Logic and fallacies
Good arguments rest on sound reasoning. Recognising logical fallacies protects you from weak arguments and keeps your own reasoning honest.
Deductive reasoning
Deductive reasoning moves from general premises to a specific conclusion: 'All mammals breathe air; whales are mammals; therefore whales breathe air.' If the premises are true and the logic valid, the conclusion must be true. In everyday arguments, premises are often contestable.
Inductive reasoning
Inductive reasoning moves from specific observations to a general conclusion: 'Every swan I have seen is white, so swans are white.' Conclusions are probable, not certain, and can be overturned by new evidence (black swans exist in Australia). Most research arguments are inductive.
What a fallacy is
A fallacy is an error in reasoning that makes an argument weaker than it appears. Fallacies can be persuasive because they exploit emotions or shortcuts. Spotting them helps you evaluate arguments and avoid undermining your own credibility.
Fallacies of relevance
Ad hominem attacks the person instead of their argument ('Why listen to her climate claims, she drives a car?'). A straw man misrepresents an opponent's view to make it easier to attack. Appeal to emotion substitutes feelings for evidence. Red herring introduces an irrelevant issue to distract.
Fallacies of weak induction
Hasty generalisation draws a broad conclusion from too little evidence ('My two friends hated the course, so it's terrible'). False cause (post hoc) assumes that because one thing followed another, the first caused it. Slippery slope claims, without evidence, that one step will inevitably lead to extreme consequences.
Fallacies of presumption
False dilemma presents only two options when more exist ('Either we ban cars or the planet is doomed'). Begging the question assumes what it should prove ('This law is unjust because it is unfair'). Bandwagon argues something is right because many people believe it.
Appeals to authority
Citing experts is legitimate when they are genuine experts in the relevant field and represent expert consensus. It becomes a fallacy when the 'authority' lacks relevant expertise (a celebrity endorsing medical treatments) or when one expert's view is presented as settled against a broader consensus.
Worked example
'Since the new principal arrived, test scores have fallen. Clearly her policies are hurting students. Anyone who disagrees just doesn't care about kids.' This contains false cause (scores may have fallen for other reasons, such as a harder test) and an ad hominem-style attack on those who disagree. A stronger argument would compare similar schools and examine specific policies.
Key takeaways
Deduction gives certain conclusions from true premises; induction gives probable conclusions from evidence. Fallacies are reasoning errors that seem persuasive. Common fallacies include ad hominem, straw man, red herring, hasty generalisation, false cause, slippery slope, false dilemma, begging the question, bandwagon, and misused authority. Avoiding them strengthens your credibility.
Flashcard
General premises to specific conclusion
Check your understanding
'If we let students redo one test, soon they'll expect to redo everything and grades will mean nothing.' This is an example of: