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Belief Revision: The Resubsumption Theory 343

potential. Over the life span, a person’s stock of beliefs grows vast. To keep his
belief base globally coherent, a person would have to test every new belief for
consistency against every other belief; indeed, against every consequence of
already adopted beliefs; in fact, against every consequence of every possible
conjunction of already adopted beliefs. If the number of beliefs is, for example,
500,000, and the person’s brain could carry out one comparison per second, it
would require a lifetime to test the belief base for global consistency after add-
ing a new belief, and the operation would have to be repeated every time a new
belief is created. Maintaining global coherence is impossible and people other
than philosophers do not spend a significant proportion of their waking time
worrying whether their worldview is coherent.
As P. Thagard has emphasized in multiple articles and books, human
cognition is characterized by local rather than global coherence. The striv-
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ing for coherence operates within tightly interrelated groups of beliefs. This
Principle of Local Coherence is consistent with the function of declarative
knowledge: People activate their beliefs, not to contemplate their epistemic
worth, but to perform cognitive tasks, and those tasks arise within particular
contexts and situations. An attempt to perform a statistical analysis does not
activate my knowledge of SCUBA diving, and an attempt to cook dinner does
not activate my knowledge, such as it is, of the mummification practices of the
ancient Egyptians (perhaps just as well). Grouping beliefs into interconnected
but bounded subsystems facilitates retrieval of beliefs that are relevant for a
person’s current purpose without also flooding his limited-capacity working
memory with marginally relevant beliefs.
Local Coherence implies that a person is likely to remain unaware of con-
flicts among his beliefs if those beliefs pertain to semantically distant domains
or areas of experience. If the concepts and principles of two informal theo-
ries Th(A) and Th(B) are never present in working memory simultaneously,
and hence never applied to the same event, object or situation at the same
time, then any conflict between them remains undetected or latent. The con-
flict between the two theories exists in the eye of an omniscient observer but
not yet for the person whose theories they are. Rokeach put it this way: “It
may be assumed that in every person’s value-attitude system there already exist
inherent contradictions of which he is unaware for one reason or another. …”
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In this situation, further growth of the two theories need not be hindered or
complicated by dissonance-reducing processes. The question of how we can
absorb new information that contradicts prior concepts when that informa-
tion has to be understood in terms of those very concepts does not arise. The
new information can be absorbed without dissonance because it is consistent