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For certain kinds of claims, it seems obvious in a way to not believe in them due to their lack of evidence or unfalsifiability. For example, one may not believe in undetectable goblins or fairies.

But for other kinds of claims, I also fail to see “no evidence” and yet many would still believe them.

For example, how reasonable is it to believe that the earth is a sphere? Is it reasonable to believe that the earth is a sphere? What is the degree of belief you should have that the earth is a sphere?

For all three of these claims, how would you test that you’re wrong? An ardent skeptic can look at all the evidence, state “well, I could be being tricked” and claim that it’s not reasonable to believe in a spherical earth. Why exactly would he be wrong? You can’t even show that he’s wrong to an nth degree! Even intuitive considerations of the form “well I can’t put an exact probability to it, but surely it’s above 80% likely for the earth to be a sphere” can’t be falsified.

Why is “there are invisible ghosts” any less falsifiable than “it is reasonable to believe the earth is a sphere with X% or X-Y% probability”? If not, why believe in either of them?

Of course, for pragmatic reasons, one would ultimately always have to make a choice and make decisions. No one is arguing against this. But that arguably says more about our actions and what we would do as a matter of fact than belief (unless one just defines belief to just be how we would act in given scenarios).

thinkingman
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  • How does the dogmatic skeptic know that it's possible to be wrong in the first place? What justifies believing in the possibility of being tricked? (This is not a response to Pyrrhonian skepticism, incidentally.) Quine did contend that the web of our beliefs is too vast and intricate for naive falsfiability to be a stable criterion for adding to or taking away (at least almost) any/all of the strands in the web. – Kristian Berry Aug 29 '23 at 18:07
  • He doesn’t know that it’s possible to be wrong. But he doesn’t know that it’s impossible to be wrong either. Hence, he doesn’t have to believe, although pragmatically, he can simply not consider that hypothesis. Arguably, he has no choice to – thinkingman Aug 29 '23 at 18:24
  • How does he know that he doesn't know that it's impossible, though? If he doesn't know what knowledge *per se* is, might he not know things without knowing that he knows them? – Kristian Berry Aug 29 '23 at 18:42
  • It depends on how you define “know”. I suppose some would say you don’t know something if you can doubt it. – thinkingman Aug 29 '23 at 19:03
  • I suppose then that, "I believe that I have defined the word 'knowledge' in such-and-such a way," is justifiable and unfalsifiable in some sense. Or, "I believe that I believe all that," etc. But on the other hand, we would not usually speak of stipulative definitions as falsifiable in this manner (and on yet another hand, we do often speak of bizarre thought experiments as falsifying (or at least undermining) this or that analysis of a concept). – Kristian Berry Aug 29 '23 at 19:16
  • Yes. It is perfectly reasonable to believe that the universe follows the same physical laws even beyond our light cone, for example, even though we can never interact with that. Many well established theories make predictions that cannot be verified or falsified in addition to those that can be, and we are justified in believing the former on the basis of the latter. And yes, one of the definitions of belief is as that upon which one is prepared to act (Peirce). – Conifold Aug 29 '23 at 19:37
  • There is plenty of evidence that the earth is round. Astronauts have flown round it and taken photographs for one thing. But there are credible propositions that are unfalsifiable. Some of these are consequences of reliable theories or are statistically highly probable. E.g. sentences about the remote future or the remote past. Others might be considered *a priori* knowable. Others might be general principles such as moral principles. Also, tautologies are unfalsifiable, though I expect that is not what you had in mind. – Bumble Aug 29 '23 at 21:04
  • The answer, as with all your questions, is probability. Nothing is ever absolutely falsified. There is always a chance the earth is a cube. The evidence just makes it *likely* that it's round. To say something is falsifiable just means that there is the potential for evidence that could make it arbitrarily unlikely. – causative Aug 29 '23 at 21:23
  • Saying that there is a chance the earth is a cube implies you know beforehand that that’s possible. What justification do you have for this? A dice has a chance of landing on 1/6 because we know it’s apriori possible for it to land on that side. Where is the notion of this with respect to the earth being a cube? – thinkingman Aug 29 '23 at 21:52
  • @thinkingman Anything you can imagine is a priori possible. We ideally begin with a universal Bayesian prior assigning nonzero probability to every possible world, like in Solomonoff induction. In fact it even extends to analytic truths, because from the perspective of someone who does not know that a mathematical theorem T has been proved, both T and ~T are possible (receive >0 probability) until the proof becomes known. And in fact, even after the proof becomes known, both T and ~T still have >0 probability, because it is always possible the proof contains an undetected mistake. – causative Aug 29 '23 at 22:31
  • you've already asked this question, i refer you to my earlier answer, [fwiw](https://philosophy.stackexchange.com/a/97863/67521) – forlove1 Aug 29 '23 at 23:24
  • Imagining something just makes it…possible to imagine. It has no bearing on whether it’s possible as a matter of mind independent reality. – thinkingman Aug 29 '23 at 23:25
  • There are some gradations, here: if conceivability is different from imaginability, and non-contradictoriness above conceivability, then if non-contradictoriness means possibility, so would conceivability and imaginability. @thinkingman But Kant did make your claim, saying that the possibility that something is not contradictory is not the same as the possibility of its objective/material reality. It often seems that you would find Kant quite appealing, although you also seem to prefer coming down hard in favor of empiricism, whereas Kant had some room for rationalism, of course. – Kristian Berry Aug 30 '23 at 00:06
  • Yeah I mean these kinds of discussions ultimately end up devolving into semantics anyways. I just question the use of the word “possible” to mean non contradictory since “possible” in most contexts is used in a metaphysical sense atleast in ordinary parlance. We already have terms for things we can conceive of without contradiction, such as “logically possible.” To say that it is possible for fairies to exist seems to be making a metaphysical claim without evidence to me – thinkingman Aug 30 '23 at 00:10
  • @thinkingman If you cannot rule something out, you must treat it as possible. What does "possible as a matter of mind independent reality" even mean? There may be just a single mind independent reality, containing only a single fixed timeline of events, and if that's the case, "possible as a matter of mind independent reality" is virtually meaningless. In practice, possibility is entirely a matter of what we don't already know for sure isn't the case. – causative Aug 30 '23 at 00:22
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    Yeah I concur. I have a problem with the notion of possibility in the first place. There’s no such thing. I believe in one mind independent reality where things happen or have happened or they don’t. – thinkingman Aug 30 '23 at 00:27
  • @thinkingman You're the one who introduced the term "possible" to this discussion, and now you're saying you have a problem with it? Make up your mind. You said, "A dice has a chance of landing on 1/6 because we know it’s apriori possible for it to land on that side." Were you wrong then, or wrong now? – causative Aug 30 '23 at 00:33
  • I meant I don’t like the word possible personally. However, in ordinary parlance, possible means a state of affairs of the world that doesn’t contradict known laws. To say that fairies are “possible” is a metaphysical claim that must be proven. You can’t just assert that – thinkingman Aug 30 '23 at 01:06

