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Critical Thinking Fallacy: Denying The Antecedent

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    (intro music)
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    Hello, I'm Matthew Harris.
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    I'm a philosophy graduate
    student at Duke University,
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    and today I'll be discussing
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    the formal fallacy of
    denying the antecedent.
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    Denying the antecedent
    is a formal fallacy,
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    meaning that the argument has a flaw
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    contained in its logical form.
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    This is important because
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    whenever this pattern of argument occurs,
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    regardless of topic or content,
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    the argument will always be invalid.
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    So how can we tell when
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    the fallacy of denying
    the antecedent occurs?
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    Well, it happens when we mistake
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    the direction of a conditional,
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    or confuse it for a biconditional.
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    And it starts with the denial of
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    the conditional statement's antecedent,
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    then concludes the
    denial of its consequent.
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    The logical form of arguments that commit
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    the fallacy of denying the
    antecedent look like this:
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    "If P, then Q.
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    "Not P. Therefore, not Q."
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    Now, let's take a look
    at this conditional:
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    "If you are a ski instructor,
    then you have a job."
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    The antecedent statement
    of this conditional
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    is "you are a ski instructor,"
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    and the consequent is "you have a job."
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    But suppose someone made an argument
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    with this conditional
    as its first premise.
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    Premise (1): If you are a ski
    instructor, then you have a job.
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    Premise (2): But you are
    not a ski instructor.
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    Conclusion: Therefore,
    you do not have a job.
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    Here, the second premise is
    a denial of the antecedent.
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    This premise does not tell us
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    that only ski instructors have jobs.
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    So, even if the conditional
    statement is true
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    (that ski instructors have jobs),
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    it cannot be inferred that if
    you are not a ski instructor,
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    then you are unemployed.
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    A conditional could validly
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    be used to argue for the
    truth of this consequent
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    by affirming the antecedent.
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    We find this in the arguments
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    of a form called "modus ponens."
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    It is also valid to argue from
    the denial of a consequent
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    to a denial of the antecedent.
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    But it is never, ever valid
    to deny the antecedent
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    to reject its consequent.
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    Let's try another example:
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    "If you are a property
    owner, then you are a human.
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    "But you are not a property owner.
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    "Therefore, you are not a human."
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    The antecedent, that you are
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    a property owner, is being denied.
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    Even though you need to be
    a human to own property,
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    this has no bearing on humans
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    who do not own property at all.
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    For example, graduate students.
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    Let's consider one last example:
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    "If anyone is watching this video,
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    "then they are on the internet.
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    "Some people are not watching this video.
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    "Therefore, they are not on the internet."
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    Again, denying the antecedent
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    by pointing out that not everyone
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    is currently watching this video
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    does not validly demonstrate
    the denial of the consequent,
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    that they're not on the internet at all.
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    These have been a few cases
    that I hope will come in handy
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    in avoiding this formal
    fallacy in your own arguments.
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    For more related to the fallacy
    of denying the antecedent,
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    I recommend that you take a
    look at the other related videos
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    on informal and formal fallacies,
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    the fallacy of affirming the
    consequent, and conditionals.
Title:
Critical Thinking Fallacy: Denying The Antecedent
Description:

In this video, Matthew C. Harris (Duke University) explains the fallacy of denying the antecedent, the formal fallacy that arises from inferring the inverse of a conditional statement. He also explains why graduate students might also be humans.

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Video Language:
English
Duration:
03:37

English subtitles

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