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This article makes a familiar error in thinking that I routinely see when it comes to the subject of extraterrestrial life. It goes like this: 1) There exist b

This article makes a familiar error in thinking that I routinely see when it comes to the subject of extraterrestrial life. It goes like this: 1) There exist billions and billions of stars, each of which can be thought of an independent Bernoulli trial with probability P as to whether intelligent life exists in that star system. 2) P must be a non-negligible quantity, because we ourselves exist 3) Therefore intelligent life "must" exist elsewhere in the universe due to the sheer number of trials. This is an epistemological error that echoes the sort of error made by proponents of intelligent design. They conclude that the chance of thinking beings evolving is prodigiously unlikely, and therefore our existence is proof of a creator. But in fact, you cannot post-hoc conclude that our existence says anything at all about P, because if we didn't exist, there would be no one to ask the question. Therefore the fact that we are wondering about the value of P means our existence is an implicit axiom of the logical system itself. One way to think of it is there could be infinitely many possible universes with no life at all, and by definition the question of P can only discussed in the one branch, in all that lifeless space of possibilities, where thinking beings actually exist. So P could, in fact, be made arbitrarily small, and Earth could indeed be the only place in the universe with intelligent life. Note that this doesn't claim that extraterrestrial life _doesn't_ exist. It merely points out the truth in all of this P-hacking: that we have no evidentiary basis for believing one way or the other.

mrkramer 5y ago
I think instead of taking billions of stars we should look at the "alien life" from the aspect of chemistry and biology. What is the basic chemistry and biology of life? Chemicals, heat, water, single celled organisms, multi cellular organisms, bacteria, viruses, genetic materials, gene transfers etc. All of that in the context of universe, stars and planets. I would like to see a research from that point of view. If anybody knows of such research papers I would like to read them.
- dennis_jeeves 5y ago
  >I think instead of taking billions of stars we should look at the "alien life" from the aspect of chemistry and biology. I _thought_ that was the underlying assumptions of all these probability figures including Drake's equation. I could be wrong...
- dredmorbius 5y ago
  I tend to agree, on pragmatic bases. However in alternative universes, with different underlying constants, chemistry and biology might differ somewhat, possibly in unexpected ways. That said, in this particular Universe, I suspect life is reasonably likely. Intelligence may be impossible, however.
inimino 5y ago
I'm not sure I can agree with this analysis. Consider two hypothetical universes. In one, p = 10^-1000, but by a statistical miracle, here we are. We would certainly be alone in this universe, because the number of stars in the visible universe is inconsequential compared to this number. In the second one, the universe is the same size, but p is such that there are 100 planets with intelligent life like ours. If you cannot say "anything at all about P" from our existence, then we should be indifferent between these two hypotheses, on the basis of the evidence of our existence. However, this is not the case. The evidence supports the second hypothesis much better than the first. If X occurred, and is more likely under hypothesis B than A, then X is evidence for hypothesis B, otherwise you don't have a consistent system of probability. Now you can further imagine that God comes down from heaven and tells you that in fact both of these two universes actually exist, and they are the only two universes that exist, so you are definitely in one of the two, but doesn't tell you which one. Is your posterior probability now 50% between the two? "God" here represents the anthropic principle leaning over and saying "psst, don't worry about the 10^-1000, because, well, here you are". But you still have to worry about the difference between 1 and 100. If N beings in universe A and 100 * N beings in universe B all face this same question, your only reasonable conclusion as one of them is that the odds are 100-to-1 in favor of you being in universe B. So it's not the case that our existence tells us nothing about p. Of course, p is what it is, and only physics and chemistry and biology can actually reveal it, but the fact that we are here means that theories that make our existence much less likely are themselves much less likely.
- anthuswilliams 5y ago
  This is a very thoughtful rejoinder, thanks. > If X occurred, and is more likely under hypothesis B than A, then X is evidence for hypothesis B, otherwise you don't have a consistent system of probability. The point is, this idea breaks down when neither hypothesis would exist if X had not occurred. This differs from other questions of probability because the conditional probability of an event is predicated on the ability of an observer to ask about it. We should indeed be indifferent between the two hypotheses, if our own existence is all we have to go on. (This might seem strange, if you think of hypothesis B as having 100x the prior probability, but the point here is that we really don't have a basis for a prior at all.) > Now you can further imagine that God comes down from heaven and tells you that in fact both of these two universes actually exist, and they are the only two universes that exist, so you are definitely in one of the two, but doesn't tell you which one. Is your posterior probability now 50% between the two? No, of course I would assume I was in universe B. This is a very different question. Instead of "what is the nature of our universe" the question is "of the two existing universes God has described, which do I occupy?"

