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Logic Toolkit: Quick Reference
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A compact reference guide covering all major argument forms, fallacies, evidence evaluation criteria, and reasoning frameworks from the course.
Deductive (certainty): If premises are true and form is valid, conclusion must be true. Valid forms: modus ponens (If A then B; A; therefore B), modus tollens (If A then B; not B; therefore not A). Invalid forms: affirming the consequent (If A then B; B; therefore A), denying the antecedent (If A then B; not A; therefore not B).
Inductive (probability): Conclusion is probable based on evidence. Types: generalization (specific to general), analogy (similar cases), causal reasoning (correlation to causation). Strength depends on evidence quantity, quality, and representativeness.
Key distinctions: Valid = conclusion follows from premises. Sound = valid + true premises. Strong = inductive argument with well-supported conclusion. Cogent = strong + true premises.
Relevance fallacies: Ad hominem (attack the person), straw man (misrepresent the argument), red herring (change the subject), appeal to authority (cite non-expert), appeal to emotion (substitute feeling for evidence).
Weak induction fallacies: Hasty generalization (too little evidence), false cause (correlation as causation), slippery slope (unsupported chain of consequences), weak analogy (insufficient similarity).
Presumption fallacies: False dilemma (only two options presented), begging the question (conclusion assumed in premise), loaded question (contested assumption embedded), circular reasoning (premise and conclusion say the same thing).
Watch the whole toolkit applied to a single claim: "Every dictator I can name rose to power during an economic crisis, so economic crises cause dictatorships."
Step 1 - Name the form. This is inductive (a generalization from cases), not deductive. So we judge it by strength, not validity.
Step 2 - Test the inference. It reasons from a handful of remembered cases to a universal causal claim. That is the pattern of a hasty generalization, and the leap from correlation to cause is false cause.
Step 3 - Look for the missing set. The examples are the cases that fit; crises that produced no dictator (many democracies weathered the 1930s and 2008) are invisible to the reasoner. This is selection/survivorship bias.
| Feature | Diagnosis |
|---|---|
| Reasoning type | Inductive generalization |
| Sample | Small, self-selected (only remembered dictators) |
| Correlation vs cause | Asserts cause from co-occurrence - false cause |
| Counter-cases | Ignored - hasty generalization |
| Verdict | Weak; not cogent |
Step 4 - Rebuild it honestly. A defensible version would read: "Severe economic crises raise the probability of democratic breakdown, conditional on weak institutions - as comparative studies of interwar Europe suggest." Note how the honest version adds a probability, a condition, and a source. That is the difference between a slogan and an argument.
