7 Deadly Logical Sins

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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.

Additional Resources

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  1. Hi J.D. – I like how you worded the part about logic not always being valued or influential. You nailed in in the part where you say, “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.”

    It’s not always necessary to win all arguments, is it? If we take time to listen to others we’re more apt to see the “big picture”.

  2. Here is the flip [good] side of “Fallacy of ignorance”.

    Here is the true experience I have gone through just today – someone accused me in public in something I have not done. I could go off and PROVE it is wrong. I have chosen to ignore it. Many approached me offering to “handle” it – i asked to ignore it.

    The case was closed as if it never existed. It actually has not – it was logical fallacy of the other guy so I have chosen to ignore it.

  3. J.D., great post – many of these are well recognised sales pitches. Obviously they have different names and are seen in a different light by the people using them.

    The one I dislike the most is Tautology…makes me feel as if someone is talking down to me when they do that. Like it will all make sense and fall into place if they can just find the right sequence of words to let it sink into my think skull. 😉

  4. Hi JD

    Very good explanation of it all. It’s quite frightening when you realise how many of these deadly sins one encounters daily!


  5. As Louisa, Tautology is the one that drives me crazy most, why do pepole feel they need to come in sideways with the same question or tenet, again and again?

  6. It can be useful to refer to how otehr people have evolved within themselves to acknowledge, explore and solve particular issues. How other people do thigns can be a helpful guide, yet people are encouraged to figure things out for themselves in ways that are not comparable to anyone else. This is the nature of a personal journey. It inspires courage.

  7. @ Barbara

    Right on. It’s about the big picture. Granted some people really enjoy logic, but usually people have some sort of emotional connection to information. It’s why they quickly believe it or they don’t. That’s why when they don’t believe it, logic doesn’t even help much. Having a shared frame of reference helps a lot because that’s where the emotion is.

    @ Alik

    Really good point. If you focused on it, you could have made a mountain of a molehill. There’s so many better ways to spend your energy and time and it sounds like you prioritized the right things.

    @ Louisa

    Thank you. I know what you mean. Even the name Tautology just sounds annoying. It’s too much like taunt or toot 😉

    @ Juliet

    Thank you. It’s eye opening. I’m still working on learning them, but it will take time. Some I run into way more than others.

    @ Jannie

    Kind of like the song, “You spin me right round, baby, Right round like a record, baby, Right round round round” … except, I like the song.

    @ Liara

    The beauty of personal journeys is no matter how many people have traveled the same path, each one is unique.

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