Thursday, November 20, 2008

Logic and Rhubarb Pie

We're currently studying informal fallacies in my Intro to Logic class. On the coming exam students will have to identify whether an argument commits any one of a number of informal fallacies or commits no fallacy.

In Hurley's Intro to Logic he gives, as an example of "no fallacy," the following argument.

1. Rhubarb pie is a dessert.

2. Therefore, whoever eats rhubarb pie eats a dessert.

In this argument no informal fallacy is committed. Also, it's a valid deductive argument, which is to say: if the premise is true, then the conclusion necessarily follows. So, formally, it's a valid deductive argument. But is it a sound argument? Only if P1 is true.

We had quite a debate tonight over this thing! One student even left the classroom for a few minutes and went to a computer to print out the lexical definition of "dessert."

If you're in my class I now appeal to you:

The argument commits no informal fallacy.

The argument is valid deductive.

We don't know if it is "sound" or not - that depends on P1 being true. In other words we have to know that P1 is true. It seems debatable, and I think that's where the debate was taking place tonight. As for me, I think P1 is true.

Now imagine this. You're sitting down to a Thanksgiving meal next week at your grandmother's house. She says "Let's start with an appetizer!" She then proceeds to bring out a rhubarb pie. You say to her, "But grandmother, that's a dessert, not an appetizer, because the lexical definition of "dessert" means "something served at the end of a meal."" (Privately, you also wonder about her mental competency.) She says "Oh yes, I am so sorry." It's only because you recognize rhubarb pie as a dessert that you're able to express your confusion about this. But then someone says, "Why don't we eat the rhubarb pie first?" And, getting very radical, you all do just that. You begin the meal with "dessert" and violate it's lexical definition.
Or, you go to a restaurant and on the list of entrees there appears, right below prime rib, "rhubarb pie." Hasn't something gone quite wrong here precisely because a dessert is mislocated?

Therefore, has not my point been made? :)