I've got a wad of Kleenex stuffed up my nose as I type.
Daniel said it was "impossible" to affect change on the future timeline. And it is undeniable that killing people in the past changes timelines. The argument that they could do it "because they already did it" is the chicken and egg argument.
ANYTHING they do in the past could be described as "something they already did" because as soon as they do it...they did it in the past.
I think I'm going to lay my head down on my desk for a while.
