"Absence of evidence is not evidence of absence".

[Kirbmarc]

We infer its presence, because we detect gravitational and inflationary effects consistent with its existence. This may not be a result of dark energy.

Fair enough. "Dark energy" is not detectable, and believing in it is irrational. Suggesting it as a possible explanation isn't.

[Angrboda]

We infer its presence, because we detect gravitational and inflationary effects consistent with its existence. This may not be a result of dark energy.

Fair enough. "Dark energy" is not detectable, and believing in it is irrational. Suggesting it as a possible explanation isn't.

It is not unscientific to make a guess, although many people who are not in science think it is. Some years ago I had a conversation with a layman about flying saucers — because I am scientific I know all about flying saucers! I said “I don’t think there are flying saucers”. So my antagonist said, “Is it impossible that there are flying saucers? Can you prove that it’s impossible?” “No”, I said, “I can’t prove it’s impossible. It’s just very unlikely”. At that he said, “You are very unscientific. If you can’t prove it impossible then how can you say that it’s unlikely?” But that is the way that is scientific. It is scientific only to say what is more likely and what less likely, and not to be proving all the time the possible and impossible. To define what I mean, I might have said to him, "Listen, I mean that from my knowledge of the world that I see around me, I think that it is much more likely that the reports of flying saucers are the results of the known irrational characteristics of terrestrial intelligence than of the unknown rational efforts of extra-terrestrial intelligence." It is just more likely. That is all.

— Richard Feynman

[Kirbmarc]

How does Feynman's quote contadict what I said?

EDIT: Even Feynman talks about "making a guess". There is no reference to "believing".

[Angrboda]

How does Feynman's quote contadict what I said?

EDIT: Even Feynman talks about "making a guess". There is no reference to "believing".

I didn't say it did. However, if I were to do so, I would say that it is rational to believe in what you believe to be probable and not rational to believe in what you consider improbable. Since there is a good probability that dark energy or something like it exists, it's rational to believe in it. It is also rational to believe in things we can detect and things we are certain of, but restricting belief to those categories is itself irrational. (Not to mention this is verging on the same error that the logical positivists made: we can't detect the truth value of the sentence, "It is irrational to believe in the undetectable," therefore it is irrational to believe it. [This is not to say it then becomes rational to believe in the undetectable, only that your sentence itself is self-refuting].)

[Kirbmarc]

I would say that it is rational to believe in what you believe to be probable and not rational to believe in what you consider improbable.

I'd say that is only rational to consider something more probable or improbable. To believe is to be certain. You can't be certain of something that you think is only probable. Also, what is probable today may not be as probable tomorrow.

We can't detect the truth value of the sentence, "It is irrational to believe in the undetectable," therefore it is irrational to believe it

If you define a rational belief as one that is detectable, then it is irrational to believe in the undetectable.

[Angrboda]

We can't detect the truth value of the sentence, "It is irrational to believe in the undetectable," therefore it is irrational to believe it

If you define a rational belief as one that is detectable, then it is irrational to believe in the undetectable.

Ipse dixit proofs are not rational. They're invalid or circular.

[Kirbmarc]

Ipse dixit proofs are not rational. They're invalid or circular.

It's not a proof. It's a definition of the terms "rational belief".

[Angrboda]

Ipse dixit proofs are not rational. They're invalid or circular.

It's not a proof. It's a definition of the terms "rational belief".

Fine. You're believing in a definition you can't detect to be probably bullshit.

I'm gonna call you 'my little rock and equivocater' — you love to equivocate and dance around.

Your first statement, if only true by definition, is not believable on a rational basis. You're trying to beg the question. And not doing it very interestingly.

"Only Y are X."

"Why?"

"Because only Y are X, by definition."

"Why?"

"Because I said so."

[Edwardo Piet]

If you don't have a way to detect something, believing in it is irrational.

Logically irrational yes, pragmatically irrational not necessarily. If believing in something undetectable somehow gave you the boost of confidence required to help get you through something in life - or help you avoid death itself - then it can be pragmatically rational, it can be useful like a placebo can be useful. It is pragmatically rational to believe in what helps solve the problem, whether that belief is true or false. It is only when the logical irrationality of the matter negatively affects the pragmatic rationality of the matter that it also therefore becomes pragmatically irrational and unjustifiable too.

Just out of curiosity: can we not detect the truth of something through logic? If "detect" means to more or less "discover" don't you detect something whenever you discover the truth of something through logical argument?

[Angrboda]

If you don't have a way to detect something, believing in it is irrational.

Logically irrational yes, pragmatically irrational not necessarily. If believing in something undetectable somehow gave you the boost of confidence required to help get you through something in life - or help you avoid death itself - then it can be pragmatically rational, it can be useful like a placebo can be useful. It is pragmatically rational to believe in what helps solve the problem, whether that belief is true or false. It is only when the logical irrationality of the matter negatively affects the pragmatic rationality of the matter that it also therefore becomes pragmatically irrational and unjustifiable too.

Just out of curiosity: can we not detect the truth of something through logic? If "detect" means to more or less "discover" don't you detect something whenever you discover the truth of something through logical argument?

Logic is always inference. Physical is detection. Primarily, if for no other reason, we instinctively understand why detect is reliable when reliable. Inference is not nearly as robust or certain.

(Think of Godel and the Halting problem and NP complete.)