This has to be one of the hardest things to comminicate to thoes who try to use such phrases as "Well how do you know ____ is real but ____ isn't?  Well the answer is pretty simple, we don't.  What the Mystic minded, the magically inclined, tend not to understand is that knowledge can never be perfect.  

So how do I know that Kansas exists but Heaven does not?  The simple answer is that I don't, however, I have a great deal of reaon to belive that Kansas does exist and very little reason to believe in Heaven.  For one thing there are a great deal of people who claim to live there, there is currency, a state represenitive, a place on the map, and several million people out there who can eazily go to the earth-bound territory and report if anything is suspicious about such a claim.  As for heaven, no one has come back and those who claim they have can be understood with a much simpler explanation than transition into another realm and another reality.  No artifacts have been brought back from heaven, and so far none of its represenitives (Gods/Angeles) have cared to make any confirmable public appearances.

Even with the utter lack of evidence there could still be a heaven, and there may actually not be a Kansas... can I say anything with omnicient understanding?  No, no knoweledge is ever certain, it is always up for revision and rediscovery at any given moment, that is why instead of relying on absolutes such as faith, we rely on the slowly progressing and defining wisdom that is scientific inquiry.

That is also why the faithful will, perhaps, always bite their own tounge when they call themselves humble and us prideful... what could be more humble than always questioning the illusion of certainty?