7 Deadly Logical Sins
In my previous post, I wrote about 3 ways to spot logical fallacies. In this post, I write about the families and examples of logical sins according to lessons from Jay Henirchs. In Thank You for Arguing: What Aristotle, Lincoln, and Homer Simpson Can Teach Us About the Art of Persuasion , Heinrichs teaches us about the seven deadly logical sins. In a logical debate, committing a logical sin gets you kicked out of the game.
Key Take Aways
Here’s my key take aways:
- 3 kinds of logic flaws. You can boil logic flaws down to three kinds: 1) bad proof 2) bad conclusion and 3) disconnect between proof and conclusion.
- 7 deadly logical sins. you can think of the 7 deadly logical sins as examples of the 3 kinds of logical flaws.
- Know when logic matters and when to let it go. I’m a fan of logic, but not for logic’s sake. I care about logic when I need to make a better decision based on facts. I also care about logic when I need to influence a logical argument.
- Remember that logic is not always valued and it’s not always influential. You need to read the room or situation to know whether it’s important. If you’re really good at splitting hairs, you might find yourself winning more arguments, but losing more wars or getting stuck in the weeds.
When logic does matter, I find the frame and examples Heinrichs uses are a great tool. They make it easier to look for flaws to find bad information, make more informed decisions, and build better arguments.
The Seven Deadly Logical Sins
Heinrichs names the following logic flaws:
- First Deadly Sin: The False Comparison
- Second Deadly Sin: The Bad Example
- Third Deadly sin: Ignorance as Proof
- Fourth Deadly Sin: The Tautology
- Fifth Deadly Sin: The False Choice
- Sixth Deadly Sin: The Red Herring
- Seventh Deadly Sin: The Wrong Ending
First Deadly Sin: The False Comparison
According to Heinrichs, the false comparison is the first deadly logical sin. Here’s some examples of the false comparison
- All natural fallacy. Just because something is all natural, it doesn’t mean it’s good for you.
- Appeal to popularity. “Everybody’s doing it.” Because all the other kids get to, I should, too.
- Reductio ad absurdum. This is about reducing an argument to absurdity by using an absurd comparison. The premise isn’t believable.
- Fallacy of antecedent. It’s never happened before, so it will never happen. Heinrichs provides a few responses … “That’s a long time to tease fate” … or, “Your karma must be terrible.”
- False analogy. I can do this well, so I can do that unrelated thing just as well. Heinrichs gives an example, “I’m a successful business man. Elect me mayor and I’ll run a successful city.”
- Unit fallacy. One apple plus one orange equals two apples. Think of how big the pie is and whether you are dealing with a slice of the pie or a slice of a slice. If your percentages add up to more than 100% of the pie, something is off.
All Natural Fallacy
Heinrichs provides an example of the all natural fallacy:
Plums and grapes are purple, but they don’t make purple a fruit. You need not be an Aristotle to figure that one out. But how many consumers have fallen for the same kind of fallacy? Made with all natural ingredients. It may not seem like it, but the “all natural” pitch commits the “purple is a fruit” error: because an ingredient belongs to the same group as things that are good for you (natural substances, purple fruit), the ingredient must also be good for you. But botulism is natural, too, and not at all good for you. (not to mention the sneaky syntax that implies a hyphen between “all” and “natural.” Add a gram of grape pulp and a gram of wheat germ to a doughnut’s chemical blend and voila! All-natural ingredients. Two all-natural ingredients, to be exact.
Second Deadly Sin: The Bad Example
According to heinrichs, the bad example is the second deadly sin. Here are some examples:
- Misinterpreting the evidence. The examples don’t support the conclusion.
- Hasty generalization. The argument offers too few examples to prove the point. To put it another way, the sample size is too small.
Third Deadly sin: Ignorance as Proof
According to Heinrichs, ignorance as proof is the third deadly logical sin. Here’s an example:
- Fallacy of ignorance. If we can’t prove it, then it must not exist. Or if we can’t disprove it, then it must exist.
Fallacy of Ignorance
Heinrichs illustrates fallacy of ignorance with an example:
Scientists and doctors often screw up logic by assuming that their examples cover all possible examples – a mistake appropriately called the fallacy of ignorance: what we cannot prove, cannot exist.
Doctor: There’s nothing wrong with you. The lab tests came back negative.
Proof: The lab tests are all negative. So …
Conclusion: Nothing is wrong with you.
But a logical chasm lies between the negative tests and perfect health. The proof doesn’t support the conclusion. Never mind that you happen to be doubled over in pain and seeing spots; the doctor has no data of illness, so you must be well. The only way to respond to this illogical argument, other than throwing up on their shoes, is to suggest more examples.
Fourth Deadly Sin: The Tautology
According to Heinrichs, Tautology is the fourth deadly logical sin:
- Tautology. The same thing gets repeated in different words. The proof and conclusion agree because they are the same thing. Heinrichs provides an example,: “The Cowboys are favored to win since they’re the better team.” There’s no new information, just conclusion.
Fifth Deadly Sin: The False Choice
According to Heinrichs, the false choice is the fifth deadly logical sin. Here are some examples:
- Many questions. Two or more issues get squashed into one, so that a conclusion proves another conclusion.
- False dilemma. You’re being given only a few choices when really there are many.
- Complex cause. Only one cause gets the blame (or credit) for something that has many causes.
Sixth Deadly Sin: The Red Herring
According to Heinrichs, the sixth deadly logical sin is the red herring. Here are some examples:
- The red herring. It’s a distraction from the real argument. It switches issues in midargument to throw the audience off the scent.
- The straw man fallacy. It switches topics to one that’s easier to fight. It’s a variation off the red herring.
Seventh Deadly Sin: The Wrong Ending
According to Heinrichs, the seventh deadly logical sin is the wrong ending. Here are some examples:
- Slippery slope. If you allow this reasonable thing, it’s inevitably lead to an extreme version of it.
- Post hoc ergo proper hoc (the Chanticleer fallacy.) After this, therefore because of this.
For more information, check out the following:
- Thank You for Arguing: What Aristotle, Lincoln, and Homer Simpson Can Teach Us About the Art of Persuasion
- Jay Heinrich’s blog
- Logical Fallacies and the Art of Debate
My Related Posts