological category. The latter are high-
level categories that specify the types of entities the world contains: events,
objects, processes and so on. When a phenomenon is assigned to an onto-
logical category, it inherits the properties that characterize entities in that
category. If the category is inappropriate, the inherited properties might
interfere with correct understanding. For example, electromagnetic waves
cannot be understood correctly as long as they are thought of as causal sys-
tems but need to be resubsumed under a field category. The resubsumption
and ontological shift theories share the idea that change involves discon-
necting an area of reality from one higher-order knowledge structure and
attaching it to another.
They differ in other respects. Whereas the ontological shift theory singles
out ontological knowledge as playing a special role, the Resubsumption Theory
treats all knowledge as alike and does not postulate a highest category level. All
events, objects, processes, regularities and states of affairs that appear in either
direct experience or discourse are subsumed under some concept or intuitive
theory. My perception of a particular dog is subsumed under my concept of
dogs; a statement about dogs in general is understood in terms of my beliefs
about dogs; and so on. Even such mundane assignments and categorizations
can sometimes be mistaken and require revision: I might pick up what I think
is a ballpoint pen and look for the button that makes the writing tip appear, only
to discover that it is a pencil with a missing lead. To re-categorize the object
from the category “ballpoint pen” to “pencil” is a resubsumption event (of min-
imal scope) but it is not an ontological shift. Any concept, belief or theory can
subsume new information received through perception or discourse; any piece
of information must be subsumed under some concept or another; and every
act of subsumption can, in principle, be mistaken and require revision. The
ontological shift theory emphasizes the effects of these processes with respect