by Carl Sagan
"A fire-breathing dragon lives in my garage"
"Show me," you say. I lead you to my garage. You look inside and see a ladder, empty paint cans, an old tricycle — but no dragon.
"Where’s the dragon?" you ask.
"Oh, she’s right here," I reply, waving vaguely. "I neglected to mention that she’s an invisible dragon."
You propose spreading flour on the floor of the garage to capture the dragon’s footprints.
"Good idea," I say, "but this dragon floats in the air."
Then you’ll use an infrared sensor to detect the invisible fire.
"Good idea, but the invisible fire is also heatless."
You’ll spray-paint the dragon and make her visible.
"Good idea, but she’s an incorporeal dragon and the paint won’t stick." And so on. I counter every physical test you propose with a special explanation of why it won’t work.
Now, what’s the difference between an invisible, incorporeal, floating dragon who spits heatless fire and no dragon at all?