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Belief Revision: The Resubsumption Theory 337

cycles. In short, an entire belief system, no less than a single belief, is a type
of entity to which the mind can attach a truth value. The proof that the truth
value of a system is distinct from the truth values associated with the individ-
ual beliefs is that all those beliefs need not be associated with the same truth
value. It is common among sensible people to recognize strengths in belief
systems they reject, as in I do not believe Karl Marx’s theory of history, but I
think his analysis of the causes of the French Revolution was largely accurate or
I am certain that the global climate is warming, but the critics are probably right
that the ice core measures are flawed.
A belief system can enter into relations with other belief systems. In
particular, it can appear as a component of another system of more com-
prehensive scope. Consequently, a person’s belief base exhibits a hierarchi-
cal structure, with higher-order systems of large scope having multiple belief
systems of smaller scope among their parts. For example, a person’s beliefs
about law enforcement might nestle within his broader system of beliefs about
good governance and within his system of beliefs about morals and human
nature. The hierarchical structure is clearly visible in the explicit theories of
the professions and the sciences. A geologists’ theory of earthquakes nestles
within his overarching theory of plate tectonics, and the theory of aerodynam-
ics fits within the general theories of physics. In short, a belief system or theory
is simultaneously a unit in its own right, a structure that relates units of smaller
scope and a component of one or more systems of larger scope. 14
This Janus-faced character of belief systems does not imply that a belief
base branches out from a single starting point, a single Ur-belief. A more
likely view is that a belief base is anchored in multiple parentless nodes,
representing the person’s most fundamental beliefs about material reality,
life, society and so on. Also, the multiple hierarchies growing from these
fundamental beliefs are not freestanding, separate structures, lined up in
memory like so many Eiffel Towers standing side by side without touching.
A belief can play a role in multiple belief systems. For example, the belief
that cars burn fossil fuels might play a role in a person’s belief systems about
climate change, foreign policy and public health. Consequently, the individ-
ual’s belief base as a whole is not orderly, but a mess of tangled hierarchies, a
system of center-periphery structures that share components here and there;
see Figure 10.1. 15
The upshot is that we have to distinguish between at least four system
levels in the study of belief systems: components of beliefs (propositions and
parameters), the beliefs themselves, belief systems and the entire belief base.
Change processes can operate on any one of these levels.