  In this paper, we examine the concept of modularity, an often cited advantage
of the ruled-based representation methodology. We argue that the notion of
modularity consists of two distinct concepts which we call syntactic modularity
and semantic modularity. We argue that when reasoning under certainty, it is
reasonable to regard the rule-based approach as both syntactically and
semantically modular. However, we argue that in the case of plausible
reasoning, rules are syntactically modular but are rarely semantically modular.
To illustrate this point, we examine a particular approach for managing
uncertainty in rule-based systems called the MYCIN certainty factor model. We
formally define the concept of semantic modularity with respect to the
certainty factor model and discuss logical consequences of the definition. We
show that the assumption of semantic modularity imposes strong restrictions on
rules in a knowledge base. We argue that such restrictions are rarely valid in
practical applications. Finally, we suggest how the concept of semantic
modularity can be relaxed in a manner that makes it appropriate for plausible
reasoning.
