How to assess the probability of pretty much everything

An ancient approach to effectively navigating a world of uncertainties

The Bayesian principle of updating one’s beliefs as a result of new evidence, from analyticsvidhya.com.

I’m pretty sure—almost certain, in fact—that right now I’m sitting at my desk, typing an essay on probability on my recently acquired MacBook Air M2 laptop. True, there could be a Cartesian demon messing around with my mind and creating the illusion that I’m typing on a keyboard, but I don’t think that’s very likely. It is only slightly more likely that I’m dreaming of typing on a keyboard. I’ve certainly had stranger dreams before. Then again, I can usually tell the difference between when I dream and when I’m awake, so I’ll stick with my initial assessment. I am, indeed, typing this essay on my laptop’s keyboard.

One thing I’m almost absolutely sure of is that the square root of nine is three. Unless I’ve suffered from a stroke that has impaired my basic reasoning functions. Which is unlikely, but certainly possible.

By contrast, I’m far less positive about what the weather will be next weekend in Brooklyn. That’s because I know that weather forecasts aren’t reliable if stretched over a period of more than 3-4 days. If pressed, I could guess based on Brooklyn’s typical weather this time of the year, though with climate change accelerating you never know for sure.

And so on. Our lives are made of constant assessments and re-assessments of probabilities. Rarely, if ever, can we seriously claim to have Knowledge of the Truth (notice the capitalized letters). But we also rarely, if ever, actually need Knowledge or Truth. A convincing evaluation is good enough to actually act in the world.

The word “convincing” is how the Greek term pithanon is often translated, and pithanon was the criterion for action invoked by Academic Skeptics like Carneades of Cyrene. Interestingly, the Roman philosopher Cicero translated that same word with the Latin probabilis, from which of course the English probability derives. When we say that something is probable, therefore, we mean that we have convincing evidence or reasons to provisionally accept it as true. And, more importantly, to act on it.