Post hoc ergo propter hoc

I've had an interesting story to share for some time about just how easy it is to fall into the post hoc fallacy, but I haven't posted it yet because it's also very sad and it helped to have some distance.

Back in July, my favorite cat and my first real pet as an adult suddenly got sick and died under very mysterious circumstances. It seems most likely she ate some kind of poison, but we never identified what it was -- she was an indoor/outdoor cat, so who knows what she could have gotten into? That morning, she appeared totally healthy. I clearly remember refilling the food dish that morning and seeing her chowing away, i.e. completely normal appetite. And then, that evening, her kidneys completely shut down, and even before we got to the vet I knew from her symptoms she was going to die.

She was eight years old, a little past midway point for a cat. I'm mostly over it now, but the one thing that still gets me is when I think to myself, "She could have had another eight years... all those wonderful times with Stash, multiplied by two." I tend to get kind of angry when I think that, so I try not to...

Anyway! Enough with the sad part of the story. So, it turns out that shortly before that we had decided we should put our cats on flea medication. We ordered it online, and it arrived a few days before her death. But despite my wife's helpful reminders, I kept forgetting to give it to the cats. I finally treated our remaining cat a couple weeks after Stash died.

Imagine if I had remembered to apply the medication when it first arrived..! I would be 100% convinced it was the flea medication: I put a chemical on my cat that she's never been exposed to before, and then 48 hours later she drops dead from apparent poisoning. I would have been devastated. I never would have forgiven myself.

And yet, it would all have been an illusion. Pure post hoc. Stash died for some completely different reason. I shudder when I think of the kind of unnecessary guilt and anguish I would have felt, if the chips had fallen just a tiny bit differently...