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4.5.3: Fallacies of Weak Induction

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    Post hoc ergo propter hoc

    (Latin for “After this, therefore because of this”) This fallacy happens when one argues that because X happened immediately after Y, that Y was the cause of X. Or, when concerning event types: event type X happened immediately after event type Y, therefore event type Y caused event type X. In a sense, it is jumping to a conclusion based upon coincidence, rather than on sufficient testing, repeated occurrence, or evidence.

    Example: The sun always rises a few minutes after the rooster crows. So, the rooster crowing causes the sun to rise.

    Example: Once the government passed the new gun laws, gun violence dropped by 10%, therefore the new gun laws are working and caused the occurrence of gun violence to drop.

    Fallacy of Composition

    (Also known as exception fallacy) The fallacy of assuming that when a property applies to all members of a class, it must also apply to the class as a whole.

    Example: Every player in the NHL is wealthy; therefore, the NHL must be a wealthy organization.

    However, this type of reasoning is not always fallacious: Every scene in that movie is hilarious, so the movie is hilarious.

    Fallacy of Division

    (Also known as false division, or faulty division)

    The fallacy of assuming that when a property applies to the class as a whole, it must also apply to every member of that class as well.

    Example: The US Republican Party platform states that abortion is wrong and should be illegal. Therefore, every Republican must believe that abortion is wrong and should be illegal.

    However, this type of reasoning is not always fallacious: That sculpture of Pee Wee Herman is made of

    metal, so his hand is made of metal.

    Hasty Generalization

    (Also known as argument from small numbers, unrepresentative sample) This fallacy occurs most often in the realm of statistics. It happens when a conclusion or generalization is drawn about a population and it is based on a sample that is too small to properly represent it. The problem with a sample that is too small is that the variability in a population is not captured, so the conclusion is inaccurate.

    Example: My Grandfather drank a bottle of whiskey and smoked three cigars a day, and he lived to be 95 years old. Therefore, daily smoking and drinking cannot be that bad for you.

    Begging the Question

    (Latin: Petitio Principii) The fallacy of attempting to prove something by assuming the very thing you are trying to prove. Essentially, in order for one of the premises to be true, the conclusion must already be true. This is very similar to a circular argument (see below), but it is subtly different.

    Example: The Bible is the word of God, God never lies, and the Bible says God exists, so God must exist. (Note: In order for the Bible to be the word of God, you must assume God exists – but isn’t that your conclusion?)

    Example #2: It’s always wrong to murder human beings. Capital punishment involves murdering human beings. Therefore, capital punishment is wrong.

    Note that this is NOT “raising the question” as “begging the question” is often used:

    “He always carries around a knife, that begs the question, what he is scared of?”

    Circular argument

    The fallacy of proving something that you’ve already assumed. Basically, your conclusion has already appeared as an assumption.

    Example: All of the statements in Smith’s book Crab People Walk Among Us are true. Why, he even says in the preface that his book only contains true statements and firsthand stories.


    A self-sealing argument is one that cannot possibly be wrong for one reason or another. Definitional arguments or claims and conspiracy theories are perfect examples.

    Example (“true by definition”): The following is a clear example of a self-sealing statement: Two weeks from today at 4:37 PM you are going to be doing exactly what you will be doing.

    Example #2 (“universal discounting”): Aliens exist and the only reason we don’t think they exist is because they’re making us believe that.

    So how do you deal with self-sealers? The best way is to just ignore them or not start an argument with someone who uses them in the first place. Also called “going upstairs” because instead of staying downstairs and speaking with a highly irrational person, you’d rather just go upstairs, put your headphones, and listen to some Nickelback. This would appear to be the only situation where listening to Nickelback is preferable to the alternative. (Am I committing fallacy here in this line of reasoning?)

    This page titled 4.5.3: Fallacies of Weak Induction is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Noah Levin (NGE Far Press) .