I will make a longer post shortly but decided to copy and paste this from a debate I was having.:
All things equal, Jack is standing across from some man in the present. Everybody agrees he can kill him simply because of the conditions that exist. If he could not kill him, he would look to the physical world to explain what the heck is happening, why his constant attempts are thwarted. After all, a veritable plenum of coincidences doesn’t only occur in one direction. And perhaps that’s all it is. He can’t kill him by some weird species of luck, the weather, the man’s heart, physics, etc. Jack goes back in time with these same conditions. This man is his grandfather at a time before he reproduced Jack. Except, of course, Jack is standing right in front of him. So what relevance this has is not obvious – Jack exists. Since grandfather clearly reproduced in order for Jack to travel to the “time before grandfather reproduced” the latter time can only index the time as such, and not the events occurring in it. At time X, Grandfather reproduced. At Time X, again, the product of the reproduction has returned. Two distinct events. Anyway, Jack travels back in time with his existence well intact ( as far as we’re assuming ) to stand across from grandfather. Jack tries repeatedly but fails to kill grandfather. Instead of looking out at the world, he remembers “Oh, he’s my grandfather. I can’t kill him!” Or ” Oh, the very fact that I’m in this position of stabbing him repeatedly means his wounds won’t be fatal. ” Jack doesn’t even look into the world, perform experiments, or worry about any empirical facts – he looks in the dictionary, figures out what grandfather means ( and hence the relation to him), and is satisfied. Despite the fact that the man is bleeding out right in front of him, he can’t die. Ever. At this time. Otherwise, he wouldn’t have been there to savage him in the first place! This is the picture.
If it is reasonable, a slam dunk scenario, then, in the very first scenario, Jack would be just as justified when he can’t kill a man in the present time, instead of checking, say, his equipment, or doubting his skills or even chalking up the whole thing to luck he immediately and sensibly ponders “Well, perhaps, he’s my grandfather or some ancient relative that necessitates my very existence! Maybe I”m time traveling!” This would be reasonable. Right? Then, the reason Jack can’t fatally injure this man with several point blank shotgun blasts has nothing to do with this man being wolverine, a God of some of sort, or some kind of super powerful robot, but the mere fact that he’s related semantically across time. After all, that he’s biologically real already becomes irrelevant the moment he starts to shoot so it’s not just biological relation that’s making it impossible for this man to die but a purely semantic one. One can now imagine Jack’s grandfather, a bloodied mess, seeming to fade away into oblivion suddenly beginning to cackle. An inexplicable grin comes across his shattered face as he reaches under his bed and pulls out… Schuam’s Outline of Logic. “There, you ungrateful, punk. Do what you want. I can’t die.” Even the great Kant would blush when faced with such a triumph of pure reason.