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## A Few Good Modal Paradoxes

People most often hear about paradoxes that challenge our notions of truth and falsity, like the Liar Paradox, Curry's Paradox, Russell's Paradox, Berry's Paradox, etc. But just as interesting are the paradoxes that challenges other notions we hold dear, the ones philosophers call "modal" notions: knowledge, possibility, morality. So let me present one of the most famous ones, called Fitch's Paradox of Knowability, and if people find that interesting I can talk about a few other favorites of mine.

The question we're dealing with is: Are all true statements knowable? To put it another way, is it possible for there to be some truth which can never be known, no matter how hard you try? Here's an argument that seems to answer this question. Obviously there are some unknown true statements out there; we don't know everything, do we? For instance, either "The Riemann Hypothesis is true" or "The Riemann Hypothesis is false" is one of these statements. In any case, let P be some unknown true statement. Then consider the statement Q, which says "P is an unknown truth." Then Q is obviously a truth. Is it possible for Q to be known? Well, suppose Q were known. Then we would be able to say "I know that Q is true" or equivalently "I know that P is an unknown truth" or in other words "I know that P is true and that P is unknown." But it's impossible for that to be true, isn't it? Because if you knew that P is true, then P would be known, so it would be impossible to know that P is unknown, because P is not unknown, and you can't know a false statement! Thus it's impossible to know Q, so in other words Q is an unknowable truth.

So to review, we started with the hypothesis that P is an unknown truth and we got to the conclusion that Q is an unknowable truth. So "there exists an unknown truth" implies "there exists an unknowable truth." Turning this around, "all truths are knowable" implies "all truths are known", which is crazy! Clearly it is possible for there to be some truths which we happen to be unknown right now, but might be discovered in the future. But Fitch's argument above seems to suggest that if you believe that any truth is within our grasp, you have to believe that we already know everything!
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So to review, we started with the hypothesis that P is an unknown truth ..

But even at the start, that hypothesis seems a little shaky, and rather a play on, or fluid use of, wording.

How would you know it's a truth if it's unknown ?

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 Quote by alt So to review, we started with the hypothesis that P is an unknown truth .. But even at the start, that hypothesis seems a little shaky, and rather a play on, or fluid use of, wording. How would you know it's a truth if it's unknown ?
Sorry, maybe I was unclear. We start with the hypothesis that there EXISTS some unknown truth P. Presumably we don't know what that truth is.

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## A Few Good Modal Paradoxes

IMO; For any proposition to be true, you need a criterion for its truth, and the criterion needs to be satisfied. And it is only upon verification we say that it is satisfied.

In this sense you can't have unknowable truth. "P is true, but I don't know it to be true" just doesn't make sense. "P is true" doesn't express more or less that "I know P is true". The paradox arise from abuse of language, just like any other.

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 Quote by lugita15 Sorry, maybe I was unclear. We start with the hypothesis that there EXISTS some unknown truth P. Presumably we don't know what that truth is.
No, you were clear. But I'm saying that the hypothesis is nosnensical, imo.

May as well start with the hypothesis that there exists a five legged tripod. It's a similar word play to say we have an unknown truth. You can't call it truth if it's unknown. To call it truth you would have to know it as being that.

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 Quote by disregardthat IMO; For any proposition to be true, you need a criterion for its truth, and the criterion needs to be satisfied. And it is only upon verification we say that it is satisfied. In this sense you can't have unknowable truth.
That is a view called verificationism, which states that all truths are knowable. The whole point of Fitch's paradox of knowability is to disprove verificationism
 "P is true, but I don't know it to be true" just doesn't make sense.
Knowledge is different than belief. You may believe one thing, but find out later you were wrong. On the other hand, if you know something then by definition it must be true. A common definition of knowledge used in philosophy is justified true belief. In other words, in order to know a statement P, the following three criteria must be met:
1. You believe that P is true.
2. P is true.
3. You are justified in believing that P is true, in the sense that you cannot possibly be wrong about it.
 "P is true" doesn't express more or less that "I know P is true".
These two statements are very different. To say "P is true" is the same as saying "I believe P is true", but is very different from saying "I know P is true."
 The paradox arise from abuse of language, just like any other.
No it doesn't, at least not in the straightforward way you're thinking.

