Understanding Science 8. Logical Fallacies © Colin Frayn,

What is a Logical Fallacy? Structure of a deductive argument –“Premise 1” & “Premise 2” “Conclusion” Example: –“All dogs are mammals, Rover is a dog, therefore Rover is a mammal” Can be logically sound, but with false premises –“All dogs are reptiles, Rover is a dog …” Can be logically unsound –“All dogs are mammals, Rover is a dog, therefore Rover speaks fluent English” Can be a false argument, but still true –“All dogs are mammals, Tiddles is a cat …” Inductive argument –Based on previous experience (e.g. science, largely) © Colin Frayn,

Arguments from Ignorance Appeal to Ignorance (Ad Ignorantiam) –True because you can’t prove it’s false or vice versa –“Aliens built the pyramids” Unexplained therefore Unexplainable –“You can’t explain the origin of life, therefore creationism. Personal Incredulity –“I can’t believe it’s true” (see “Intuition”!) © Colin Frayn,

Personal Arguments Personal Attacks (Ad Hominem) –Argument is false because of person stating it “Hitler was a vegetarian” –Someone is evil/untrustworthy/nasty/rude Doesn’t follow that they are wrong Arguments should be evaluated on their own merits Argument from authority –Argument is true because of person stating it “Professor A said that this was true” … “and he has a Nobel prize…” Argument from Einstein / Newton © Colin Frayn,

Empirical Arguments Argument from consequences –“Darwinism leads to Nazism” –Truth is truth, independent of consequences Correlation/causation –“Taller people are cleverer” –Also racism / sexism / ageism © Colin Frayn,

Logical Errors False Deduction (Non sequitur) –Irrational steps in arguments, often hidden –Conclusion does not follow from premises –“All dogs are mammals, Rover is a dog, therefore Rover has two legs” Begging the question (circular reasoning) –Assume the thing you’re trying to prove Then use that to prove it! –Circular argumentation –“Radiometric dating is wrong because it gives ages of the Earth that are older than 10,000 years” –Tautology © Colin Frayn,

Comparisons False analogy –Use a false comparison to disprove an argument “Evolution is like a tornado in a junkyard assembling an aircraft by chance” False continuum –Rejection of demarcations within a continuum The “slippery slope” –Rejection of a continuum “There are no bald men” False dichotomy –“Big pharma is wholly evil or wholly good” –Disprove one, and assume the other is therefore true © Colin Frayn,

Sneaky Tactics Moving the goalposts –Common with psychics “My powers won’t work with skeptics” Straw Man –Make a fake caricature of your opponent’s point, and then disprove it “Evolution says that mud turns into people” No true Scotsman –Redefining categories to match the claim “Clearly they weren’t real psychics…” The “Gish Gallop” –Throw arguments out at lightning speed –Named after Duane Gish of the ICR © Colin Frayn,

Summary All good thinking is founded on logic –Deductive arguments –Inductive arguments Logical fallacies are common Recognising them helps to avoid bad arguments Good thinking is a skill –Keep working at it © Colin Frayn,