I've often said it multiple times in the past, but: "Science fiction/Fantasy has a tendancy of becoming science FACT."[/color]
How has fantasy ever had any tendency of becoming fact? That's simply false. With science fiction there is some truth there, but this is because it tends to be focused on imagining what things might be like in the future. And since this is the some sort of thing that leads to technological progress, it's not surprising that there is some overlap. Note however, that where that saying of yours holds any truth, it has no relation to ghosts or any other "paranormal" things that'd fit under this thread.
Do we really understand enough about how the universe works to say something we don't understand is really impossible? Obviously there are some things that are
clearly so.
While we can never be sure of what is, we can often be sure of what is not. Thats how science works. That's why it's so important that claims be falsifiable. For example, imagine you have a box but you don't know what's inside. Even without looking in the box, there are still are a large number of things we can confidently say are not in there.
And, yes, you should remain open to new evidence, but the time to believe something is when there is sufficient evidence to support it. And with claims of "paranormal", that's just not the case. Not only are such claims usually non-falsifiable, but the evidence they have is incredibly weak.
For example, I've seen ghost hunter type shows where they get all excited when their recording equipment picks up strange noises that anyone that's worked with audio recording should clearly recognize as merely audio artifacts. These sounds are in no way paranormal, and yet these people will make claims of how "it sounds like it's saying this" which quickly becomes "it's saying this". There are problems upon problems with this.
For one: The phenomenon of pareidolia. People are really good at finding patterns in things even when they are not really there. And it's even easier to hear a random sound as saying a particular thing what you've primed yourself expecting to hear it. The brain is really good at making stuff up to fill in the parts that are missing. (Though not directly related, here is a neat example of how good the brain is at filling in unknown gaps with made up stuff:
http://www.blindspottest.com/)
Another common problem is that these claims usually rely almost entirely on anecdotal evidence. We know that our senses can be easily fooled. And we know that humans are very prone to faulty ways of thinking such as confirmation bias. Even worse, stories of "paranormal experiences" I've heard often occur when someone is awoken during the night. When we are sleeping, are brains are in an altered state and even for some time after we wake up, especially if woken suddenly, there are still going to be a lot of those chemicals in your brain that are released during sleep. In other words, that's a time when it's even more likely than usual for your senses to get fooled.
[edited to fix typos]