This is my second and presumably last post on the anthropic principle. The first one is here.

The anthropic principle per Wikipedia is the “philosophical consideration that any data we collect about the universe is filtered by the fact that, in order for it to be observable in the first place, it must be compatible with the conscious and sapient life that observes it.” In popular discourse, this notion often manifests something like this: “How is the outrageously unlikely fact of our existence explained? Well, if the universe weren’t consistent with human life, we wouldn’t be here to ask that question!” I have beaten this formulation with a big heavy stick before (see the foregoing link) and I’ve now figured out how to frame the issue in a different but equally clear way.

First note that probabilities from your point of view depend on how much you know. For example, there’s probability that it will rain on any random day in Boston, given no other information. Then there’s there’s probability that it will rain today in Boston, given that it rained yesterday. These are generally going to be different probabilities.

Stat folks say “conditional on” instead of “given that.” E.g. where a normal person would say “the probability that it will rain today, given that it rained yesterday,” a Stat person would say “the probability that it will rain today, conditional on the fact that it rained yesterday.” And the probability that it will rain on any random day, given no other information, is called the **un**conditional probability.

On the “anthropic principle”: When people ask things like, “How is the outrageously unlikely fact of our existence explained?” they are interested in the *unconditional* probability that the universe has properties that can support human life (and that human life actually did evolve, but let’s just stick with the first part). Whereas the “anthropic principle” answers the completely trivial question, “What is the probability that the universe can support human life, conditional on the observation that human life actually exists?” The answer to that utterly trivial question is 100%, obviously.

Literally no human being ever, in the history of the world, meant to ask, “What’s the probability that the universe can support human life, given that it actually does support human life?” Yet that is the question that the so-called “anthropic principle” answers. Seriously, here’s the Wikipedia formulation again: The anthropic principle is the “philosophical consideration that any data we collect about the universe is filtered by the fact that, in order for it to be observable in the first place, it must be compatible with the conscious and sapient life that observes it.” That is literally saying, “The probability that the universe can support life, given that there is life to observe it, is 1.”

So much for the “anthropic principle.”

So what’s the honest answer to the unconditional question? I don’t see how anyone could know, because to answer this we’d need to know the probability distribution from which the actual universe was drawn. We don’t know that. Of course one can put forth if-then propositions about it. A common one is, “Suppose all universes which are physically possible exist. (The many worlds hypothesis.) Then the probability of any particular universe existing (including ones with humans) is 1.” Sure. But we don’t know whether the many worlds hypothesis is true.