Informal Logic and Fallacies

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Inductive Reasoning
Turn from informal fallacies, which are flaws in the premises of an argument, to questions of validity, or the logical integrity of an argument. In this lecture, focus on four fallacies to avoid in inductive reasoning: selective evidence, insufficient sample size, unrepresentative data, and the gambler's fallacy.
Fallacies of Faulty Authority
Deepen your understanding of the fallacies of informal logic by examining five additional reasoning errors: appeal to authority, appeal to common opinion, appeal to tradition, fallacy of novelty, and arguing by analogy. Then test yourself with a series of examples, and try to name that fallacy!
An Introduction to Formal Logic
Logic is intellectual self-defense against such assaults on reason and also a method of quality control for checking the validity of your own views. But beyond these very practical benefits, informal logic--the kind we apply in daily life--is the gateway to an elegant and fascinating branch of philosophy known as…
Why Should We Trust Reason?
Hone your philosophical thinking by identifying the categories of fallacious reasoning that ensnare us all. Investigate examples of gut-thinking, confirmation bias, appealing to ignorance, the correlation fallacy, begging the question, and equivocation. Learn how to check your reasoning for flaws.
Fallacies of Irrelevance
Learn how to keep a discussion focused by recognizing common diversionary fallacies. Ad hominem attacks try to undermine the arguer instead of the argument. Straw man tactics substitute a weaker argument for a stronger one. And red herrings introduce an irrelevant subject. As in other lectures, examine fascinating cases of…
Fallacies of Cause and Effect
Consider five fallacies that often arise when trying to reason your way from cause to effect. Begin with the post hoc fallacy, which asserts cause and effect based on nothing more than time order. Continue with neglect of a common cause, causal oversimplification, confusion between necessary and sufficient conditions, and…
Three-Valued and Fuzzy Logic
See what happens if we deny the central claim of classical logic, that a proposition is either true or false. This step leads to new and useful types of reasoning called multi-valued logic and fuzzy logic. Wind up the course by considering where you've been and what logic is ultimately…
First-Order Predicate Logic
So far, you have learned two approaches to logic: Aristotle's categorical method and truth-functional logic. Now add a third, hybrid approach, first-order predicate logic, which allows you to get inside sentences to map the logical structure within them.
Truth Tables
Truth-functional logic provides the tools to assess many of the conclusions we make about the world. In the previous lecture, you were introduced to truth tables, which map out the implications of an argument's premises. Deepen your proficiency with this technique, which has almost magical versatility.
How Do We Reason Carefully?
Avoiding fallacious reasoning is just the beginning of philosophical thinking. Go deeper by studying the rules of deduction and induction. In the process, learn Aristotle's three axioms of logic, the difference between truth and validity, common mistakes in logical arguments, and why practically all scientific arguments are inductive.
Demonstrating Invalidity
Study two techniques for demonstrating that an argument in first-order predicate logic is invalid. The method of counter-example involves scrupulous attention to the full meaning of the words in a sentence, which is an unusual requirement, given the symbolic nature of logic. The method of expansion has no such requirement.
Truth Tables and Validity
Using truth tables, test the validity of famous forms of argument called modus ponens and its fallacious twin, affirming the consequent. Then untangle the logic of increasingly more complex arguments, always remembering that the point of logic is to discover what it is rational to believe.