V · Logic in language
Informal logic
logica informalis · ‘reasoning examined in its natural habitat’
Most argument is not symbolic. It happens in natural language, with implicit premises, contested meanings, and interested parties on either side. Informal logic studies it as it actually occurs.
Orientation
What ‘informal’ means
Informal logic is not the absence of rigour. It is the study of arguments whose validity cannot be decided by their syntactic form alone. The premises of a real political argument depend on contested empirical claims; the warrant connecting them to the conclusion is often unstated; the parties to the argument disagree about which warrants the field allows. None of that is captured by formal logic.
The discipline has two principal branches. The first is fallacy theory — the catalogue of recurring failure modes that informal arguments are prone to. The second is argumentation theory — the constructive side, concerned with the structure of arguments as they unfold in discourse rather than in isolation.
Comparison
Formal and informal, side by side
| Concern | Formal | Informal |
|---|---|---|
| Object | Inference patterns as syntactic structures. | Arguments as they appear in language. |
| Decides validity by | The form of the argument; content does not enter. | Content, context, and the relation between speakers. |
| Treats premises as | Fixed propositions, taken as given. | Defeasible, contested, sometimes unstated. |
| Strengths | Precision; decidability; reusability across content. | Reach into the arguments people actually make. |
| Characteristic failure | Cannot decide whether the premises are true or relevant. | Susceptibility to fallacy; vagueness about which rules of the game apply. |
The two branches
Where to go from here
Fallacies catalogue what goes wrong. Argumentation describes what is supposed to happen. The two together cover the field as it is taught.
- I
Fallacies
A catalogue of recurring failure modes. Three formal fallacies plus four families of informal ones — of relevance, defective induction, presumption, and ambiguity. Each entry: form, example, why it fails.
Visit - II
Argumentation
How arguments actually proceed when more than one person is involved. The Toulmin model, dialectic, pragma-dialectics, and the rhetorical triangle.
Visit
Lineage
Where this discipline comes from
Fallacy theory is older than logic in the modern sense. Aristotle's Sophistical Refutations(c. 350 BC) is the founding text — a catalogue of the tricks used by the Sophists, organised by the kind of mistake they exploit. The basic taxonomy (relevance, ambiguity, presumption, defective induction) is a refinement of Aristotle's.
Argumentation theory in its modern form begins with Stephen Toulmin's The Uses of Argument (1958), which criticised formal logic for being silent about the arguments people actually make in law, ethics, and everyday discourse. Pragma-dialectics (van Eemeren and Grootendorst, 1984) added a normative theory of how disputes ought to be conducted. The two strands together constitute contemporary informal logic.