Logic Lessons pt 7: Fallacies of Induction

Induction is when you take several instances of something to form a general conclusion. You can never get certainty from induction, but that is not the problem. The following are times where induction is improperly used.

1. Hasty Generalization = going from specific example to general principle too quickly. For example, when I was a child, my mom brought me to the old Cleveland Stadium to watch an Indians game. We were in the nosebleeds and I was bored. Then she brought us again. And after watching the pitcher throw to first base for half-an-hour, I concluded that I baseball was a boring sport. Now, it may very well be, but only 2 games may be too soon to make that conclusion.
2. Post Hoc = (post hoc ergo propter hoc) “After that, therefore caused that.” The classic example is Chanticleer the rooster who crows and then the sun comes up. He thinks that because it follow his actions that it is because of his crowing that the sun rises. This is the source of all of our superstition. I never bowl a strike. But then one day I did when I wore mismatched socks. So from now on I wear my “lucky” mismatched socks. But because something follows another thing, it does not mean that it caused the thing.
3. Hypothesis Contrary to Fact: “if x were true (which it isn’t) then y would be true.” The problem with this is that it forces you to assume something to be true that is not. For example: “If Al Gore was elected we would never have invaded Iraq.” Or “If McCain was elected, the economy would be better.” Now, both examples are possibly true. But the “if” premise of the statement is NOT true. Because it is not true, it is impossible to verify the conclusion. We have do not know what an Al Gore presidency would have been like because that is an historical possibility that is impossible to check.
4. False Analogy = assumes analogies prove something. Now it is important to note that analogies don’t actually prove anything. CS Lewis is one of the best modern Christian writers because of his ability to use an analogy effectively. But even he would acknowledge that an analogy is not an argument. It doesn't prove that a thing is true, but explains how it could be true. The fallacy occurs when using false analogies, which consist of
   a. Using a false (inappropriate) analogy. This would be making an analogous comparison to something that does not share any similar traits. For example, “Giving birth is like eating spaghetti” The two activities have nothing in common, as opposed to saying something like “Giving birth is like writing a book.” Now in both situations, something is made that hasn't been before and it requires effort. It may be a poor analogy, but it is not a false analogy.
   b. using an analogy falsely. Assuming if some things are similar in one way, they will be similar in all ways. St. Patrick used the Shamrock as an analogy to the Trinity (“3 leaves but one leaf”). But it would be wrong (using a quote from “Nuns on the Run) to say “God is like the Trinity... short, green, and split 3-ways.”
5. Argument from Silence = drawing conclusions from silence on a subject. When the Da Vinci Code was popular, some of my students would ask if Jesus was married. I said that there is no evidence for it. They respond, “That doesn't mean he wasn't.” But that doesn't help PROVE the proposition that Jesus was married. I respond, “I am going to assume that you are a serial killer because there is no evidence to say that you aren't.”
6. Selective Evidence = referring only to evidence that supports you and ignoring evidence that refutes you. This is something you see a lot in political ads. A report from the Congressional Budget Office will come out and the candidates will ignore whatever disagrees with their position and highlight what agrees with them.
7. Slanting the Question: You find this a lot with polling. Take these 2 questions about the same topic: “Do you support a woman's right to choose?” and “Do you support an unborn child's right to live?” Both questions are asking “Should abortion be legal?” But depending on how it is asked, it leads the answerer to a conclusion.