UPDATE: In a comment below this post, reader Peter Beattie calls attention to a short summary of Popper’s falsifiability criterion that he thinks will be helpful to readers who want the nuances of Popper’s views.

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This will be short. As many of us know, Karl Popper demarcated a scientific theory from a nonscientific one because the former is falsifiable—there are experiments or observations that can be done to disprove it. The “theory” of evolution, for example, could be disproven if we regularly found well-dated fossils out of the proper order (like mammals in the Devonian, for instance), if species didn’t have genetic variation to respond to selection, or if we often found “adaptations” in member of one species that were useful only for another species (e.g., a special nipple on a female mole that was only used for suckling mice).

I’m told that falsification is naive as a criterion for good science, and that scientists no longer accept or use that as a criterion. Some assert that, in contrast, a good scientific theory is one that best explains the data we have. But it seems to me that this is equivalent to falsifiability, for a theory that best explains the data we have could be shown not to explain the data we have.

At any rate, putting this musing aside, my question is this: is there any scientific fact or theory that is widely accepted despite the fact that it is not in principle capable of being falsified? I am referring to real theories here, not possible theories.

It is my impression, for instance, that string theory in physics isn’t widely accepted as true simply because we haven’t found a way to test it—to test that its predictions are verified or not. And I often hear—Anthony Grayling and Hitchens both said this, I believe—that a theory that can explain everything explains nothing (i.e., God constructed the process of evolution). In other words, a theory that can’t be shown wrong is useless. The presupposes falsifiability as a criterion for scientific truth.

Englighten me here, but note that this discussion deals with the philosophy of science, so if you think that endeavor is useless you shouldn’t be responding!