I would like to respond correctly to a post elsewhere on probability. It was said that the universe ‘must have had’ a creator. I said that, since the universe exists. The probability is 1; not 1:large-number-of-choice.

The response was that, after it happened, the probability was 1, but the probability of it happening by way of a random event before the universe was created is zero.

Is this correct? If not, could someone give me a simple answer, please?

The probability that this Universe exists is 1, because it is certain that it does exist.

The probability that a universe could pop into existence where no universe existed before is unknown. We don't know the physics behind universe creation so we can't make a reasoned estimate of it.

As an aside, if you have a quantity that is measured as a real number, the probability of any one particular exact value is always zero. Fore example, if you pick a random person off the street, the probability that they are exactly 6 feet tall (1.8288 metres) is zero.

Therein lies the answer to the fallacy. If we could measure my height to the accuracy of the Planck length e.g. 1.7272 (and another 31 digits) metres and then calculate the probability of any random person being within 10

^{-35} metres of that height, the probability is going to be very very small. But here I am, existing with my exact height at a probability of 1. This is a variation of Douglas Adams' example of how a puddle might marvel that the indentation in which it is sitting is exactly the right shape for it.

So you have a two pronged attack. First challenge them to justify their probability of 0. Then go with the above, if they come up with anything sensible.