You can't. It's a universal philosophical limitation on science. One humorous way to phrase it would be, "there could always be a gnome hiding behind the camera."
You have got to see the implicit, unspoken qualifiers that are on all statements like that. "It would be counter to all known patterns if...", "it would have to be extraordinarily small, beyond unimaginable sensitivities, if..." and "we can't imagine any practical distinction between what it seems like and what it might be," are all reasonable interpretations of the statement "it isn't." The only unreasonable interpretation would be the literal one.
(P.S. It is not actually fundamental, it is made up of right-electrons and left-electrons, coupled together.)
You have got to see the implicit, unspoken qualifiers that are on all statements like that. "It would be counter to all known patterns if...", "it would have to be extraordinarily small, beyond unimaginable sensitivities, if..." and "we can't imagine any practical distinction between what it seems like and what it might be," are all reasonable interpretations of the statement "it isn't." The only unreasonable interpretation would be the literal one.
(P.S. It is not actually fundamental, it is made up of right-electrons and left-electrons, coupled together.)