7. Informal Fallacies - An Introduction

 

Summary

This session begins a multi-part tour of informal fallacies. Normally a logic course teaches formal (deductive) logic first, but the order is reversed here because informal fallacies are immediately applicable: they are the errors you see constantly in articles, news broadcasts, and everyday conversation. The lecture opens with a list of flawed sample arguments (e.g. "she's blonde, so she must be stupid"; "same-sex marriage should be illegal because most people are against it") to revisit and diagnose once the categories are learned.

Formal vs. informal logic

• Formal logic concerns the form or structure of an argument. Deductive arguments are judged valid/invalid and sound/unsound; with a correct form and true premises, the conclusion is certain.
• Informal logic concerns the matter or content. Inductive (and scientific) arguments are never valid or sound, only strong or weak, and their conclusions are at best highly probable, never certain.

What a fallacy is

A fallacy is a mistake in reasoning that often looks correct until examined. Any fallacious argument is a non sequitur ("it does not follow"): the conclusion does not follow from its premises, so the argument must be rejected.

Two kinds of fallacy

• Formal fallacies – errors in the structure of an argument, even when the premises are individually fine. Examples: the four-term fallacy in categorical syllogisms (a valid syllogism needs exactly three terms, usually failing for lack of a linking middle term), and denying the antecedent in a hypothetical syllogism ("if it's a circle, it's a geometric figure; it's not a circle", from which nothing follows).
• Informal (material) fallacies – errors in the content, arising from unclear meaning, irrelevance, or unsupported presumption.

The three categories of informal fallacy

The course organizes informal fallacies into three families (a "fallacy family tree" descending from fallacy → formal/informal → the three categories):

1. Fallacies of ambiguity – the argument is vague or imprecise; terms are misused or grammar is faulty, making the content incoherent or inaccurate.
2. Fallacies of relevance – a premise has little or nothing to do with the conclusion. Three subdivisions:
   • Attacking the source (ad hominem) – attacking the arguer instead of the argument.
   • Appeal to emotion – swaying opinion through pity, anger, fear, or desire, all irrelevant to truth.
   • Distraction – shifting focus by changing the subject or proving something beside the point.
3. Fallacies of presumption – the argument rests on a hidden, unsupported, or questionable assumption. These split into non-causal (general unsupported assumptions) and causal / inductive ones that interfere with a reliable inductive process.

Caveats

Textbooks organize and name these fallacies differently, and a single argument often commits more than one fallacy at once, so two careful analysts may legitimately spot different errors in the same example.

The next episode begins with the first and smallest (and often most humorous) family: the fallacies of ambiguity.