[ Music ]

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

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

First, consider the following arguments.

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

Premise two: Jane doesn't eat peanut butter.

Conclusion: therefore Jane
is allergic to peanuts.

Consider this other argument.

Premise one: a feather is light.

Premise two: what's light cannot be dark.

Conclusion: therefore a feather cannot be dark.

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

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

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

This defect can be unintentional or intentional.

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

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

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

and identify distinct fallacy types

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

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

Consider this argument.

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

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

This is a valid argument.

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

over 20 articles, among her other works.

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

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

And this is a valid argument form.

The conclusion necessarily
follows from the premises.

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

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

for instance, instead of a philosopher.

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

We can also have an invalid argument form.

Let's return to our first argument.

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

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

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

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

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

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

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

that Jane is allergic to peanut butter.

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

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

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

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

check out the video on affirming the consequent.

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

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

because of the poor form of the argument.

Any argument of these forms will be invalid.

So anything can be plugged
into these bad argument forms

and the argument would be invalid.

For instance, if meow, then splat.

Splat, therefore meow, is invalid.

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

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

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

The form may at first glance seem valid.

That is if we ignored the
meaning of the content.

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

For instance, premise one: rain is wet.

Premise two: what's wet cannot be dry.

Conclusion: therefore rain cannot be dry.

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

and one that lacks defects in its content.

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

lightweight and light color
-- yields some problems.

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

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

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

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

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

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

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

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

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

then we could not detect this fallacy.

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

with the content of the arguments.

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

because of a defect in its content.

Hence, an argument might have true premises

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

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

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

Which might also yield a defect in its form.

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