Informal Logic

Glossary of Grammatical and Rhetorical Terms

informal logic
Informal logic is sometimes used as a label for the study of informal logical fallacies. Similarly, informal logic is often equated with critical thinking. (Thomas Barwick/Getty Images)

Informal logic is a broad term for any of the various methods of analyzing and evaluating arguments used in everyday life. Informal logic is commonly regarded as an alternative to formal or mathematical logic. Also known as non-formal logic or critical thinking.

In his book The Rise of Informal Logic (1996/2014), Ralph H. Johnson defines informal logic as "a branch of logic whose task it is to develop non-formal standards, criteria, procedures for the analysis, interpretation, evaluation, criticism and construction of argumentation in everyday discourse.


  • Many informal logicians have adopted an approach that does seem to be a response to the need to acknowledge a rhetorical dimension to argumentation. This dialogical approach, which was initiated by C.A. Hamblin's (1970) writings on fallacy, is a hybrid of logic and rhetoric, and has adherents in both fields. The approach acknowledges that argumentation does not occur in a rhetorical vacuum, but should be understood as a series of dialectical responses that take a question-and-answer form."
    (Don S. Levi, "Logic," Encyclopedia of Rhetoric, ed. by Thomas O. Sloane. Oxford University Press, 2001)
  • Rhetorical Argumentation
    "A more recent model of argument that looks to wed the logical with the dialectical is that of [Ralph H.] Johnson (2000). Along with his colleague [Anthony J.] Blair, Johnson is one of the originators of what is called 'informal logic,' developing it on both the pedagogical and theoretical levels. Informal logic, as here conceived, attempts to bring the principles of logic into accord with the practice of everyday reasoning. At first this was done through an analysis of the traditional fallacies, but more recently informal logicians have been looking to develop it as a theory of argument. Johnson's book Manifest Rationality [2000] is a major contribution to that project. In that work, 'argument' is defined as 'a type of discourse or text—the distillate of the practice of argumentation—in which the arguer seeks to persuade the Other(s) of the truth of a thesis by producing the reasons that support it' (168)."
    (Christopher W. Tindale, Rhetorical Argumentation: Principles of Theory and Practice. Sage, 2004)
  • Formal Logic and Informal Logic
    - "Formal logic has to do with the forms of argument (syntax) and truth values (semantics). . . . Informal logic (or more broadly argumentation)), as a field, has to do with the uses of argumentation in a context of dialogue, an essentially pragmatic undertaking.

    "Hence the strongly opposed current distinction between informal and formal logic is really an illusion, to a great extent. It is better to distinguish between the syntactic/semantic study of reasoning, on the one hand, and the pragmatic study of reasoning in arguments on the other hand. The two studies, if they are to be useful to serve the primary goal of logic, should be regarded as inherently interdependent, and not opposed, as the current conventional wisdom seems to have it."
    (Douglas Walton, "What Is Reasoning? What Is an Argument?" The Journal of Philosophy, 1990)

    - "Formal logicians of a radical stripe often dismiss informal logical techniques as insufficiently rigorous, precise, or general in scope, while their equally vehement counterparts in the informal logic camp typically regard algebraic logic and set theoretical semantics as nothing more than an empty formalism lacking both theoretical significance and practical application when not informed by the informal logical content that formal logicians pretend to despise."
    (Dale Jacquette, "On the Relation of Informal to Symbolic Language." Philosophy of Logic, ed. by Dale Jacquette. Elsevier, 2007)

    See also: