Fallacies: Affirming the Consequent

0:00(intro music) Hello, I'm Matthew Harris, and I'm a philosophy grad[br]student at Duke University. And today, I'll be discussing[br]the formal fallacy of affirming the consequent, and why you sometimes cannot conclude that you should bathe[br]in a tub of peanut butter.

0:16Affirming the consequent occurs when someone tries to infer the truth of the antecedent of a[br]conditional statement from the truth of the[br]conditional and its consequent. But let's see what this means in more detail. There are two kinds of logical fallacies: formal and informal.

0:30Both kinds are defective[br]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[br]with affirming the consequent that they have defects in[br]the forms of the argument and that they are invalid.

0:47Just to be clear, let's go[br]over a few more definitions. We make conditional[br]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,"[br]whether or not they're at the beginning, middle[br]or end of sentences.

1:07If 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.

1:26So let's take the following[br]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[br]that if the neighbors had eaten the parrot, then[br]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.

1:51Perhaps 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[br]follow from the conjunction of the true conditional[br]and the true consequent that the antecedent is true.

2:07Let'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.

2:27Maybe he just doesn't know any better way to get attention. Now, let's take a look[br]at one more example. Consider this conditional[br]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[br]allergic to peanut butter."

2:50Just because it is true that it would be a bad idea to bathe in[br]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[br]peanut butter in general, does not mean that you are[br]allergic to peanut butter. If you were to conclude this, then you would be committing the fallacy of affirming the consequent.

3:07So that's the formal fallacy of affirming the consequent, and a few examples that you[br]could use in the future. Subtitles by the Amara.org community