“Post hoc ergo propter hoc” is a Latin phrase for a well-known mistake in generalizing. It literally means “After that therefore caused by it.” It refers to the mistake of inferring that something that came before something else must necessarily have caused it. Here is an obvious example of it. “Yesterday I got a stomach ache after doing algebra. I will avoid doing algebra in the future.” Here is another: “Yesterday my son was in a car crash. Right before it happened, I had a feeling something bad was going to happen. This proves that you can hurt the people you love by thinking negative thoughts about them.”
The fact is that before any given event happens, other things happen, usually many, many things. That does not mean that all of these earlier events caused what came afterwards. Monday comes before Tuesday, but does not cause it. Summer comes before Fall, but does not cause it. I put on my shoes before I eat my breakfast, but putting on my shoes does not cause me to eat my breakfast.
EXAMPLE:
The last time we had a major strike of teachers, a student died in a fight. That just shows you the irresponsibility of teachers striking.
COMMENT:
Sorry, but no causal connection has been established. The mere fact that the strike occurred prior to the fight is no good reason to consider it a cause.
EXAMPLE:
“Last time I had dinner at Jack’s house, I had a bad stomach ache the very next day. The food at Jack’s house must have been bad.”
COMMENT:
I’m afraid we must decline this “post hoc” generalization about the nature of the food at Jack’s house. There are many other possible causes of the stomach ache in question. You could check and see whether the other people who ate at Jack’s house also got sick.