Elizabeth: You know what? Maybe it was plastic surgery. Or an alien able to take on any shape he wanted. We've considered all your theories, Walter, and none of them have brought our son back. It happened. It just happened, and it's inexplicable.
Walter: Nothing is inexplicable, Elizabeth. Science seems like magic to primitives. Something happened in this house and we have to go over it, and over it, until we find out what we're missing!