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Belief Revision: The Resubsumption Theory 351

their actions, initiatives and votes are consistent with those values and prin-
ciples. Like any adult, our hypothetical everyman-turned-politico also has an
intuitive theory about trade, about buying and selling, about one person offer-
ing something that has a certain worth or value to somebody else, who might
therefore be ready to offer something of equal value in return. This theory is
normally evoked in the context of deciding on a purchase or a sale, not politics.
But even modest experience with politics as actually practiced might cause a
person to re-conceptualize the work of politicians in terms of trades. Nothing
can be accomplished unless a politician is ready to trade favors, and the effec-
tive politician knows the political worth of every favor traded and never trades
away more than he gains. The veridicality of this view of politics is not the issue
here, only the fact that a change from the Civics Theory to the Trade Theory
illustrates how a theory formed in the course of everyday activities like buying
groceries and shopping for a new car can turn out to provide a better structure
for understanding a superficially very dissimilar domain of experience than a
theory that was originally formed with reference to that domain.
As an example with scientific subject matter, consider biological evolu-
tion via genetic variation and natural selection. Darwin invented this theory
to explain the pattern of similarities and differences among living species, their
geographical distributions, the fossil record and other features of the living
world. As originally stated, his theory was embedded in the biological domain;
it was about organisms, species and, after the modern synthesis, genes and
mutations. However, the schema of variation and selection, once invented, can
subsume other domains as well. At one point, the theory that the immune sys-
tem develops antibodies that are targeted for a particular virus – which raises
the question of how the immune system derives information from the virus in
order to concoct the right antibody – gave ground to the idea that antibodies
evolve through variation and selection. The immune system issues antibod-
ies of great variety, and it makes more of those antibodies that match foreign
particles in the body. In this way, the functioning of the immune system can
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be resubsumed under the variation-selection principle. The first person who
thought of the immune system in this way underwent conceptual change, and
given the likelihood that he or she already knew the theory of evolution, this
was an instance of resubsuming the facts of immunology under the theory
originally developed to explain the history of life.
To verify that this was not an isolated instance, consider two other
resubsumptions involving variation and selection. First, Walter G. Vincent
has proposed that the variation-selection theory can be applied to the his-
tory of aviation: Aeronautical engineers developed a variety of wing shapes,