I saw this quote and it made me think of Doctor Who (especially the trailer for Season One).
When people talk about travelling to the past, they worry about radically changing the present by doing something small (like stepping on a butterfly).
But rarely does anyone in the present think they can radically change the future by doing something small.
Go out there and change the future! 
Ooh. I like this!
Just stop deliberately stepping on butterflies! What has butterflies ever done to anyone to be the constant victim of this thought experiment? 
And just to be pedantic, is it actually possible to change something that hasn’t happened yet? 
I don’t think I’ve ever stepped on a butterfly. I imagine it’s difficult to do even if you tried.
Yes you can change the future! It happens all the time in Doctor Who!! 
DOCTOR: What the hell are you doing!?
AMY: Changing the future. It’s called marriage.
Sure, but without access to a TARDIS how would you know you had changed something and not merely played along with what would have happened anyway.
Going to the past and changing something would then change the future which is the contemporary present. But how would you know you have changed the future future be actions taken in the present and not merely taken actions that creates better conditions for the future? (Which is probably what the original quote means )
I should really stop trying to make sense of time travel theory, it does my head in
I did say I was being pedantic