I Exist Therefore I Am, I Think.

They say the problem with time travel is that you might create a paradox by killing your grandfather or by stepping on the one crazed prehistoric sea creature to finally get fed up with all that swimming about under the ocean only to have to keep coming up for gulps of precious air, instead deciding that it would be far better to simply stay on land where it could breathe as often as it liked and develop a fondness for recklessly expelling air in contented sighs. However, the real problem with time travel is that if everyone did it, then neither grandfathers nor crazed sea creatures would have even the slightest idea if they were real or not, as they would continually wink in and out of existence as a result of all those people mucking about the time line in search of finding their long-lost loves, asserting their virility by hunting dinosaurs, or just looking for a nice place to sit down and ponder the nature of their existence, if indeed they have one at all.