Fallacies And Errors·Rigorous·5 lessons·65 practice activities·~340 min

Fallacies and Reasoning Errors

From fundamental inferential failures to specific species across modes of reasoning

Start the first lesson Back to curriculum

What you'll learn

By the end of this unit, you can…

Classify fallacy families.
Identify species by reasoning mode.
Repair fallacious reasoning.
Apply charitable reading before diagnosis.

Lessons

Lesson sequence

1
Fundamental Fallacy Families
Introduces a structural taxonomy of fallacies by fundamental inferential defect rather than by disconnected labels, and establishes the diagnosis-and-repair routine used throughout the unit.
15 activities5 worked examples
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2
Deductive and Formal Fallacies
Examines fallacies driven by formal structure, quantifier scope, class reasoning, and invalid deductive patterns, and shows why these errors demand structural rather than evidential repair.
18 activities5 worked examples
Student Pro
3
Inductive, Abductive, and Best-Explanation Fallacies
Shows how support miscalibration and option-space failure produce different species across evidence-based reasoning, and why good diagnosis has to track the mode of inference.
15 activities5 worked examples
Student Pro
4
Bayesian and Problem-Solving Fallacies
Extends the taxonomy to probabilistic updating and practical strategy selection, showing how base-rate neglect and option-space failure behave across both domains.
15 activities5 worked examples
Student Pro
5
Capstone: Diagnosing, Repairing, and Communicating Fallacies
An integrative lesson that asks students to run the full diagnostic cycle on arguments drawn from the wild: name the fallacy, explain the mechanism of failure, and propose a repair or a steelman when one is available.
2 activities1 worked example
Student Pro

How to study

Three moves that work for this unit

Read the explanation

Each lesson opens with a guided walkthrough — read it before the activity.

Study the worked example

Look at why each step follows, not just what the answer is.

Practice with the target in mind

Know which rule applies and what would make the response weak before you start.

Reference materials

Optional context for the unit. Each lesson surfaces the concepts and rules it uses — these are here when you want the bigger picture.

Concept map (7 terms)

Fundamental Fallacy

A broad family of inferential failure that can generate several more specific named fallacies.

Species of a Fallacy

A more specific recurring error pattern that instantiates a broader fundamental inferential defect.

Relevance Failure

A family of errors in which the offered consideration does not bear on the conclusion in the required way.

Support Miscalibration

A family of errors in which the degree or type of support claimed is stronger, weaker, or otherwise different from what the evidence actually warrants.

Scope or Structure Failure

A family of errors caused by misreading logical form, quantifier scope, class structure, or formal relation.

Option-Space Failure

A family of errors in which relevant alternatives, constraints, or strategy revisions are ignored.

Diagnosis and Repair

The practice of pairing any fallacy identification with a description of what change would make the reasoning acceptable.

Rules and standards (4)

Trace the Species Back to the Family. A correct fallacy diagnosis should identify both the specific named error and the broader inferential defect it exemplifies. Common failures: The learner memorizes labels without understanding the violated standard.; Two fallacies are treated as unrelated even though they share the same deeper defect..
Diagnose by the Mode of Reasoning. The same surface error term may need different analysis depending on whether the reasoning is deductive, inductive, abductive, Bayesian, or practical. Common failures: A probabilistic error is judged as though it were purely deductive.; A problem-solving failure is mislabeled with a rhetorical fallacy name while ignoring strategic structure..
Repair Rather Than Merely Label. A strong fallacy analysis should explain what inferential repair would be required for the argument to meet the relevant standard. Common failures: The fallacy is named but not explained.; No corrected version of the reasoning is offered..
Charitable Reading Before Fallacy Accusation. Before diagnosing a fallacy, reconstruct the strongest plausible version of the argument and check whether the error survives that reconstruction. Common failures: Attacking a careless phrasing instead of the best version of the argument.; Declaring a fallacy prematurely without checking for an implicit premise that would rescue the reasoning..

Formalization patterns (2)

Fallacy Taxonomy Map. From argument_or_reasoning_case to family_species_repair_analysis — Identify the mode of reasoning involved.; State the standard that should govern the case.; Determine the fundamental fallacy family violated.; Identify the most specific species if one applies.; State how the reasoning would need to be repaired..
Cross-Mode Fallacy Comparison. From paired_reasoning_cases to shared_family_different_species_analysis — Identify the reasoning mode in each case.; State the governing standard for each case.; Identify the shared fundamental family.; Explain how the species differ by context.; Compare the repairs required..

Full mastery and assessment guidance

Mastery requirements

Classify fallacy families. Successful Classifications · 20_successful_classifications
Identify species by reasoning mode. Percent Consistent · 80_percent_consistent
Repair fallacious reasoning. Successful Repairs · 10_successful_repairs
Apply charitable reading before diagnosis. In 8 Cases · demonstrated_in_8_cases

Assessment advice

What fundamental inferential defect is present here?
Which specific species best fits the case, and why does a rival label fit worse?
What repair would the argument need to meet the relevant standard?
Using a familiar label without explaining the deeper structural failure.
Skipping the repair step.
What formal standard is violated here?
Is the problem one of connective form, term distribution, or scope?
Would the argument be invalid even if every premise were true?
Describing a deductive failure as though it were just weak evidence.
Skipping the structural repair step.
Is the evidence strong enough for the degree of support claimed?
Were live alternatives compared before the conclusion was chosen?
Would a weaker conclusion be acceptable on the same evidence?
Treating the first plausible explanation as the best explanation.
Generalizing from a sample that is too small to matter.
Was the probability update calibrated to both the prior and the evidence?
Did the practical analysis consider relevant constraints and alternatives?
What would tell the reasoner the plan or belief needs to be revised?
Assuming a practical plan is good merely because it is decisive.
Treating test accuracy as a direct probability of the hypothesis.
Did I read the argument charitably before naming a fallacy?
Did I explain the specific mechanism of failure?
Is my repair proposal faithful to the original topic?
Using fallacy names as rhetorical weapons rather than diagnostic tools.
Assuming that an argument you disagree with must be fallacious.

Historical context (3)

Aristotle. In Sophistical Refutations, catalogued recurrent deceptive patterns in argument and distinguished apparent refutations from genuine ones. Structured fallacy diagnosis and the distinction between apparent and genuine reasoning.
John Stuart Mill. Analyzed inductive fallacies in A System of Logic and distinguished causal reasoning errors from purely deductive ones. Modern categorization of inductive and causal fallacies alongside deductive ones.
C. L. Hamblin. In Fallacies (1970), critiqued simplistic textbook treatments and pushed for context-sensitive, standards-based analysis of real reasoning. The contemporary approach of diagnosing fallacies by the standards of the relevant reasoning mode rather than by stock examples.