We explore the use of argumentation for justifying
claims reached by plausible reasoning methods in
domains where knowledge is incomplete, uncertain, or
inconsistent. We present elements of a formal theory
of argumentation that includes two senses of
argument, argument as supporting explanation and
argument as dialectical process. W e describe a partial
implementation of the theory, a program that
generates argument structures that organize relevant,
available, plausible support for both a claim and its
negation. Then we describe a theory of argument as
dialectical process, where the format of a two-sided
argument is used to intertwine the strengths and
weaknesses of support for competing claims, so
arguments can be refuted and directly compared.