Again, I could easily be wrong about all this.<p>I think this entire discussion is sensitive to exactly what one means by &quot;The Anthropic Principle&quot;. If one&#x27;s definition of the Anthropic Principle in this context simply means that an observed universe will be consistent with a universe that <i>can</i> be observed, then I find it to be trivially true and applicable to a single universe scenario.<p>I subscribe to a definition of the anthropic principle that invokes selection bias as its core mechanism. I think that, in order to have a selection bias, one needs to actually be <i>selecting</i> from a population. Hence my assertion that you <i>need</i> a multi-verse (with different constants in each universe) or a constant that is variable in space or time [2].<p>The difference, to me, is this:<p>If you are <i>selecting from a population</i> the Anthropic Principle can actually help explain <i>why</i> a value is what it is. Under our understanding of planetary bodies, we often invoke the Anthropic Principle to explain <i>why</i> the earth falls in the habitable-zone. Earth falls in the habitable-zone <i>because</i> — we think — life is most likely to arise on a planet in that zone.<p>However, if you are <i>not</i> selecting from a population, I don&#x27;t find that the Anthropic Principle explains <i>why</i> a value is what it is. In the scenario where there is only <i>a single</i> value, I think the Anthropic Principle simply demonstrates that <i>that particular</i> value must support life.<p>Put another way, if <i>all</i> planets we observed were the exact same distance from their star as Earth [1], then we couldn&#x27;t <i>use</i> the Anthropic Principle to explain why Earth is that particular distance from the sun. We would have to find other astronomical reasons to explain why planets form at that specific distance from a star. In this single-distance scenario the only thing the Anthropic Principle actually does for us is confirm that <i>such a distance supports life</i>.<p>[1] Well, not distance per se, but placed such that they received the same amount of stellar energy in their orbits.<p>[2] This final sentence of the introduction of the Anthropic Principle also makes some reference to this saying: &quot;Most often such arguments draw upon some notion of the multiverse for there to be a statistical population of universes to select from and from which selection bias (our observance of only this universe, compatible with life) could occur.&quot;