Black Swans,
or cats?
2009–08–18
Black Swans—
a logical assumption that,
when reality bites,
leads to…
well see where it got me.
alarms
Last week the alarm went off in my annex, well the annex of the school that I work in to be precise, but it is my annex, in my mind. Nothing terribly unusual there, alarms are the bells that cried wolf—the classic false positive. The alarm has gone off? It’s broken, real thieves would have got around it.
So, no great cause for concern? Well…there were some oddities— it had double-knocked [it had detected two movements within a short space of time] and then it had re-armed itself, and gone off again, and again. [I won’t give you a lecture on how alarm systems work, you’ll just have to trust me that this is odd.]
Problem one—the neighbors, they were a tad upset about the continuous ringing. Problem two—the engineer and I couldn’t see why it was happening. Three PIRDs [Passive Infra-Red Devices] were firing, firing again and again. Problem three—it kept happening, night after night.
Now, given the title of this post you’ll know exactly what was happening, and it was our [the engineer and I] first thought too. But. Yes but—there is no place to hide in my annex, it’s a featureless plain currently occupied by about three hundred actors and technicians putting on a fringe performance. An animal was not a possibility. Someone would have seen it.
Unless, of course, the cat had made a home for itself under the stage.
why are you telling me this?
Because it’s what we do when we’re hacking code—make assumptions, hide from reality, don’t see the obvious.
But perhaps it’s more subtle than that. There were two logical reasons for this problem. Two, good, logical reasons that were at odds with each other:
- No self-respecting cat would want to set up home in my annex. There’s nothing to eat and ,unless someone opens a door, there’s no way in and out [or is there?]. And even if it had moved in someone would have seen it.
- If the alarm goes off and there hasn’t been an obvious break-in then the alarm is knackered.
The engineer and I ruled out the impossible and checked out the wildly improbable, but still possible—three PIRDs had failed simultaneously. What bit us was that the impossible was reasonable, but we didn’t think so.
Until I saw a cat.
black swans
Many, if not most, of my bad experiences with de-bugging have been due to me writing an axiom in my head, an axiom that doesn’t accord with reality—
All swans are white.
That is Black.
Therefore…
Remember the cat! Check everything, especially that bit that you know is perfect.