Bayesian Update

Most people treat their beliefs like cement. Once it sets, they never look at it again. A thinking person treats beliefs more like a kitchen ledger. As new evidence comes in, you update the numbers. The fancy name for this is Bayesian updating. The simple name is changing your mind in proportion to what you just learned.

An example you live every Pakistani summer. At 4 pm a rumour reaches your phone: 'tonight there will be no load shedding in Defence till 11 pm because of a special order'. How likely do you think this is true? Maybe 30 percent, because we have heard such rumours before, and half the time they were nonsense. That 30 percent is your prior, your starting belief.

Now a piece of new evidence arrives. You open the LESCO Lahore website and see the official feeder schedule for the night. It clearly shows your feeder is on a planned outage from 8 pm to 10 pm. The schedule is a primary source from the people who actually flip the switches. How does your belief in the rumour change? It collapses. It should now sit somewhere around 5 percent. The new evidence pulled the belief down hard, because the source was strong.

The honest test of a thinking person is the third rule. Updating against your own side is hard. If you believed cricket bats from Sialkot were the best in the world, and then a careful test showed Australian Kashmir willow bats outperformed them in moisture retention, would you update? A small update is enough; the bats are still excellent. But pretending nothing happened is the cement-mind path. Do not take it.

A mental practice for the week. Pick one belief you hold strongly. It can be 'private schools are better than government schools', 'Karachi food is better than Lahori food', anything. Write it down. Next to it, write what evidence would actually change your mind, and by how much. If you cannot write that, your belief is not really a belief; it is a flag you wave.