I can think of what an alien looks like in my head right now, even though i've never seen it before, I can still make it up in my mind conciously, why couldn't I when i'm sleeping?
Heres an interesting quid: imagine a color never seen before - now thats impossible.
"Aliens" are portrated in many different ways, so how have you seen one?
Maybe Earth is just an alien experiment.
Really, we will never really know until we get much much much more sophisticated technology, but personally I don't think we'll ever develop the kind of technology we need to confirm any of this.