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The two claims are different in an important way. The existence of invisible fairies (I'll assume they are also unhearable, untouchable, etc., to the point they have no detectable impact on the world) is inherently unfalsifiable. There is no possible difference it would make in the world. The proposition that the earth is a sphere--or flat, or a cube--is in principle falsifiable. The truth of the matter makes a detectable difference in the world.

However, you are apparently coming from a standpoint of personal falsifiability. You cannot personally tell the difference between a flat earth and a round one--it's too big for you to have direct experience of it.

For most people, the answer would be that the round earth is attested to by a variety of things external , but that I can see--photos, historical records, scientific experiments. All those things could be faked, but it would take a concerted conspiracy. Therefore, I am willing to extend personal credulity to the round earth, but not the invisible fairies.

You are free to be radically skeptical about anything you don't have direct personal experience of, but it would make for a very limited set of things you could be said to know.

Chris Sunami
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There are plenty of propositions that are both falsifiable and rational to believe. This is because falsifiability concerns evidence, but there are other forms of epistemic justification other than evidentiary ones. So there are beliefs that can be both justified without being empirically justified; and there can be beliefs that are both justified and cannot in principle be empirically justified.

For example, consider a priori truths (truths we can justify through reason alone), such as the claim that '2 + 2 = 4'. This claim is unfalsifiable, because there is no possible state of affairs that could prove it false (consider: if we found that two objects and two objects, when bundled together, made five objects, we would not think that 2 + 2 = 5; we would think that an extra object had been created). Yet no one would seriously suggest that it is irrational to believe that 2 + 2 = 4. This general pattern applies to many if not all a priori propositions, such as mathematical or logical rules of inference or the truths that follow from them.

604adrian
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