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 Quote by alt No, you were clear. But I'm saying that the hypothesis is nosnensical, imo. May as well start with the hypothesis that there exists a five legged tripod. It's a similar word play to say we have an unknown truth. You can't call it truth if it's unknown. To call it truth you would have to know it as being that.
I think you still don't understand what I'm saying. I'm not saying that there is a particular truth which we know to be unknown. Rather, I'm saying that there EXISTS an unknown truth out there, even if we don't know what it is. Surely you agree that we don't know everything, don't you? Like we don't know whether the number of hairs on Obama's head is even or odd. Yet either "the number of Obama's hairs right now is even" or "the number of Obama's hairs right now is odd" must be true, and yet presumably no one knows which one. But one of these is surely an unknown truth, so we can at least say that there exists an unknown truth, can't we?

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 Quote by lugita15 That is a view called verificationism, which states that all truths are knowable. The whole point of Fitch's paradox of knowability is to disprove verificationism
That's ridiculous. The whole point of requiring a criterion for truth is that one rejects the notion of true statements being true simply in virtue of their meaning. So there "existing unknown truth out there" is meaningless. Propositions require a well-defined criterion for truth. Fitch's paradox doesn't disprove anything in this regard, it is just playing around with words.

 Quote by lugita15 Knowledge is different than belief. You may believe one thing, but find out later you were wrong. On the other hand, if you know something then by definition it must be true.
The point is that by asserting a proposition, you can't deny that you believe it. Saying "P is true and I believe P is false" is simply meaningless. "P is true" and "I believe P is true" has no different criterion for truth, so it's impossible to assert one of them are deny the other. Many paradoxes arise from this kind of abuse. In the same fashion, asserting that "I know P to be true and P is false" is meaningless.

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 Quote by lugita15 I think you still don't understand what I'm saying. I'm not saying that there is a particular truth which we know to be unknown. Rather, I'm saying that there EXISTS an unknown truth out there, even if we don't know what it is. Surely you agree that we don't know everything, don't you? Like we don't know whether the number of hairs on Obama's head is even or odd. Yet either "the number of Obama's hairs right now is even" or "the number of Obama's hairs right now is odd" must be true, and yet presumably no one knows which one. But one of these is surely an unknown truth, so we can at least say that there exists an unknown truth, can't we?
Well in that case, you can reduce a great many (perhaps all) things to your definition of unknown truth. The number of atoms making up your computer screen for instance. An unknown truth. The number of cells in your left ear. Same. The exact number of cents that flowed through the American economy between 9 AM and 10.29 AM today. The number of raindrops that fell on Tokyo between 1934 and 2011. All unknown truths. This however, is reduction to the ridiculous, as is your example of Obamas hairs.

So if reduction to the ridiculous is your thing, then I suppose Fitch's paradox is attracive.

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 Quote by disregardthat That's ridiculous. The whole point of requiring a criterion for truth is that one rejects the notion of true statements being true simply in virtue of their meaning. So there "existing unknown truth out there" is meaningless.
Don't you think that either "The number of hairs on Obama's head is even" or "The number of hairs on Obama's head is odd" is an unknown true statement?
 Propositions require a well-defined criterion for truth. Fitch's paradox doesn't disprove anything in this regard, it is just playing around with words.
It's not just playing with words, at least not in the sense you're talking about, because it can be formalized symbolically using epistemic logic. See here. (That's a great article, and it has numerous proposed resolutions to Fitch's paradox. If anyone is interested I can discuss my preferred resolution.)
 The point is that by asserting a proposition, you can't deny that you believe it.
I agree.
 Saying "P is true and I believe P is false" is simply meaningless.
It's not meaningless, it's just wrong.
 "P is true" and "I believe P is true" has no different criterion for truth, so it's impossible to assert one of them are deny the other.
I agree, they mean the same thing, so to assert one and deny the other would be wrong.
 Many paradoxes arise from this kind of abuse.
As I said, Fitch's paradox does not arise from at least that kind of abuse of language, because it can be expressed in symbolic language which avoids all the ambiguities and vagaries of English.
 In the same fashion, asserting that "I know P to be true and P is false" is meaningless.
It's not meaningless, again it's just contradictory and hence false.

