"What is that?"
"It's just a scrap of paper."
"Then why are you looking at it so closely?"
"I'm reflecting on how I see it."
"I see it too. What's the big deal?"
"I'm not sure, but it strikes me that seeing isn't as straightforward as we take for granted. Well, not just seeing, but perceiving in general. Here, feel the paper."
"It feels like paper. I still don't get it."
"Well, what is it to see, or feel, the paper?"
"I know what it is to see. I'm seeing right now. I see the paper. Ditto for feeling."
"Sure, of course. But what, exactly, is happening now that wasn't before you looked at the paper?"
"That's easy. Light reflected by the paper is now hitting my eyes, and my brain extracts information from that light — or something like that. I'm not a scientist."
"A camera processes reflected light too, but it doesn't see."
"Who says it doesn't see?"
"Well, okay, perhaps there's some way in which a camera 'sees', but quite obviously there's a big difference between you and the camera. The camera isn't conscious. What it is for you to see the paper seems to have something to do with the way you're conscious of the paper."
"What do you mean by 'conscious'?"
"Exactly! If I had a good answer, I wouldn't be thinking so hard about what it is to see. It seems obvious that when you or I see the paper, something happens that doesn't happen when a camera merely registers and encodes the light reflected from the paper. But, it's hard to say what that something is. Here, perhaps, is a first try. You and I, but not the camera, have a private 'inner' perspective from which we see the paper. In contrast, the camera, but not us, is 'dark' 'on the inside'. Although it's a different example, pain might help make this clear. If you smashed my hand with a hammer, I would feel pain. If you smashed the camera, it would feel nothing, even if you rigged it up with sensors comparable to the nociceptors in my hand. Anyway, the way in which you or I see (or feel) the piece of paper seems deeply tied up with this private inner perspective of ours. When we see the paper, it's as if the paper is somehow intruding into our private inner perspective, in something of the way that pain sensations (unwelcomely) intrude when we're injured."
"Well, the pain example shows that answer can't be right. Take feeling. When you feel the paper, what you're actually feeling are just sensations generated by touching the paper, not the paper itself. It's always sensations that 'intrude' into our minds, not stimuli themselves."
"Even if that's true, it's certainly not how things strike us as we use our senses. When I look at the piece of paper, it is for me as if the paper itself is intruding, not merely 'sensations' caused by the paper. That, precisely, is the mystery. What is it about my experience that strikes me as if the paper itself is intruding into that experience? Besides, the appeal to 'sensations' doesn't get us very far — what, exactly, is a sensation?"
"I suppose that is the mind-body problem, is it not?"
"Yes, indeed."