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The Formation of Belief 293

THE QUESTIONS OF CONVERSION
Beliefs vary in scope from fundamental to trivial. The former include meta-
physical, moral, philosophical, political, scientific and religious principles. As
i use the term “belief” it also includes assertions of narrow scope pertaining to
the mundane details of everyday life. There is a subway station at the corner of
X and Y streets is an example. A belief consists of, among other components,
a proposition, the content of the belief, and its truth value. For example, a
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person may or may not have encoded the existence of a subway station at the
X/Y corner into memory; if he has, call his mental representation of this fact
P. The person might decide that P accurately describes reality, and so assign it
the truth value true.* The resulting memory structure, true (P), is a belief;
the proposition P itself, shorn of its truth value, is not. if the person decides
that the subway station is located somewhere else, he might assign P the value
false instead; false (P) is also a belief. Conversion is a switch in truth value,
from true (P) to false (P) or vice versa.
Beliefs tend to be grouped by topic or theme; cognitive scientists prefer the
term domain. examples include art, fashion, gardening, health, international
relations, law enforcement, money, morals, parenting, pets, politics, relatives, reli-
gion, romantic love, sports and the weather, to mention only a few of the domains
about which the average person is likely to have at least some beliefs. The set
of beliefs about a domain, whether fundamental or trivial, broad or narrow and
objectively true or false, is a person’s belief system for that domain. Although this
term usually refers to belief systems of great scope and consequence, with politi-
cal and religious beliefs as the prototypical examples, i will use the term to cover
sets of beliefs about any topic. For example, if someone believes that coffee con­
tains caffeine, that brand XYZ offers the best coffee and that coffee grows somewhere
far away, then those three beliefs constitute his belief system regarding coffee,
however impoverished it may appear to the knowledgeable caffeine aficionado.
The totality of all his beliefs, regardless of topic or domain, is a person’s
belief base. There is no method for accurately estimating it, but the number
of beliefs held by an average educated adult must be in the tens of thousands,
probably in the hundreds of thousands. A psychological theory of belief should
explain how this vast system is organized and how it grows and changes over
time.
A belief can be expressed in discourse to answer a question, communi-
cate information, articulate an argument and for other purposes. However, the

* i use small capitals to symbolize subjective truth values.