I've heard of them, and don't envy them a single bit. Talk about Hell on Earth.
I know - depending on how stringent our definition of "know" is, of course ;) - that I have experienced pain in the past. Because I have uniformly experienced pain in response to stimuli that would be expected to produce such an effect, I induce that I am very, very likely to experience pain again in the future. But do I know that I will? The evidence for it is of the same order of the evidence that the sun will rise tomorrow ;)
OK, but that's not what I'm asking. I'm asking if you know that you do experience pain. In other words, do you know what it actually is? And I'm not asking for a description, because I don't think you can objectively describe it. I just need to know if you're familiar with the beast. Maybe you don't know it'll happen to you again. Maybe you don't even know for sure that it ever happened to you before. But you should still know what it is - in the sense that if someone were to mention the word to you, you would have direct knowledge of what goes along with that word. Am I coming through clearly now?
Computers cannot reliably fool us into thinking they are human just yet. But interestingly, humans can fool us into thinking they are computers without too much trouble at all ;)
"D'Ohh!", by any other name...
Okay, I think I'm with you. I know that I have experienced it before, because I remember experiencing it and I assume that my memories are valid. I remember it quite clearly in a few instances - I once had a load of boiling water accidentally poured on me. I came to understand entirely new horizons in pain that day, but fortunately for me, the water all landed on my legs, and just south of the important parts, if you catch my drift. ;)
So, as a result, I think I can say that I know what pain is in the sense that it is generally intended.