ABSTRACT: “Informal logic» is the name given to an approach to arguments
and argumentation emerging in Canada and the United States in the 1970s and
consolidated there for three decades. It developed initially independently of the
influence of The Uses of Argument and La Nouvelle Rhétorique. Over the past decade
especially, however, these works have become increasingly pertinent to informal logic
theory. The paper begins with a brief foray into the history of ideas and the sociology
of knowledge to partly explain the genesis, and hence the nature, of informal logic.
Then it traces the focus of informal logic on defeasible reasoning and arguments
to the difficulties fatal to the assumption that formal deductive logic provides
adequate theory for the interpretation and evaluation of arguments. Turning to the
influence of Toulmin, the most significant ideas for informal logic are his concepts
of warrant interpreted as inference license, relevance conferrer, and argument
scheme embodiment; of the field or topic dependence of backing; and of qualifiers
and conditions of rebuttal interpreted as signifying the defeasibility of arguments
employed in much argumentation. Perelman and Olbrechts-Tyteca’s rejection of formal
logic as the theory of argumentation, their emphasis on the importance of audience,
the dialectical features of their rhetoric, their concepts of particular and universal
audience, and their empirical focus on argument schemes are all significant for the
informal logic perspective on arguments.−The discussion of the influence of these
two works is not historical for the most part, but rather reflects this author's views
about the significance of some of their doctrines for current theory at the beginning
of the 21st century