Post Hoc Ergo Propter

We learned in Lesson 14 that to make a strong causal argument you need the cause to precede the effect. In other words, if problem A causes result B, cause A had to occur before result B. However, this is not the only factor in determining cause. Just because one event precedes another does not mean that it caused it. When you wrongly make that assumption, you commit the fallacy known as post hoc, ergo propter hoc.

This fallacy, like the chicken and egg, has to do with cause and effect. Often called post hoc, it means in Latin, "after this, therefore because of this," and occurs when an assumption is made that, because one event precedes another, the first event must have caused the later one. The fallacy, sometimes referred to as false cause, looks like this:

1. Event A precedes event B.

2. Event A caused event B.

To make a strong causal argument, you must account for all relevant details. For example, every time Ahmed tries to open a video program on his computer, it crashes. He concludes that the program is causing the computer to crash. However, computers are complex machines, and there could be many other causes for the crashes. The fact that the opening of one program always precedes the crash is a good possibility for cause, but it cannot be maintained as the one and only cause until a stronger link is made. To avoid the post hoc fallacy, he would need to show that all of the many other possibilities for the cause of the crashing have been evaluated and proven to be irrelevant.

Superstitions are another example of post hoc fallacies. Some superstitions are widely held, such as "if you break a mirror, you will have seven years of bad luck." Others are more personal, such as the wearing of a lucky article of clothing. However, all of them are post hoc fallacies because they do not account for the many other possible causes of the effect. Bad luck could happen to someone who breaks a mirror, but bad things also happen to those who do not. The superstition does not account for why the breaking of the mirror causes something bad to happen to the person who broke it. In these cases of superstitions, the real cause is usually coincidence.

How can you strengthen an argument and keep it from becoming an example of the post hoc fallacy? First, show that the effect would not occur if the cause did not occur. For example, if I don't strike the match, it will not catch on fire. Second, be certain there is no other cause that could result in the effect. Are there any sources of flame near the match? Do matches spontaneously catch fire? Is there anything else that could cause it to catch fire? If the answer is no, then there is no post hoc fallacy.


■ I took three Echinacea tablets every day when my cold started. Within a week, my cold was gone, thanks to the Echinacea.

■ I wanted to do well on the test, so I used my lucky pen. It worked again! I got an A.

■ Last night I had a dream that there was a car accident in my town. When I read the paper this morning, I found out a car accident did happen last night. My dreams predict the future.

Friendly Persuasion

Friendly Persuasion

To do this successfully you need to build a clear path of action by using tools if necessary. These tools would be facts, evidence and stories which you know they can relate to. Plus you always want to have their best interests at heart, in other words, you know what is good for them

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