by Geoffrey Schmitt

Created on: May 13, 2010   Last Updated: May 16, 2010

Those who want to convince us of things by appealing to our emotions prefer us not to use our minds to analyze the logic behind their arguments. This is true of car and insurance salesmen, politicians, and even our friends and family. There are a few tried and true, even “classical” logical fallacies we should all know and be able to recognize.

Some of these go by their Latin names. The “non sequitur” is Latin for “it does not follow.” Just because x is part of y does not mean that if something is not x it is not also y. 

For example, all Mustang automobiles are made by Ford. It does not follow that if your car is not a Mustang that it is not a Ford. Let’s take a more subtle political message. Many Illegal immigrants come from Mexico. If you are from Mexico you came here illegally. Now that could be a damaging “no sequitur.”

The next logical mistake was even a title of one episode of The West Wing television series: “Post hoc, ergo propter hoc.” In English it is “after this, therefore because of this.”

Just because one thing follows another does not necessarily mean the first caused the second. I came into the room and the lights came on. This does not mean that I somehow caused the lights to turn on.

It might just be possible that there was a motion detector switch that I tripped. However, without further investigation, it is not logically true. The illogic of this fallacy can wreck havoc on the stock exchange.

A favorite ploy of arguments is “Petitio principia” to “beg the question.” “Have you stopped beating your wife?” assumes that you were beating her. This is a fact not in evidence.  

A close relative of begging the question is circular reasoning “circulus probando”.  You should get eight hours of sleep because it is good for you. This may be true but the argument is circular. Let’s have some real evidence. This is also called an unsupported assertion.

Argument from analogy is another ploy fraught with illogic. It is used currently in the debate about gay rights. We have liberated the blacks and given equal rights to women, therefore it is time to give them to homosexuals.

You cannot logically compare “apples and oranges.” Regardless of the true merits of gay rights argument by analogy proves nothing.

If you will be on the alert for just these five logical pitfalls you will be in a much better position to get at the real evidence of an argument. 

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