Informal Logic: Issues and Techniques
Wayne Grennan
Acumen Pub Ltd
This critique of informal logic and critical thinking strategies and techniques examines argument evaluation techniques being used in both formal and informal logic texts and, finding them lacking, proposes a new system for evaluation. This evaluation is based on two criteria - logical adequacy and pragmatic adequacy for evaluating everyday arguments. The work asserts that the common formal logic systems, while logically sound, are not very useful for evaluating everyday inferences, which are almost all deductively invalid as stated. Turning to informal logic, it is pointed out that while more recent informal logic and critical thinking texts are superior in that their authors recognize that everyday arguments are inductive, they typically cover only inductive fallacies, ignoring the inductively sound patterns used frequently in successful persuasion. To redress these problems, the text introduces a variety of additional inductive patterns. Concluding that informal logic texts do not encourage precision in evaluating arguments, a new argument evaluation procedure is proposed, one that expresses judgments of inferential strength in terms of probabilities. Based on theories of Stephen Toulmin, Roderick Chisholm, and John Pollock, the proposed system uses a probability-based rating scheme to arrive at a more precise judgment of the persuasive force of arguments.