Thursday, 6 February 2014

Post hoc ergo propter hoc

For tonight’s lesson, I told The Girl, I want to tell you two sentences. Listen carefully, and let me know if they are true:

Snow White’s mother wished she would have a daughter, and she got her wish.

“That’s true,” The Girl said emphatically, and I nodded. Okay, I said, what about this:

Snow White’s mother wished she would have a daughter, and her wish was granted.

“Oh! That one’s not right!” she said. Why not? I asked.

“Because …” she said, trying to find the words. “Because who granted it? Just because she had a girl, it doesn’t mean someone made her have a girl.”

Right, I said. Even if you assume God grants wishes, it doesn’t mean he – or something – intervened.  It’s a mistake people make all the time, thinking that because something happened after something else, that one caused the other. In Latin it’s called ‘post hoc, ergo propter hoc.’

“Post-duck… um…” The Girl began.

Just think of a hawk that delivers our post, I said. It can fly, but the other hawk is too proper to go through the air -- post-hawk air-go, proper hawk. Can you think of another example?

“Witches!” The Girl said, and there was a pause. Okay, I said, keep going with that thought.

“Well, they used to burn witches, right?” she said. An old woman would walk by your field, and all the slugs would get your crops, and you could say the old woman caused it, that she put a hex on you.”

Excellent, I said; I’m really proud of you. With things like that it's obvious, but it's harder in things like experiments, because you’ll often see things happen together – called correlation -- and it will be tempting to think that one causes the other. Correlation doesn’t mean causation.  

I’ll give you another example: a lot of fake medicines -- ‘herbal’ pills and such -- do the same thing, saying they’ll cure your cold. You take it, and your cold gets better.

“But it would have got better anyway?” The Girl asked. 

Sure, I said – it’s a cold.

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