mrkramer 5y ago

I think instead of taking billions of stars we should look at the "alien life" from the aspect of chemistry and biology. What is the basic chemistry and biology of life? Chemicals, heat, water, single celled organisms, multi cellular organisms, bacteria, viruses, genetic materials, gene transfers etc. All of that in the context of universe, stars and planets. I would like to see a research from that point of view. If anybody knows of such research papers I would like to read them.

dennis_jeeves 5y ago
>I think instead of taking billions of stars we should look at the "alien life" from the aspect of chemistry and biology. I _thought_ that was the underlying assumptions of all these probability figures including Drake's equation. I could be wrong...
dredmorbius 5y ago
I tend to agree, on pragmatic bases. However in alternative universes, with different underlying constants, chemistry and biology might differ somewhat, possibly in unexpected ways. That said, in this particular Universe, I suspect life is reasonably likely. Intelligence may be impossible, however.

inimino 5y ago

I'm not sure I can agree with this analysis. Consider two hypothetical universes. In one, p = 10^-1000, but by a statistical miracle, here we are. We would certainly be alone in this universe, because the number of stars in the visible universe is inconsequential compared to this number. In the second one, the universe is the same size, but p is such that there are 100 planets with intelligent life like ours. If you cannot say "anything at all about P" from our existence, then we should be indifferent between these two hypotheses, on the basis of the evidence of our existence. However, this is not the case. The evidence supports the second hypothesis much better than the first. If X occurred, and is more likely under hypothesis B than A, then X is evidence for hypothesis B, otherwise you don't have a consistent system of probability. Now you can further imagine that God comes down from heaven and tells you that in fact both of these two universes actually exist, and they are the only two universes that exist, so you are definitely in one of the two, but doesn't tell you which one. Is your posterior probability now 50% between the two? "God" here represents the anthropic principle leaning over and saying "psst, don't worry about the 10^-1000, because, well, here you are". But you still have to worry about the difference between 1 and 100. If N beings in universe A and 100 * N beings in universe B all face this same question, your only reasonable conclusion as one of them is that the odds are 100-to-1 in favor of you being in universe B. So it's not the case that our existence tells us nothing about p. Of course, p is what it is, and only physics and chemistry and biology can actually reveal it, but the fact that we are here means that theories that make our existence much less likely are themselves much less likely.

anthuswilliams 5y ago
This is a very thoughtful rejoinder, thanks. > If X occurred, and is more likely under hypothesis B than A, then X is evidence for hypothesis B, otherwise you don't have a consistent system of probability. The point is, this idea breaks down when neither hypothesis would exist if X had not occurred. This differs from other questions of probability because the conditional probability of an event is predicated on the ability of an observer to ask about it. We should indeed be indifferent between the two hypotheses, if our own existence is all we have to go on. (This might seem strange, if you think of hypothesis B as having 100x the prior probability, but the point here is that we really don't have a basis for a prior at all.) > Now you can further imagine that God comes down from heaven and tells you that in fact both of these two universes actually exist, and they are the only two universes that exist, so you are definitely in one of the two, but doesn't tell you which one. Is your posterior probability now 50% between the two? No, of course I would assume I was in universe B. This is a very different question. Instead of "what is the nature of our universe" the question is "of the two existing universes God has described, which do I occupy?"