i think he wouldnt remember you either. in fact, i wonder how he would remember why he was even there and how to get back to the present...

whether there is evidence that we existed is irrelevant as to if we did in fact exist, in my opinion. all we could do is show circumstantial evidence anyway-birth and death records, graves, even bodies can be falsified.

the interesting thing is how would history have changed because of your grandfather being murdered, and your not being born ... we can never know how far reaching the effects are of a single human life, no matter how insignificant it may seem to be.

Honestly, I don't even think that you would need a time machine to pull that issue off. There are people that existed thousands if not tens of thousands of years ago who had their own distinct personalities and traits and (generally speaking) are forgotten in time. I don't even think there are many people who can name more than five or six generations of their own lineage. But not only that, there are beings such as Icarus and Pegasus which have never existed yet have some level of existence as far as we are concerned. Another issue may be the fact that things exist without us knowing about them. India (to borrow from Jihindra Mohanty) exists even though you have never been there.

Before anything, I would at least say that your question is an ontological inquiry. Not to go too much into it, but the definition of existence refers to a set sum of total reality (in the logical sense, that is, everything that exists). This is above all a relative notion though, which is probably why there are so many threads on this subject on the forum with so many various and unique conceptions. Unfortunately, I have not yet come across a thread that linked the Aristotelian sense of "being" in the sense "being qua being" to blunt existence yet. It seems like it would be a great venue to discuss. On that note, I would think that anything we attribute as "existing" is a second level predicate. Speaking in terms of ontological arguments, "existing" or "exist" are predicable, that is, they are attributable to something else. Like in the case of Aristotle's Metaphysics, the underlying substance is ontologically deeper than the predicates that comprise its properties, like the "redness," or "roundness" of a given object. Simply, saying that something "exists" expresses the property of a property, not the ontological root of the object. Labeling that which exists as "existing" is in many ways fallacious even though we have no other way of explaining it. LOL!

There is however a very fascinating venue opened by David Lewis called modal realism, which essentially states that things that may not have existed but exist only as parts of our conceptual memory (like Pegasus for example) exists as non-actual objects (compared to actual-world objects) in different possible worldsYouTube - Flight of the Conchords Ep 3 Hiphopopotamus vs. Rhymenoceros

TurboLung, I don't think your question matches up with the thread title. Oh by the way, great name.

So I have played the same thought game as you before, well many times actually. It's a great mind puzzle. I'm with salima on this one, the fact that you knew the person whom you just erased from history, would in fact erase the memory of them from your mind as well. I also think it might effect you traveling back in time as well. Even though your trip might not have been specifically about killing the person's grandfather but it has to do with the information itself. That information would be deleted from existence and the actuall effect of that deletion might effect many events in your history time line. Even the most insignificant person can have major effects on outcomes. So there would be a drastic alteration of the history by erasing one individual from existence.

EDIT: Let's say the person whom you errased was the person who invented the time machine you used to go back in time with?

To answer the tread question. Yes I think you can never truely exist.

The actual idea we form, and consequently reason concerning, when we talk about "going back in time" doesn't appear to be any different than if we were to imagine letting time continue to operate normally while we used some unimaginably amazing machine to physically rearrange the entire universe to sufficiently resemble a previous state.

In situations like this I like to remind myself that our idea of time is derived from the succesion of our perceptions. An idea of time by itself can never be conceived alone by the mind, but always attends some perceptible changes in our ideas or impressions.

Form an idea of a white obelisk, for instance. Consider this idea in and of itself, unchanging, and it will never give the impression of time. Now imagine some changeable object with which to compare it to, the clouds moving behind it, and the idea of time presents itself. But as the idea of time is not distinguishable and seperable by the mind, to be considered on its own, I conclude that it would be more proper to call time a sensation. Time is a sensation we receive when we perceive change.

Why am I rambling on about our idea (sensation?) of time? Because if we are to have any consistency in our reasoning concerning time-travel, we must be able to form clear and concise ideas about what time-travel is or might be, and that starts with an understanding of our (idea?) sensation of time.

Proceeding in this method then I run into difficulties LONG before I find myself pondering a time-travel causation paradox. Indeed, I'm not even sure it is necessary to assume the existence of an external counterpart to our internal sensation of time.

of course there is the concept of time not being linear-which opens up the possibility of going into both the past and the future because they are all in the now. that also opens up the realm of cause and effect-if there is no linear time how can there be cause and effect? but i thought that was a bit more than what was being asked. anyway i really get confused pondering stuff like this-how can there be free will if the future already exists etc...i am still working on the small stuff!

i suspect time is non-linear in another aspect of reality but it is linear in the world in which we live and operate our computers etc. if i could i would like to reach into the future and get my computer after linux was installed and switch it with the one i am looking at right now.

i have read all posts with interest and agree with all posts, even though most don't agree with each other.

i don't know if you guys are understanding me 100% though. if the person who was killed in the past does not lead to a person being born AND you lose memory of that person, then, there is no information of that individual in the entire universe - no matter where you look, so, he does not exist, however, somewhere along the line we know he DID exist. this is the conundrum. i guess the poster talking about alternate universes has the closest answer, although, we are not talking about the other universes, just the same one which has been altered somewhat.

now, what if you could pear into the future and see a person who has not been born yet. could you say that he exists in the present time, because, you know what he looks like and you know he is coming?

I think you are confusing knowledge of something and the actual existence of something. Just to know that someone will exist in the future does not make them all of a sudden exist because you are aware of their future existence. This is only the knowledge OF THEM that exists and not them self existing.

Time is a human experience, not a physical phenomenon. Therefore thinking "before" and "after" changing something in the past makes no sense.
If you are ever going to kill someones grandfather, that person (the grandson) will already never have existed. So obviously have no reason to kill his grandfather in the first place.
Which leaves us with the old time travel paradox.
Consequently, the only thing you can change in the past is something that when changed will not effect your decision to go back and change it.