Before I go on with this post, the assertion in the title is actually logically sound and consistent, which I find beautiful.

Now, more seriously, this is something I have noticed in most answers. The answer is almost always one of those things:

  • "No because of that very simple fact" (usually but not always, a fundamental principle of thermodynamics)
  • "Yes and it actually exists" (or has existed in the past).
  • "This is a technology we expect to have soon".

A note on the Fermi paradox: if you rephrase "is life on another planet possible" as "is life on a planet possible", this falls in the "Yes and it exists" category.

I find this both mind-boggling and beautiful. In a sense, it's quite understandable as nature hates void and we like to realize anything that seems possible (allegedly sometimes just "because it's hard").

Are there some answers where it says "No because of that complicated fact"? Was there an answer like "yes, it's possible, but it didn't happen on earth because ..." ?

Also, am I just underinformed? Some elders may have knowledge about good answers to share...

  • $\begingroup$ I've actually noticed this trend myself. $\endgroup$
    – Ranger
    Commented Dec 13, 2016 at 16:35
  • 1
    $\begingroup$ You asked, we have $\endgroup$
    – MolbOrg
    Commented Dec 19, 2016 at 0:36

3 Answers 3


I think a major factor in this effect is the "Open World Assumption." I find the Open World Assumption to be very popular among people developing worlds because it states that anything which is not proven impossible may be true. Contrast this with the Closed World Assumption, which is the assumption that anything you cannot prove to be true is false. These two assumptions sit on opposite ends of a giant spectrum of different approaches to life.

If one believes their listener is using a closed world assumption, any "maybes" they may say will get turns into falsehoods by the closed world assumption in the listener's mind. After this happens enough times, I think it's natural to flip it around and turn "maybe true" into "true," just to challenge the closed-world listener to pony up and think about the topic enough to generate a counter.

Perhaps once the closed-world-minded person spends enough time on the topic, there will be an opportunity for them to meet in the middle in some of the more nuanced positions that a debate can take between closed and open worlds.


Logically speaking, possibility does in fact imply existence.

As long as we take into account the following:

  • The universe is infinite
  • The requested item/concept/idea doesn't break the laws of physics

For as long as the universe is infinite, you can check different places for the existence of a valid item/concept/idea - which means you can check until you find it and never run out of places to check. You'll find it eventually, just like how you can eventually count to any number, as all numbers are contained within the limits if negative infinity and positive infinity.

But why is this an issue? What does it matter that all reality-check answers are of those three categories? I suspect it's exactly because the question needs an answer of that type, that all the answers end up being of that type. After all, if you asked "does X exist in real life?" and I said "Monkeys like bananas", that wouldn't quite make sense, would it?

  • $\begingroup$ First, the observable universe is finite, and (except for astronomy questions) existence is found on earth. Also, you may want to learn about uncountable sets. I didn't thought about this as an issue, more as a good discussion topic. $\endgroup$
    – PatJ
    Commented Dec 15, 2016 at 15:36
  • $\begingroup$ @PatJ The observable universe only comprises of the area of space that light and signals have had time to reach Earth since the big bang, meaning there is an "non-observable universe" which isn't finite. If you're limiting your logic to 'within the observable universe, then yes, possibility =/= existence. You've lost me with uncountable sets, I don't think it applies here. $\endgroup$
    – Aify
    Commented Dec 15, 2016 at 16:57
  • $\begingroup$ You can search for an infinite time without finding what you're looking for. You cannot "eventually count to any number" if your set is uncountable. $\endgroup$
    – PatJ
    Commented Dec 16, 2016 at 1:59
  • $\begingroup$ @PatJ You may not ever find it, but the possibility of it existing at the next place is always there - besides, the universe is countably infinite, so uncountable set logic doesn't apply. $\endgroup$
    – Aify
    Commented Dec 16, 2016 at 4:55
  • $\begingroup$ @Aify is it though? How would you go about making a list of every single point in the universe? For any given two points in space there are infinite points between them. Much like for any two real numbers there are infinite real numbers between them, the most often used example of an uncountable set. The number of planets (or particles) in the universe is still countably infinite though. Not that this would matter. It's impossible to count to pi but that doesn't mean that it doesn't exist. $\endgroup$
    – Annonymus
    Commented Dec 18, 2016 at 17:35
  • $\begingroup$ @Annonymus you've proved my point exactly: The number of planets is countably infinite. $\endgroup$
    – Aify
    Commented Dec 19, 2016 at 3:05

I think you are mischaracterizing a lot of those answers.

Is something realistic? If it has existed in the real world before, or still exists now, then obviously it is a member of the realistic club.

The Fermi paradox is not "is life on another planet possible." The paradox is that all of our computations using fairly modest assumptions suggest the universe should have a great deal of extraterrestrial life, but, as Fermi notes, we've never seen any evidence of them. The paradox is in the disparity between a near certainty they exist and the glaring lack of evidence they exist.

But to take your point, and ignoring all the "infinity" excuses, if the question is "Is life possible on other planets," the reality-check is that, since life exists on our planet, and to the best of our knowledge it evolved here rather quickly and there is nothing particularly unique about our planet, then yes, life is possible on other planets.

It is not that possibility implies existence; it does not. But something actually having existed is a stronger argument than just "it is possible": If something has existed in real life then that thing is automatically realistic. No infinite-possibility arguments are needed.


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