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false cause

This very common error in logic involves an assumption about cause. In this fallacy, when two events occur one after the other, the writer assumes, without enough evidence, that the first event caused the second.

A lot of superstitions are based on this kind of reasoning:

  • “My friend gave me a crystal charm, and both times I wore it, I won at cards. But the time I forgot it at home, I lost. I have to remember to wear that crystal if I want to win.”

But this type of reasoning is not found only in superstitions:

  • Two employees attended the weekend conference. They were both sick afterwards. The food served at the conference must have been bad.

In this example, although the employees’ illness may have been caused by food poisoning at the conference, there is not enough evidence to draw a conclusion. The fact that the sickness occurred after the conference does not prove that something at the conference was the cause.

This fallacy is sometimes called a post hoc fallacy, from the Latin phrase post hoc, ergo propter hoc (after this, therefore because of this).