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>> 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.