First we must deal with logic. The easiest way to start in logic is to begin with a syllogism. You've seen these before--both on SATs and in real life. A syllogism is a form of argument, with at least a premise and a conclusion. To adapt an illustration from J.P. Moreland and William Lane Craig: suppose your friend wants to buy a new CD from a store. You tell your friend, "You won't be able to do that." He responds, "Why not?" "Because it is a Sunday, and that store is closed on Sundays." Essentially, you have made a syllogistically sound argument. It would look like this:
Premise 1: If it is a Sunday, then the store is not open.
Premise 2: It is a Sunday.
Conclusion: Therefore, the store is not open.
This is a formally valid argument. This means that the structure, or form, is sound. If the argument is formally valid, and the premises are taken as true, then the "therefore" statement is always true.
Now we must deal with two logical fallacies: denying the antecedent and affirming the consequent. Let us first demonstrate the denying the antecedent from the previous example:
1. If it is a Sunday, then the store is not open.
2. It is not a Sunday.
3. Therefore, the store is not open (or not not-open, ie open).
Either conclusion will not work, since premise 1 does not tell us what happens if we negate it. Now let's try affirming the consequent. This is stating the positive of the second part of an if-then statement.
1. If it is a Sunday, then the store is not open.
2. The store is not open.
3. Therefore, it is a Sunday.
The problem is that we were not told "if and only if it is a Sunday," so we really cannot conclude it is a Sunday simply because the store is not open. Try this example for a clearer picture:
1. If Jim is at the mall, then he is having fun.
2. Jim is having fun.
3. Therefore, Jim is at the mall.
Obviously, Jim could be having fun somewhere other than the mall. These are simply two formal fallacies; other logical fallacies that are more "reasoned" or "argumentative" in nature will be discussed. Check back frequently for site updates!
