What is Post Hoc Ergo Propter Hoc?

The idea that one event causes another can be a logical misstep when you are making an argument. If you’ve ever taken a critical thinking course you may recognize this as the Latin phrase post hoc ergo propter hoc, or you may have heard something referred to as a post hoc argument or comment. The Latin term translates to “after this, therefore caused by this,” and this determination of causation, also called false cause or correlation by coincidence is considered a logical fallacy.

Post hoc ergo propter hoc is an easy assumption to make, and it’s not always based on illogical thinking patterns. If the first time your puppy hears fireworks he hides under the bed, you might assume that fireworks scare the puppy. This may or may not be true. You’d have to test it ought several times before you assume that fireworks were a direct cause of the pooch’s fear. Moreover, simply because the puppy is afraid of fireworks the first time it hears them doesn’t mean it will remain afraid of them, particularly if you train him to not mind the sound.

Additionally, sometimes people make the leap with post hoc ergo propter hoc thinking in believing they can eliminate a problem by eliminating the supposed cause of the problem. You might think: “If I don’t set off fireworks this year, the puppy will not be afraid.” That’s not entirely true: the puppy could be fearful of lots of other things that have nothing to do with fireworks. Even if its true that fireworks are one cause of the dog’s fear, they are not the sole cause, and a car backfiring, a door slamming or someone shouting might find said puppy hiding under the bed again.

The essential structure of post hoc ergo propter hoc is the following:

We could make a very strong case using this fallacy that drinking water may cause cancer. We might look at all the data on cancer patients and find that all of them had had a glass of water at least once in their lives. Using post hoc ergo propter hoc, we would then assume that drinking water causes cancer. You can see the inherent problems with this, because plenty of people who drink water don’t get cancer. Simply because something occurs doesn’t mean it has any relationship to something that occurs at a later point.

On the other hand, if you wanted to correct this fallacy, you might establish a correlation between one event and another, or be able to prove cause through a huge number of examples. When Erin Brokovich took on the Pacific Gas & Electric Company in California for allowing residents to live near a water supply (which they used) that was tainted with hexavalent chromium, there was a clear correlation, given the number of cases of cancer emerging there, that drinking water tainted with hexavalent chromium raised the risk of getting cancer. Not all people who drank or bathed in the water had cancer, but the preponderance of cases helped establish correlation between drinking the tainted water and higher risk of cancer.

Additionally, Brokovich and Edward Masry were able to use scientific data to strengthen their argument. In the end, Brokovich’s argument won because it was not a simple post hoc ergo propter hoc assumption. It was not just “A caused B.” Instead it was based on plenty of evidence that there was a direct relationship between A and B.