Fallacies: Affirming the consequent

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Fallacies: Affirming the consequent

(intro music) Hello, I'm Matthew Harris, and I'm a philosophy grad student at Duke University.

And today, I'll be discussing the formal fallacy of affirming the consequent, and why you sometimes cannot conclude that you should bathe in a tub of peanut butter.

Affirming the consequent occurs when someone tries to infer the truth of the antecedent of a conditional statement from the truth of the conditional and its consequent.

But let's see what this means in more detail.

There are two kinds of logical fallacies: formal and informal.

Both kinds are defective argumentative patterns.

First, we have informal fallacies, which lack support for the conclusion because of a flaw in its content.

We also have formal fallacies, which all have in common with affirming the consequent that they have defects in the forms of the argument and that they are invalid.

Just to be clear, let's go over a few more definitions.

We make conditional statements all the time.

They're generally easy to spot because they usually are of the form "if P, then Q."

Here, "P" is the antecedent. An easy way to spot antecedents is to remember that they typically come after the word "if," whether or not they're at the beginning, middle or end of sentences.

If you need help remembering that, just remember that the antecedent comes before the other logically, and that it sounds a lot like "ancestor."

The consequent of the conditional is the part that typically follows after the word "then."

It should be easy to remember because it sounds like "consequence" and basically is just that.

So let's take the following conditionals for examples.

Suppose someone tells you the following true conditionals and statement: "If the neighbors ate Susan's parrot, "then Susan is angry," and "Susan is angry."

Just because it is true that if the neighbors had eaten the parrot, then she would have been angry, and it is also true that she is angry, does not mean that she's angry because they ate her parrot.

Perhaps she's mad because her parrot isn't very interesting.

Or maybe she's angry that it doesn't know how to use the toy car that she spent all afternoon building for it.

Nevertheless, it does not follow from the conjunction of the true conditional and the true consequent that the antecedent is true.

Let's look at a few more examples: "If Tom has a good reason to complain, "then Tom will complain tomorrow."

Now, maybe you know Tom well, so you know that this is true.

Maybe you even know that it's true that he will complain tomorrow.

But it would not follow that Tom has a good reason to complain.

Maybe he just doesn't know any better way to get attention.

Now, let's take a look at one more example.

Consider this conditional and the assertion: "If you are allergic to peanut butter, "then it would be a bad idea "to bathe in a tub of peanut butter," and "it is a bad idea to bathe "in a tub of peanut butter;

therefore, you are allergic to peanut butter.

Just because it is true that it would be a bad idea to bathe in a tub of peanut butter if you are allergic, and it is also true that it is a bad idea to bathe in a tub of peanut butter in general, does not mean that you are allergic to peanut butter.

If you were to conclude this, then you would be committing the fallacy of affirming the consequent.

So that's the formal fallacy of affirming the consequent, and a few examples that you could use in the future.

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Fallacies: Affirming the consequent발음듣기

(intro music) Hello, I'm Matthew Harris, and I'm a philosophy grad student at Duke University.발음듣기

And today, I'll be discussing the formal fallacy of affirming the consequent, and why you sometimes cannot conclude that you should bathe in a tub of peanut butter.발음듣기

Affirming the consequent occurs when someone tries to infer the truth of the antecedent of a conditional statement from the truth of the conditional and its consequent.발음듣기

But let's see what this means in more detail.발음듣기

There are two kinds of logical fallacies: formal and informal.발음듣기

Both kinds are defective argumentative patterns.발음듣기

First, we have informal fallacies, which lack support for the conclusion because of a flaw in its content.발음듣기

We also have formal fallacies, which all have in common with affirming the consequent that they have defects in the forms of the argument and that they are invalid.발음듣기

Just to be clear, let's go over a few more definitions.발음듣기

We make conditional statements all the time.발음듣기

They're generally easy to spot because they usually are of the form "if P, then Q."발음듣기

Here, "P" is the antecedent. An easy way to spot antecedents is to remember that they typically come after the word "if," whether or not they're at the beginning, middle or end of sentences.발음듣기

If you need help remembering that, just remember that the antecedent comes before the other logically, and that it sounds a lot like "ancestor."발음듣기

The consequent of the conditional is the part that typically follows after the word "then."발음듣기

It should be easy to remember because it sounds like "consequence" and basically is just that.발음듣기

So let's take the following conditionals for examples.발음듣기

Suppose someone tells you the following true conditionals and statement: "If the neighbors ate Susan's parrot, "then Susan is angry," and "Susan is angry."발음듣기

Just because it is true that if the neighbors had eaten the parrot, then she would have been angry, and it is also true that she is angry, does not mean that she's angry because they ate her parrot.발음듣기

Perhaps she's mad because her parrot isn't very interesting.발음듣기

Or maybe she's angry that it doesn't know how to use the toy car that she spent all afternoon building for it.발음듣기

Nevertheless, it does not follow from the conjunction of the true conditional and the true consequent that the antecedent is true.발음듣기

Let's look at a few more examples: "If Tom has a good reason to complain, "then Tom will complain tomorrow."발음듣기

Now, maybe you know Tom well, so you know that this is true.발음듣기

Maybe you even know that it's true that he will complain tomorrow.발음듣기

But it would not follow that Tom has a good reason to complain.발음듣기

Maybe he just doesn't know any better way to get attention.발음듣기

Now, let's take a look at one more example.발음듣기

Consider this conditional and the assertion: "If you are allergic to peanut butter, "then it would be a bad idea "to bathe in a tub of peanut butter," and "it is a bad idea to bathe "in a tub of peanut butter;발음듣기

therefore, you are allergic to peanut butter.발음듣기

Just because it is true that it would be a bad idea to bathe in a tub of peanut butter if you are allergic, and it is also true that it is a bad idea to bathe in a tub of peanut butter in general, does not mean that you are allergic to peanut butter.발음듣기

If you were to conclude this, then you would be committing the fallacy of affirming the consequent.발음듣기

So that's the formal fallacy of affirming the consequent, and a few examples that you could use in the future.발음듣기

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