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Belief Revision: The Resubsumption Theory 339

It cannot be both at the same time. Does the sentence, they are eating apples
mean that the apples are for eating or that someone is already using them that
way? Do the data imply that the global climate is heating up or cooling down?
Incompatibility forces the mind to resolve conflict by choosing one represen-
tation over the other.
It is not obvious why some alternative representations are mutually incom-
patible while others are not. What distinguishes pairs of beliefs that are mutu-
ally incompatible from pairs of beliefs that are compatible? The shape of the
Earth is a case in point. The world has to be in some physical state or another
at each moment, but this fact does not imply that our representations of it need
to be mutually exclusive. A globe depicts the Earth as spherical while a map
depicts it as flat, but we feel no need to choose between globes and maps. We
go back and forth between them or use both as circumstances dictate. Why,
then, must our mental representation of the Earth depict it as either spherical
or flat?
Sometimes incompatibility is a matter of definition, as in the classical
example of a married bachelor. This type of incompatibility is merely a side
effect of how certain words are used. In logic, a contradiction is a conjunction
of two propositions that assign different truth values to the same proposition,
as in “true (P) & false (P).” Neither the definitional nor the logical type of
impossibility plays a central role in the theory proposed in this chapter.
In interesting cases of cognitive conflict, the conflicting beliefs specify
properties or states of affairs that cannot hold simultaneously. To continue
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the planetary example, the Earth is both round and heavy; no contradiction
there. Why can it not be both round and flat? The sphere and the disc repre-
sentations of the Earth are incompatible because it is impossible in a material
sense for the Earth to be an instance of both. The fact that the Earth cannot
be both flat and round is not an assertion about the words “flat,” “round” and
“Earth,” but about shapes and planets. A planet cannot be both flat and round
because, briefly put, matter either does or does not occupy a given region of
space. Similarly, an animal cannot be both a cat and a dog. This is not because
the propositions X is a dog and X is a cat logically contradict each other; they
do not. Instead, an animal cannot be both a cat and a dog because, briefly put,
it only has one set of parents, a core piece of our beliefs about mammalian
reproduction. The type of conflict that drives cognitive change is not logical
contradiction but material incompatibility, states of affairs that we believe can-
not hold simultaneously.
The source of this type of incompatibility is contextual: It is grounded in
our theories about the relevant areas of experience. This observation implies