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[ Music ]

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>> Hello. I'm Paul Henne, and I'm a
philosophy gradate student at Duke University.

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And in this video, I'm going to introduce
you to formal and informal fallacies.

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First, consider the following arguments.

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Premise one: if someone is allergic to
peanuts, then she doesn't eat peanut butter.

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Premise two: Jane doesn't eat peanut butter.

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Conclusion: therefore Jane
is allergic to peanuts.

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Consider this other argument.

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Premise one: a feather is light.

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Premise two: what's light cannot be dark.

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Conclusion: therefore a feather cannot be dark.

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These arguments probably seemed a
bit off to you, and you're right.

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Each argument contains a fallacy,
though each has a different kind.

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A fallacy, as we've seen in other wireless
philosophy videos, is a defect in reasoning.

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This defect can be unintentional or intentional.

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That is, I could simply make
a mistake in my reasoning.

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Or I could want to trick you into believing
something by using deceptive reasoning.

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Either way, it's probably a good
idea that we are able to understand

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and identify distinct fallacy types

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so that we can adequately correct
any defects in our reasoning.

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But before we talk about fallacy types,
let's recall what a valid argument is.

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Consider this argument.

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Premise one: if someone is a philosopher,
then she publishes articles on philosophy.

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So Tamar Gendler is a philosopher, therefore,
Tamar Gendler publishes articles in philosophy.

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This is a valid argument.

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Actually, although it isn't bearing on this
argument's validity, Gendler has published

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over 20 articles, among her other works.

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This form of argument is called modus ponens,
and it can be represented as the following.

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Premise one: if X, then Y. Premise
two: X. Conclusion: therefore Y.

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And this is a valid argument form.

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The conclusion necessarily
follows from the premises.

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Remember though, this valid form does
not mean that the premises are true.

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It would still be a valid argument if Tamar
Gendler had decided to become a train conductor,

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for instance, instead of a philosopher.

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See the videos on soundness and validity
if you have questions about this.

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We can also have an invalid argument form.

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Let's return to our first argument.

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This argument, as you might have suspected,
contains a fallacy, a formal fallacy.

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And a formal fallacy is exactly what it sounds
like: a defect in the form of the argument.

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In other words, there are certain
forms of arguments that are invalid.

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This invalid argument form is a formal
fallacy called affirming the consequent.

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This fallacy is apparent if you consider
that Jane just might not like peanut butter.

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So simply because Jane doesn't like
peanut butter, she doesn't eat it.

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Given just the fact that she doesn't eat it and
the condition in premise one, we cannot conclude

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that Jane is allergic to peanut butter.

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This form of fallacy can be
represented in the following way.

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If X, then Y. Y, therefore X. And this form
of fallacy is one that you don't want to use.

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It's actually one of the fallacies
frequently tested on the LSAT.

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So if you're interested and want more
practice in identifying this fallacy,

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check out the video on affirming the consequent.

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We will talk more about this and other
formal fallacies in upcoming videos.

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But for now, we need to see that
formal fallacies are fallacies

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because of the poor form of the argument.

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Any argument of these forms will be invalid.

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So anything can be plugged
into these bad argument forms

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and the argument would be invalid.

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For instance, if meow, then splat.

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Splat, therefore meow, is invalid.

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So now that we have a general understanding
of formal fallacies, what about informal ones?

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Lt's look at our other example
from the beginning of this video.

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Notice that this fallacy arises out of the
content of the argument, not out of the form.

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The form may at first glance seem valid.

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That is if we ignored the
meaning of the content.

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Consider that we can use a seemingly similar
argument form to make a valid argument.

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For instance, premise one: rain is wet.

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Premise two: what's wet cannot be dry.

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Conclusion: therefore rain cannot be dry.

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And here we have a valid argument that seems
to have a similar form as the fallacious one,

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and one that lacks defects in its content.

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But the content of our fallacious
argument, given the two meanings of light --

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lightweight and light color
-- yields some problems.

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That is, if we understand the meaning of
the terms -- the two meanings of light,

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for instance -- we realize that the argument
actually has an invalid argument form.

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Initially, without considering the meaning
of the content, it looks like the form

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of the argument is something like, all X are
Y. What is Y cannot be Z. Therefore no X are Z.

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But actually, given the shifting
meaning of light, the form is more like,

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all X are Y. What is W cannot be Z. Therefore
no X are Z. This fallacy is called equivocation.

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And see the video on this if
you'd like to know more about it.

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The only way we can understand this defect
is by examining the content of the argument.

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If we just looked at the form
without understanding the content,

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then we could not detect this fallacy.

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So as we now know, informal
fallacies occur because of problems

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with the content of the arguments.

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So an argument might have a seemingly
valid form but commit an informal fallacy

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because of a defect in its content.

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Hence, an argument might have true premises

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and a seemingly valid form,
yet also commit a fallacy.

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To reiterate, a formal fallacy means that
the argument has a defect in its form,

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while an informal fallacy has a
defect in the argument's content.

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Which might also yield a defect in its form.

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So now that's the difference between
formal and informal fallacies.