Informal Logic

Overview

Informal logic is the study of the norms of argument as it actually occurs in natural language — in conversation, journalism, politics, law, and everyday reasoning. It arose in the mid-twentieth century from a dissatisfaction with the gap between formal logic and real argumentation. Formal logic, with its precise languages and proof systems, handles idealized argument forms perfectly; but the arguments people actually make are elliptical, ambiguous, loaded with implicit premises, and embedded in context. Informal logic asks what it means for such arguments to be good, and how ordinary reasoners can recognize when they are not.

The name can mislead: informal logic is not logic without rules, nor is it mere rhetoric. It is the systematic study of the informal standards by which natural-language arguments are evaluated — relevance, sufficiency, acceptability of premises, and the strength of inferential links. Its practitioners develop theories of fallacies, taxonomies of argument schemes, and methods of argument analysis that can be taught and applied. In this respect informal logic is the theoretical backbone of the critical thinking movement.

Historical Development

The tradition of evaluating real arguments is as old as philosophy itself. Aristotle wrote the Rhetoric and the Sophistical Refutations, cataloguing the sophisms — the "refutations that seem to be refutations" — that still form the core of the modern list of fallacies. The Sophists against whom he argued were masters of persuasive but defective reasoning, and Aristotle's response was to distinguish valid argument from apparent argument. Socrates modeled the other pole: the patient examination of claims through questioning, which remains the ideal of informal inquiry.

The modern discipline took shape in the 1950s and 1960s. Stephen Toulmin's The Uses of Argument (1958) proposed a model of argument — claim, data, warrant, backing, qualifier, rebuttal — that abandoned the syllogism as the universal form and attended to how arguments function in practical fields. Charles Leonard Hamblin's Fallacies (1970) revived the systematic study of fallacies after decades of neglect, distinguishing dialectical from formal approaches. Howard Kahane's Logic and Contemporary Rhetoric (1971) argued that logic courses should engage with real-world arguments — advertising, editorials, politics — and helped launch the informal logic movement in North America. [Douglas Walton] and others subsequently developed argumentation theory: the analysis of dialogue types, argument schemes, and the critical questions that test them.

Core Ideas

Fallacies as Dialectical Failures

The heart of informal logic is the theory of fallacies. A fallacy is a pattern of argument that seems good but is systematically defective — not because it violates a formal rule (it may be formally valid) but because it fails in relevance, sufficiency, or acceptability. The ad hominem attacks the person rather than the argument; the straw man misrepresents the opponent's position; the appeal to authority relies on an unreliable or irrelevant authority; the false dilemma presents two options when there are more; the slippery slope asserts without evidence that one step leads inevitably to disaster. Hamblin's insight was that many fallacies are best understood dialectically: they violate the rules of rational dialogue between questioner and respondent rather than the rules of a formal system.

Argument Schemes

Argumentation theory systematizes the recurring patterns of everyday inference — argument from expert opinion, argument from analogy, argument from consequences, argument from correlation to cause — and attaches to each scheme a set of critical questions. An argument from expert opinion, for example, is assessed by asking whether the expert is genuinely expert in the relevant field, whether the claim falls within that field, and whether the expert is reliable and disinterested. This approach bridges informal logic and the analysis of scientific reasoning, where arguments from authority and from evidence are ubiquitous.

Formal vs Informal Standards

Informal logic does not reject formal logic; it supplements it. Formal validity is one standard of argument goodness, but in natural language, most arguments are enthymematic — they rely on unstated premises — and are best evaluated as to whether the premises, made explicit, provide adequate support. Informal logic thus shifts the focus from truth-preservation to cogency: the overall quality of an argument as a piece of reasoning in context, including the plausibility of its premises and the strength of the inference to its conclusion.

Key Thinkers

Aristotle (384–322 BCE) founded the study of fallacies in the Sophistical Refutations. [Stephen Toulmin] (1922–2009) transformed argument analysis with his field-dependent model of argumentation. [Charles Leonard Hamblin] (1922–1985) reestablished the systematic theory of fallacies in Fallacies. [Howard Kahane] (1928–2001) made informal logic the basis of critical thinking pedagogy. John Dewey (1859–1952) supplied the philosophical background with his account of reflective inquiry.

Influence

Informal logic has reshaped education: the modern critical thinking curriculum — fallacies, argument analysis, source evaluation — descends directly from it. It informs law, where arguments are evaluated in adversarial dialogue; journalism, where standards of relevance and evidence are at issue; and the study of rhetoric and argumentation in communication studies. For the philosophy of logic, informal logic raises the question of whether validity itself should be relativized to fields and dialogues; for epistemology, it studies the norms of real belief-formation. Wherever arguments are made in natural language — which is everywhere — informal logic supplies the standards by which they can be judged.

Sources

1. Stanford Encyclopedia of Philosophy, "Informal Logic." The authoritative survey of the discipline and its history.
2. Stanford Encyclopedia of Philosophy, "Fallacies." The systematic treatment of the theory of fallacies.
3. Internet Encyclopedia of Philosophy, "Critical Thinking." A practical overview of the applied tradition that informal logic grounds.