Saturday, November 5, 2011

Logic 101- Formal Fallacies


In this edition of Logic 101, I felt that we should go back to the beginning (well, since "Logical Absolutes" is the absolute beginning, this is more like the semi beginning) and talk about the most common formal fallacies. It is important to get these out of the way before speaking about the informal ones, because with these you don't need to examine the content of the argument to recognize that it is false. It is the construction of the argument itself that is flawed.

All Formal fallacies are a form of non-sequitor (latin for "does not follow")

A Non Sequitir, in formal logic, is an argument in which the conclusion does not follow from its premise. This has no bearing on the truth or falsehood of that premise, it merely demonstrates that you can't reach that conclusion using the argument that is posited because there is a logical disconnect between them. 

A common example is;

1. If I am a cat, then I am a mammal

2. I am a mammal

3. Therefor I am a cat.

This doesn't follow as it doesn't fully distribute the middle term in the categorical syllogism. Nowhere does it say that all mammals are cats, just that all cats are mammals.

See what I mean?

There are other forms of non-sequitirs such as affirming the consequent, or denying the anticedent.  These are arguments where a connection is drawn between A and B, then being told that A is either true or false, and assuming the truth or fallacy of B as a result.  It is a jump in logic, or "doesn't follow".

Enough of that.

Now we will move on to a few of the more common logical fallacies and detail them so that you can avoid this in any argument. The three most popular formal fallacies are that of appeals to Authority, Probability, and Popularity. Remember, that Authority, Probability, and Popularity can sometimes be used as EVIDENCE to sway your audience (or opponent) but are not logical grounds for reaching said conclusion so it is best to stay away from them.

Appeal to Authority-
Appeals to authority are always deductively fallacious. Pointing to an authority and claiming that makes it true is akin to saying "He was right about other stuff, so he must be right about this". It is an assumption, but this logical fallacy is usually only asserted when the "Authority" is not an expert in that area of knowledge, or if the information expressed from a recognized authority goes against the expert concensus.

Appeal to Probability- This is an argument that fails in its structure in that it assumes that since a probability is high, that an event will inevitably happen, or since the probability is low it will never happen. Creationists try to use this to disprove evolution by saying it is stastically unlikely, therefor impossible. That is a conclusion that does not follow from it's premise. Even statistically unlikely things happen all the time. 

This cheque is too unlikely, so it can't be real.  Am I having a stroke?

Appeal to Popularity- Appeals to Popularity are VERY common in debate. This fallacy usually takes the form of something like this; "90% of the people in the world believe in a god, in fact most people in the history of the world do. They can't all be wrong!".

I like to demonstrate the falseness of this argument with the following comical statement;

"One MIllion Lemmings can't be wrong....JUMP!"

Seems like a good idea but I'm not taking any chances
Just because a lot of people have bought into an idea does not give it any merit. At one time everyone thought the earth was flat, or that the sun was carried across the sky by an unseen hand. Popular opinion has been, time and time again, proven to be wrong in many arenas of knowledge.

So there you have it, three logical fallacies and the explanation of a Non-Sequitir.  These are arguments that fail even before the content of your argument is examined, so if you want to be heard, avoid them at all costs.

Next time in Logic 101- The Argument from Ignorance

No comments:

Post a Comment