Thought of the Day 02.13.09

One of the most basic formal fallacies in logic is called “affirming the consequent.” Though it’s difficult to explain, it’s easy to illustrate. Here are some examples:

"If a woman is pregnant, she will develop a big belly. Sue has a big belly. Therefore Sue is pregnant." Obviously Sue could just be overweight.

"If a man drinks poison, he will die. Jim is dead. Therefore, Jim drank poison." Obviously Jim could have died from many other causes.

"If I watch a boring movie, I fall asleep. I am asleep. Therefore, I watched a boring movie." Obviously, I’ve successfully slept many times without the sedative help of a bad movie.

The fallacy flows from the fact that qualifying for the broader category does not logically qualify you for the narrower subset of that category. Here’s a more poignant example:

"If a person loves God, he will do lots of good things. I do lots of good things. Therefore I love God."

Tragically, the entrance requirement for God’s eternal companionship is loving Him, not merely imitating someone who does.