Reality

It has been said that you cannot change the past. We are locked in to whatever has happened; whatever is – is. If you traveled to the past to affect a change, that change would cancel out the attempt to make a change.
Is there a past! Is there a Future? Is there just the "Here, and Now?"

The Greek philosopher, Parmenides, said that what really exists is a unity that is unchangeable. This conclusion seems to not make sense because we observe change everywhere, but Parmenides isn't talking about what merely appears to us, he is talking about reality itself. What is reality, is reality. Parmenides argument begins as a reduction ad absurdum, or reduced to absurdity.

In other words; we cannot prove the past was, or the future will be, we can only prove that now is. I cannot prove the yesterday existed, or that tomorrow will be. I cannot say with any certainty that the things in my mind which tell me of things past are not just creations of the present. I won't know if tomorrow exists until I exist in it.