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[ Music ]

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

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And in this video, I'm going to introduce[br]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[br]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[br]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[br]bit off to you, and you're right.

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

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A fallacy, as we've seen in other wireless[br]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[br]a mistake in my reasoning.

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

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Either way, it's probably a good[br]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[br]any defects in our reasoning.

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But before we talk about fallacy types,[br]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,[br]then she publishes articles on philosophy.

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So Tamar Gendler is a philosopher, therefore,[br]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[br]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,[br]and it can be represented as the following.

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

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And this is a valid argument form.

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The conclusion necessarily[br]follows from the premises.

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

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It would still be a valid argument if Tamar[br]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[br]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,[br]contains a fallacy, a formal fallacy.

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

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

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

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

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

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Given just the fact that she doesn't eat it and[br]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[br]represented in the following way.

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

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

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So if you're interested and want more[br]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[br]formal fallacies in upcoming videos.

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But for now, we need to see that[br]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[br]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[br]of formal fallacies, what about informal ones?

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

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Notice that this fallacy arises out of the[br]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[br]meaning of the content.

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Consider that we can use a seemingly similar[br]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[br]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[br]argument, given the two meanings of light --

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lightweight and light color[br]-- yields some problems.

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

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

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

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

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

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

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

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

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If we just looked at the form[br]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[br]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[br]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,[br]yet also commit a fallacy.

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

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while an informal fallacy has a[br]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[br]formal and informal fallacies.