It really comes down to parsimony, economy of explanation. It is possible that your car engine is driven by psychokinetic energy, but if it looks like a petrol engine, smells like a petrol engine and performs exactly as well as a petrol engine, the sensible working hypothesis is that it is a petrol engine. Telepathy and possession by the spirits of the dead are not ruled out as a matter of principle. There is certainly nothing impossible about abduction by aliens in UFOs. One day it may be happen. But on grounds of probability it should be kept as an explanation of last resort. It is unparsimonious, demanding more than routinely weak evidence before we should believe it. If you hear hooves clip-clopping down a London street, it could be a zebra or even a unicorn, but, before we assume that it's anything other than a horse, we should demand a certain minimal standard of evidence.
-- Richard Dawkins, in "Science, Delusion and the Appetite for Wonder," The Richard Dimbleby Lecture, BBC1 Television (12 November 1996)

Either it is true that a medicine works or it isn't. It cannot be false in the ordinary sense but true in some "alternative" sense. If a therapy or treatment is anything more than a placebo, properly conducted double-blind trials, statistically analyzed, will eventually bring it through with flying colours. Many candidates for recognition as "orthodox" medicines fail the test and are summarily dropped. The "alternative" label should not (though, alas, it does) provide immunity from the same fate.
-- Richard Dawkins, The Devil's Chaplain (2004)