If I cannot disprove god or fairies or space monkeys, does this mean that these theories are possible?

If it doesn’t mean that, and one cannot prove that they are possible or impossible, what is the correct attitude to take towards these theories?

Should one adopt disbelief? Or in Bayesian terms, should one attach a very low probability to them? The problem with the Bayesian belief is that a non zero probability implies possibility. But that seems to be the same fallacy committed all over again.

  • Theories make predictions. Look at what predictions are being made and see if they match what you see happening.
    – Scott Rowe
    Aug 17, 2023 at 10:09
  • Some times they are disprovable since god's and angels break laws of physics.etc
    – r0k1m
    Aug 18, 2023 at 16:04

3 Answers 3


The relationship between possibility and proof is complicated, and the subject of much philosophical analysis. A good article in the SEP on the topic is "The Epistemology of Modality". Let's start with the simple:

Carl Sagan who famously discussed SETI and alien intelligence, IIRC, famously coined the phrase "Absence of evidence is not evidence of absence". One cannot search the earth and say, I have not seen alien beings, therefore alien beings do not exist, because to do so deductively leads to a fallacy. Obviously, if we were to search 1,000 planets in the local neighborhood, then we would have a much stronger case, but still an uncertain one because new evidence might defeat our conclusion. In logic, such evidence is called a defeater. Falibilism recognizes there are always things we don't know we don't know.

There is a real world example of this involving white and black swans, and to reason without recognizing that unknown unknowns are problematic for certain conclusions, one can be surprised by black swan events. From WP:

The black swan theory or theory of black swan events is a metaphor that describes an event that comes as a surprise, has a major effect, and is often inappropriately rationalized after the fact with the benefit of hindsight. The term is based on an ancient saying that presumed black swans did not exist – a saying that became reinterpreted to teach a different lesson after they were discovered in Australia.

We cannot prove the negative, but is possibility predicated on lack of disproof? You ask:

Is it a fallacy to say that something is possible if not disproven?

That depends on your views of conceivability. From the SEP article above:

Both Stephen Yablo (1993) and David Chalmers (1996, 2002, 2004, 2010: Ch. 6) have developed independent accounts of conceivability as a source of modal knowledge (for yet another account based on response-dependence, see Menzies 1998). Yablo holds that, when adequately understood, conceivability provides defeasible evidence for possibility.

More simply put, it's rational to presume something is possible if you can merely imagine it.

So, no, it's not fallacious necessarily to believe something is possible if it is not disproven, but it is important to remember that lack of proof isn't proof it isn't possible either. This is of course is a general rule of thumb, and depends on other metaphysical presuppositions.


Ham, The Space Ape

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If something hasn't been proven to be impossible then it doesn't imply that it is possible. Conversely, if something hasn't been proven to be possible, it doesn't imply that it's impossible. This is a mistake I make, unfortunately, on a regular basis; it's almost a reflex now. Remember to distinguish metaphysics from epistemology.

  • 1
    technically, that's an ape in space, rather than a space ape
    – user67302
    Aug 17, 2023 at 4:00
  • @legoman What is the conceptual difference? Would it count if you'd sent a pregnant ape into space?
    – haxor789
    Aug 18, 2023 at 13:01
  • @legoman I see a human hand. I don't think the chimp is in space at all.
    – J D
    Aug 18, 2023 at 15:42
  • 1
    stop ruining my joke @JD
    – user67302
    Aug 18, 2023 at 22:02
  • 1
    @legoman LOL Done.
    – J D
    Aug 18, 2023 at 22:37

Suppose you're blind and asked to investigate whether an object is blue without any tools or whatnot aiding you in your quest. So that it's technically impossible for you to tell whether that object is blue, that in return does not mean that the object in front of you is blue.

However that inability is a different form of inability from idk a mathematician who has tried every possible option within a given space of options and was not able to disprove something, in that case the exhaustive search of ways to negate can be a proof.

So the strength of that argument can vary drastically between having check 10 out of billions of options or having tested every reasonably available option so that only blind faith and hope keep the theoretical possibility alive.

I guess the conceptual problem in that question is that these are not fixed value statements, but rely on the current perspective which is something able to evolve with data.

So suppose you have a black bag with objects inside of it. You don't know what these objects are. So at this point (and from your perspective) anything is possible to be in that bag. Now you reach into it an feel that there are numerous balls in it. Now despite previously being possible, now the existence of cubes in that bag has moved from possible to impossible.

So yeah if you do not have the evidence to outright accept or reject a hypothesis it's probably best to not pass judgement yet, but to consider it possible but with a probability reflecting your current level of evidence (for example pretty low), which in the case of all the possible options that are probably already overwhelming means not considering it at all until it passes the threshold of relevancy.

Possible in that sense signifies a lack of certainty of an existence which can and will be revised later and does not indicate existence. If possible requires objective possibility. Like idk for a coin to land heads, the coin needs to have a heads side, then it's probably best to postpone even that assignment of possibility till you're able to do that.

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