Suppose I have a thought about your invisible dragon. I claim it's real, though I have no evidence to show you it's there.

"What is this thought?" you ask.

"Oh, it's right here in my mind," I say. "I neglected to say that it's an invisible thought."

"Maybe you could speak your thought," you prompt.

"Ah, but you would have no way of discern whether I spoke the thought or something different from it," I say with sorrow.

"Maybe we could do an MRI on your head and show what parts of the brain are lighting up?" you offer.

"Indeed, it's always fun to go toward scientific evidence. But I fear the MRI will only tell you which parts of my brain are active at the time of the test and not really tell you what my thought is."

And so on. No matter what plans of mice and men, there is no evidence of the thought inside my head about your post.

Now, I wonder, what's the difference between the reality of my thought, that doesn't exist anywhere in space or is empirically observable, and no thought at all? There is no conceivable experiment that can be concocted to read it, decipher it, or even confirm its existence, and yet I know the thought is truly there. It truly exists and is real despite no material evidence. The conclusion is: whether you believe I have a thought or not is no warrant of its existence or not. All quest for scientific evidence will fall short of guaranteeing for you what my thought is about your post. And yet my thought truly does exist.

Suppose I seriously make such an assertion to you. Surely you'd want to check it out, see for yourself. There have been innumerable stories of dragons over the centuries, but no real evidence. What an opportunity!

"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? 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. Claims that cannot be tested, assertions immune to disproof are veridically worthless, whatever value they may have in inspiring us or in exciting our sense of wonder. What I'm asking you to do comes down to believing, in the absence of evidence, on my say-so.