Carl Sagan says it very Well
THE DRAGON IN MY GARAGE.
As an example of skeptical thinking, Sagan offers a story concerning a fire-breathing
dragon who lives in his garage. When he persuades a rational, open-minded visitor to meet the dragon, the visitor remarks that they are unable to see the creature. Sagan replies that he "neglected to mention that she's an invisible dragon". The visitor suggests spreading flour on the floor so that the creature's footprints might be seen, which Sagan says is a good idea, "but this dragon floats in the air". When the visitor considers using an
infra-red camera to view the creature's invisible
fire, Sagan explains that her fire is heatless. He continues to counter every proposed physical test with a reason why the test will not work.
Sagan concludes by asking: "Now what's the difference between an invisible, incorporeal, floating dragon who spits heatless fire and no dragon at all? If there's no way to disprove my contention, no conceivable experiment that would count against it, what does it mean to say that my dragon exists? Your inability to invalidate my hypothesis is not at all the same thing as proving it true."