Informal Logic: Issues and Techniques

Charles R. Crawford, formerly an associate professor of computer science
at York University, is a computer-programming and mathematics
consultant.

The word “logic” is usually used in ordinary language in connection
with making decisions. We do something or choose something if the
arguments for it outweigh those against it. Informal Logic investigates
how ordinary people weigh arguments and proposes a way to systematize
the process.

The book begins with a technical analysis of arguments as speech acts
and proceeds to a review of argument evaluation systems, including the
author’s. The next six chapters deal with different types of
arguments, such as obligation claims, physical claims, and mental-world
claims. The various evaluation systems are discussed in connection with
each argument type. The final chapter reviews the issues addressed by
the book and the highlights of the author’s evaluation system.

The author’s system relies on numerical evaluations of the components
of the argument. These numbers are described as probabilities or
confidence levels, and they are combined in the same manner as
probabilities to arrive at a final evaluation of the argument. The
author discusses the problems ordinary people might have in assigning
these numbers and recommends expanding the two-value “true–false”
system to 3 or 5 and finally to 11 levels.

Although this book concerns the logic of ordinary people, it is not a
textbook. The writing style is technical and assumes some background in
logic and philosophy. The author does, however, discuss the problem of
teaching logic at the undergraduate level, and his evaluation system

is intended as a classroom tool.