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 Quote by alt Well in that case, you can reduce a great many (perhaps all) things to your definition of unknown truth. The number of atoms making up your computer screen for instance. An unknown truth. The number of cells in your left ear. Same. The exact number of cents that flowed through the American economy between 9 AM and 10.29 AM today. The number of raindrops that fell on Tokyo between 1934 and 2011. All unknown truths.
Yes, we can find a lot of examples of unknown truths.
 This however, is reduction to the ridiculous, as is your example of Obamas hairs.
I agree that these are silly examples, but there's nothing fundamentally wrong with them. They're just a way to illustrate that there are such things as unknown truths.
 So if reduction to the ridiculous is your thing, then I suppose Fitch's paradox is attractive.
The reasoning in Fitch's paradox is not as ridiculous as you think. I suggest you examine Fitch's logic more closely.

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 Quote by lugita15 Don't you think that either "The number of hairs on Obama's head is even" or "The number of hairs on Obama's head is odd" is an unknown true statement?
Absolutely not. I personally believe it is a very basic misconception of logic. Let me explain:

The logical conjunction "The number of hairs on Obama's head is even OR the number of hairs on Obama's head is odd" is true by virtue of being a logical tautology. There is no need for any criterion here.

But either of the statements P: "The number of hairs on Obama's head is even" and Q: "The number of hairs on Obama's head is odd" requires criteria for truthfulness, such as the result of counting the hairs being even or odd. The truth of P is realized by satisfying such a criterion.

It's tricky when it comes to time: If the criterion for a proposition P (which does not depend on time) is satisfied tomorrow, it doesn't make it correct to assert "P is true now" today. It would however be correct to assert "P was true yesterday" tomorrow. The statements have a different sense. So we could say "that the truth(-value) of P was unknown yesterday" tomorrow, but it wouldn't be correct to call it an unknown truth now.

This form of verificationism is very much alike the way we use ordinary language, and the way we treat scientific hypotheses and evidence. It is only in the platonic pits of formal logic or shaky metaphysics one end up with such silly paradoxes.

 Quote by lugita15 It's not meaningless, again it's just contradictory and hence false.
Contradictory, meaningless, useless. All the same to me. It isn't false in the sense of failing to satisfy its criterion, because there is no criterion, none can be given.

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 Quote by disregardthat But either of the statements P: "The number of hairs on Obama's head is even" and Q: "The number of hairs on Obama's head is odd" requires criteria for truthfulness, such as the result of counting the hairs being even or odd. The truth of P is realized by satisfying such a criterion. It's tricky when it comes to time: If the criterion for a proposition P (which does not depend on time) is satisfied tomorrow, it doesn't make it correct to assert "P is true now" today. It would however be correct to assert "P was true yesterday" tomorrow. The statements have a different sense. So we could say "that the truth(-value) of P was unknown yesterday" tomorrow, but it wouldn't be correct to call it an unknown truth now..
OK, forget about truths that are unknown in general. Do you at least agree that there are truths that you do not know, but perhaps that other people do know? Because even with that assumption we can carry through Fitch's paradox, and use it to disprove the statement "Any truth can be known by you."

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 Quote by lugita15 OK, forget about truths that are unknown in general. Do you at least agree that there are truths that you do not know, but perhaps that other people do know? Because even with that assumption we can carry through Fitch's paradox, and use it to disprove the statement "Any truth can be known by you."
What you are suggesting is that if P is a proposition known to be true by others, but not by me, and I realize that it is known to others and hence true (since it is supposed to be knowable by hypothesis), then upon realization (that it is known to others) I simultaneously can assert that it is unknown to me and known to me at the same time?

This time you can't deny you are playing with words, or more specifically you are ignoring the temporal aspect of the situation:

When I realize something I didn't know before, I am made aware of that I didn't know in the past. Not that I don't know now.

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 Quote by disregardthat What you are suggesting is that if P is a proposition known to be true by others, but not by me, and I realize that it is known to others and hence true (since it is supposed to be knowable by hypothesis), then upon realization (that it is known to others) I simultaneously can assert that it is unknown to me and known to me at the same time?
No, I'm suggesting something really obvious, namely that there is a statement P known to others and not to you, and that you do not know that P is known to others, but later you can come to know that P is true, at which point it will be simply be known to you, not known and unknown at the same time. Or if you prefer, you can later come to know that P is known tto others, at which which point you can conclude that P is true, so P will be known to you, not simultaneously known and unknown. What I'm saying is just trivial.
 Quote by disregardthat When I realize something I didn't know before, I am made aware of that I didn't know in the past. Not that I don't know now.
You and I are in complete agreement on that point.
 disregardthat, do you believe there is such a thing as objective truths? Or do you think things can only be true to people? I'm having trouble understanding your objections.

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