Aridas
Crazy Little Green Dragon
"One does not, by knowing all the physical laws as we know them today, immediately obtain an understanding of anything much."
"The more you see how strangely Nature behaves, the harder it is to make a model that explains how even the simplest phenomena actually work."
Richard Feynman
Yeah...obviously...we know what existence is. NOT!
....you skeptics are so....deluded.
Ugh. This tangent has degenerated far, far too much, on both sides of it, I think, if generalized insults are being tossed around.
To chime in, though, in quick review, Navigator's right, at least partially. As a rule of thumb, science does not and cannot say anything about what are, in effect, untestable and unfalsifiable hypotheses. It certainly can say something about testable and falsifiable hypotheses that are relevant to the slightly different and more philosophical issue of how reasonable it is to hold a particular position in comparison to another. To be more specific for a moment, given that we seem to have no reason to accept that there actually is an afterlife, though, other than claims that rest completely upon completely untrustworthy and consistently failing methods of gathering information about what we can observe about reality, it's no more of an issue when someone claims that there is no afterlife than it is when someone claims that invisible, inaudible, and intangible beings aren't behind you making fun of you right now. Certainly, a person could be completely wrong about either or both claims, but, until an argument more solid than "you could be wrong," which is just pointing out the obvious about all statements and positions that one holds, is presented, arguing about it is likely to not get anyone anywhere and tends to degenerate.
Now, to more directly address the quoted post, a "scientific" definition of existence was asked for. A somewhat reasonable potential one was given. By nature, a scientific definition will be concerned with testable and falsifiable traits, which may or may not be all the traits that something has with a less restricted definition. Your "objection" falters and fails, once one understands that.