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Belief Revision: The Resubsumption Theory 335

The present theory borrows M. Rokeach’s principle, introduced in Chapter
9, that belief systems exhibit a center-periphery structure. Core beliefs tend
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to be very general and few in number, and they are often held implicitly. An
example is the belief that diet is important for one’s health. Such core beliefs
have a broad scope; that is, they cover a wide range of possible instances.
Arranged in concentric rings around the core there are belts of successively
more specific beliefs of lesser scope. Continuing the health example, the belief
that diet is an important determinant of health is more fundamental than any
specific beliefs about exactly which foodstuffs are good or bad: fat is bad for
you; chocolate is good for you; and so on. In each successive belt, the beliefs are
more specific and more numerous, but also less important. At the periphery of
the belief system there are highly particular beliefs of very narrow scope such
as the claim that dark chocolate, specifically, is better for one’s health than reg-
ular milk chocolate.
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Specific beliefs are subsumed by beliefs closer to the core. That is, the spe-
cific beliefs are consistent with the core beliefs but contain additional detail.
Educational psychologist D. P. Ausubel stated this clearly: “The major organi-
zational principle [of a person’s knowledge] is that of progressive differentia-
tion of trace systems of a given sphere of knowledge from regions of greater
to lesser inclusiveness, each linked to the next higher step in the hierarchy
through a process of subsumption. It is incorrect, however, to conceive of this
mode of organization as deductive in nature.” 12
The center-periphery structure of a belief system is reminiscent of the
axiom-theorem structure of formal theories in logic and mathematics, Euclid’s
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axiomatic theory of plane geometry perhaps being the most familiar example.
However, subsumption is not logical implication. A peripheral belief is an
instance, not a deductive consequence of the relevant core belief. For example,
adopting the belief that diet affects health does not logically obligate a person
to also believe that a modest consumption of dark chocolate improves health.
The core belief does not by itself specify the effects of any particular diet. The
core belief is consistent with different, even contradictory specific beliefs: The
two beliefs chocolate is good for you and chocolate is bad for you are both sub-
sumable under diet affects health even though they contradict one another,
conclusive proof that the relation between subsuming and subsumed beliefs is
not logical deduction. A transition along a subsumption link adds additional
detail, while deduction only makes explicit what was implicit in the premises.
However, once one has adopted the belief that diet affects one’s health, one
can no longer ignore the question of the health effects of this or that foodstuff
without breach of local coherence. The core belief does not enforce adoption