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Unfalsifiable statements are said to be important in science. An example of such a statement:

All swans are white.

In plain English, that's simply a truthful statement.

So what's the difference between unfalsifiable and true?

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    "All swans are white" is falsifiable by finding a swan that isn't white. It's also false, because we have done so. – Hurkyl Jun 3 '18 at 7:07
  • So if every known swan was white, the statement would be true, but it would still be falsifiable because there's a chance we might find a swan that isn't white? – David Blomstrom Jun 3 '18 at 7:38
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    According to Popper whose philosophy of science he characterized in terms of "conjectures and refutations" [see a book of his with this title] theories that are in principle unfalsifiable are not science. For a theory to be considered scientific it should be such that it is in principle open to falsification. There are theories that were falsified and yet are scientific and important - such as Newton's theory (that was falsified by Einstein). – Jordan S Jun 7 '18 at 12:51
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    Interesting. It would be really helpful to have a list of examples - scientific theories that are either based on falsifiable/unfalsifiable statements or that were disproved by falsifiable/unfalsifiable statements. Maybe I can make that a separate question; but I'll do some research first. – David Blomstrom Jun 7 '18 at 20:24
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    @DavidBlomstrom, if it is interesting for you, perhaps explore Karl Popper's views on philosophy of science through taking a look also at his "The Logic of Scientific Discovery" and his "The Open Society and its Enemies." – Jordan S Jun 8 '18 at 2:14
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The basic answer has been given several times: a theory is falsifiable if there is some way it could be shown to be false, but not every falsifiable theory has been shown false.

Of course we do not consider every theory to be true until it is shown false. Lots of theories are currently genuinely open questions. For example proton decay. It is not even clear how that could be falsified by experiment, since any amount of very careful searching which reveals no decay could be dismissed as not having looked long enough!

Other theories are accepted as true, although some conceivable evidence could refute them. There may even be well known evidence against them which is just not considered important enough. General Relativity is an accepted theory though, like all serious theories, it has some known problems. It is considered true, and falsifiable.

An important point: "falsifiable" per se is quite vague. Many people believe you can prove evolutionary theory false by showing it contradicts their reading of the Bible. Karl Popper used a more specific notion: "empirically falsifiable." And he noted this is a feature of how you view a theory, not of the theory itself: If you understand a theory in such a way that certain experimental outcomes would make you reject it, then you are taking that theory to be empirically falsifiable.

Some one else, though, might take that very same theory and insist that in the face of any conceivable contrary evidence, they would find some way around the evidence rather than reject the theory. Then they are taking that theory not to be empirically falsifiable.

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A theory is considered true, until it is proven false. The black swan, as mentioned in the comments (also an excellent book: Nassim Nicholas Taleb, The Black Swan), is a great example of this. Until a certain time, saying all swans are white was a true statement, relative to Europeans. It wasnt until Europeans discovered Australia - and black swans - that it was proven false. One can find innumerable observations to support a theory, but it only takes one to disprove it.

Something is unfalsifiable, if it cannot be proven false through measurable or repeatable means. Take the statement "Gravity exists everywhere." While we currently know this to be true and it is entirely possible that it is true, it is impossible to check every location in the universe for gravity. Due to our current understanding of physics, we have to accept it as true, as it is unfalsifiable.

  • "A theory is considered true" I would add that unsatisfiable statement can't be a part of theory. Therefore something unfalsifiable is not considered true. – rus9384 Jun 3 '18 at 11:16
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    Do you have any references to philosophers who take a similar position about falsifiability? This is a way to strengthen your answer and give the reader another place to look for more information. I edited the answer to add the author and name of the book you linked for clarity. – Frank Hubeny Jun 3 '18 at 21:16

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