The importance of logic

Shortly after the "stupid people are happier" comment, another conversation took place which included a logical fallacy. Can you spot it?

Person A: "How are you today?"
Person B: "I'm doing fine."
A: "You look quite happy."
B: "Are you trying to say I'm stupid?"

Though this was a joke, this logical fallacy, called "affirming the consequent" is actually very pervasive and often quite hard to detect.

Logically, here's the form:
If P then Q
Q
Therefore P

This works in some situations:
If Bob is a human, then Bob is a Homo sapien
Bob is a Homo sapien
Therefore, Bob is a human

But it does not work in others:
If Jill got hit by a train, then she needs serious medical attention.
Jill needs serious medical attention.
Jill got hit by a train.

Jill is probably glad this is not sound reasoning, or else she'd have a hard time with doctors ("Jill, it appears that your appendix is about to burst. I must again advise you to stay away from railroad tracks."). 

The reason why the first argument works is that the first statement is actually an "if and only if" statement (which in formal logic is usually indicated by an "iff", which I always dug). P implies Q, but Q also implies P, so the statement is valid by standard if-then reasoning, technically called "modus ponens."

It seems like these would be very easy to recognize, but they can be very subtle. TV News personalities love them. Although ad hominem and slippery slope probably could be the most popular, from my limited exposure. Counting fallacies is pretty much the only way I can stand to watch Fox News.

Fallacies can also be very funny, however. Some friends and I recently laid the groundwork for our Fallacious Fitness Program. We noted that, if you get done with your workout and you aren't tired, then you didn't exercise hard enough. So the purpose of a workout program is to make you tired. Thus you can be more fit if you incorporate as many tiring activities as possible into your daily life. Some suggestions would be: using the stairs, parking far away, eating deep-fried foods, riding your bike to work, driving long distances, reading very challenging books, watching basketball for 6+ hours, and staying up really late. As long as you're tired, you must have worked out.

This can be combined with the fallacy of constitution to provide an eating regime. The rule: As long as the food item contains something healthy, it itself is healthy. Some examples might include: fruit roll-ups, chocolate-covered cherries, fried potatoes, deep-fried mushrooms, broccoli-cheddar soup, corn flakes, and so on.

So I hope this has helped drive home the importance of being aware of logical fallacies. The greatest minds throughout history have all been very aware of fallacies, so if you develop you awareness of them, you could be an amazingly intelligent person as well.