Occasional reflections on Life, the World, and Mathematics


People in the know are starting to think a disastrous “no deal” Brexit is now not at all unlikely. According to UK trade secretary Liam Fox

I have never thought it was much more than 50-50, certainly not much more than 60-40.

The Latvian foreign minister is only slightly more optimistic:

The chances of the UK securing a Brexit deal before it leaves the European Union in March are only 50:50, Latvia’s foreign minister has said ahead of talks with Jeremy Hunt.

Edgars Rinkevics said there was a “very considerable risk” that, with time rapidly running out, Britain could crash out of the bloc without a withdrawal agreement.

But not to worry. Rinkevics went on to say that

having said 50:50, I would say I am remaining optimistic.

I suppose, technically, he is more optimistic than Hunt. Why so gloomy, Jeremy, with your exaggerated estimate of 60% chance of disaster? I think it’s more like 50 percent. That’s a glass half full if ever I saw one…

Of course, an “optimist” is usually thought to be someone who thinks the chances of disaster are significantly less than a coin flip.

This is an example of what I called elsewhere part of the “folk theory of probability”: All probabilities are either 0 or 1, but we are uncertain about which. Credit agencies and politicians have this way of talking, where there is a probability estimate, but also a trend in the probability. As I wrote then

From a probabilist’s point of view, the reports of credit agencies seem batty. I mean the ones of the form: “Consolidated Counterfactuals bonds are rated BBB-, but we have a “watch” on it, and we are likely to downgrade it soon”. I take a credit rating to be a way of describing a range of probabilities of the company or country defaulting on its loans. What can it mean to say, “The probability of a default is currently 0.1, but there is a significant chance of it increasing to 0.2 in the next month. This could only make sense if there is some event X that we are waiting for, and if X happens the probability of default goes up, and if it doesn’t the probability will go down, and it all averages out to 0.1…

I’m reminded of the Robert Heinlein novel, The Moon is a Harsh Mistress, which I read as a teenager. Lunar colony secessionists go to war against Earth, guided by a sentient computer that compulsively computes odds of their ultimate success based on current status. At one point there is a discussion that even then troubled me: The computer has  just lowered the estimate of the probability of success to one in nine. “Getting worse?” the protagonist asks. The computer replies: “They’ll get worse for months. We haven’t reached the crisis.”

Optimism is like the “watch”, in a positive direction. Or, to put it differently, all our fates are riding on this coin flip. So I think we can just get out there and by God win that flip!

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