In your dream you sit down at a piano and compose the most beautiful piece of music anyone has ever heard. Now in reality you have no talent for composition and can’t even play the piano. Nevertheless this is an entirely plausible dream. You can imagine waking up the next morning and reporting it to someone. You can imagine feeling wistful, wishing you could hear that music again. But if someone says, “What did it sound like?,” one of two things will happen. Either you will begin to hum the song and realize it was actually just “Für Elise” or “Benny and the Jets”, and your dreaming brain had fooled itself into thinking you had composed it. Or you won’t be able to recall the music at all. You may be able to bring to mind the sound of a piano or some echo of the feeling it stirred in you, but the song itself will remain an atomic whole called “the most beautiful piece of music anyone has ever heard”, unresolvable into an actual melody.
So did you actually compose an original and beautiful piece of music that cruelly fled upon waking? Say we use electrodes to hook you up to a device that records your dreams. No such device exists, but it could. It’s well within the boundaries of a thought experiment. Imagine a device that displays moving images of what you “saw” in the dream on a monitor and plays audio of what you “heard”. Movie cameras have existed in the waking world for over a century now, and though there’s plenty they can’t capture (taste, temperature, odor), we still take them to be adequate recorders of reality. But during playback would the piece of music that so moved you the dreamer would strike others as equally sublime? If it did, that means anyone, anywhere has the talents of a great composer locked away in their subconscious. This seems mystic and improbable. More likely that the playback would just be some preexisting piece of music, or vaguely piano-like noise, or just silence. All the dreamer could do is point at the screen and say, “Trust me, that part was great.”
Maybe you find this unsurprising because no video recorder can capture something so elusive as the feeling of sublime beauty. So imagine another, even more typical, dream scenario. You are walking down a city street. In the dream you have been there before, but on waking know the place is a fiction. What makes it convincing as a street? There are buildings, store signs, windows, parked cars, fire hydrants. Is this what we would see in the playback? Could we peer through doorways, read ads, take note of litter and cracks in the sidewalk? If there was a brick wall, could we zoom in on it close enough to see the individual bricks? Close enough to see the pits in the mortar? There would have to be a limit to the level of detail, right? Your one sleeping brain couldn’t imagine a entire city street down to the square millimeter level. (Our waking brains, in any event, are incapable of managing that quantity of information.) So again things must bottom out into unresolvable wholes into which we could zoom so far and then no farther. But what would those wholes be? A wall, a street, a window, a sign? Or would the playback be just a blur of color, and all the dreamer could do is point at the screen and say, “Trust me, this was a familiar street.”
In dreams we cheat. At some point what we believe to be details are just placeholders. One way the waking world is different is that if you and I walk together down a street one of us can say “This is a street” without having to add “Trust me.” Reality inclines us towards consensus. But do we cheat any less?