Specifically, [these recent books that deal with parallel universes] argue that if some scientific theory X has enough experimental support for us to take it seriously, then we must take seriously also all its predictions Y, even if these predictions are themselves untestable (involving parallel universes, for example).

As a warm-up example, let's consider Einstein's theory of General Relativity. It's widely considered a scientific theory worthy of taking seriously, because it has made countless correct predictions -- from the gravitational bending of light to the time dilation measured by our GPS phones. This means that we must also take seriously its prediction for what happens inside black holes, even though this is something we can never observe and report on in Scientific American. If someone doesn't like these black hole predictions, they can't simply opt out of them and dismiss them as unscientific: instead, they need to come up with a different mathematical theory that matches every single successful prediction that general relativity has made -- yet doesn't give the disagreeable black hole predictions.

-- Max Tegmark, Scientific American guest blog, 2014-02-04