← Think Like a Detective: Logic and Thinking Errors
Lesson
Lesson 3. More tricks and training your eye
Add more common fallacies (appeal to popularity, appeal to authority, slippery slope, “after this, therefore because of this”) and practice finding several fallacies in one argument.
Four common tricks
A quick field guide
— Appeal to popularity (ad populum). “Everyone thinks so / it's popular, so it's true.” But millions of people can be wrong: once “everyone” believed the Sun revolved around the Earth.
— Appeal to authority (a misplaced one). “A well-known/famous person said it, so it's right.” Authority is appropriate in its own field (a doctor on health), but a movie star is no expert on nutrition, and a diploma doesn't make every statement true. What matters is the reasons, not the title.
— Slippery slope. “If we allow A, we'll inevitably slide all the way to terrible Z.” The chain “A → B → … → disaster” is presented as unstoppable, although every link still has to be proven.
— After this, therefore because of this (post hoc). “B happened after A, so A caused B.” But “after” doesn't mean “because of”: the rooster crowed before dawn — it didn't make the sun rise. Coincidence in time proves nothing by itself.
These four round out the detective's arsenal. Next comes practice: one argument often contains several fallacies at once.
Lesson notes
A quick field guide
— Appeal to popularity (ad populum). “Everyone thinks so / it's popular, so it's true.” But millions of people can be wrong: once “everyone” believed the Sun revolved around the Earth.
— Appeal to authority (a misplaced one). “A well-known/famous person said it, so it's right.” Authority is appropriate in its own field (a doctor on health), but a movie star is no expert on nutrition, and a diploma doesn't make every statement true. What matters is the reasons, not the title.
— Slippery slope. “If we allow A, we'll inevitably slide all the way to terrible Z.” The chain “A → B → … → disaster” is presented as unstoppable, although every link still has to be proven.
— After this, therefore because of this (post hoc). “B happened after A, so A caused B.” But “after” doesn't mean “because of”: the rooster crowed before dawn — it didn't make the sun rise. Coincidence in time proves nothing by itself.
These four round out the detective's arsenal. Next comes practice: one argument often contains several fallacies at once.