I always hate it when people say that life can't exist after death because death is the absence of life. That is using narrow human definitions of words to attempt to disprove a complex, objective thing. Rather than say an afterlife is impossible, lets just narrow it down to some criteria it must pass, and some devil's advocate questions in italics for Yanniru and Paul to answer. :) Now, to commence the random rambling:
For an afterlife to exist...
Life must have something fundamentally different about it than the rest of the universe. That is, it has something structurally unique to itself. If there is nothing basically special about life, then assuming an afterlife exists for this one SUBSET of creation is absurd. Will a rock or a star experience life after death? No, because a rock or a star will never "die." Just change form. Same thing with life. Unless something seperates us from the rest of creation, then our "death" is merely the decay of a physical configuration neccesary to produce consciousness, nothing more. Death is simple a semantic term meaning change.
So, what is this fundamental difference?
Life must not be self-sustaining. The human brain cannot sustain itself, or else souls are irrelevant. A soul must be present for the brain to function. Why? Because all energy in the physical mind is eaten and digested by scavengers when it ceases to function. So, in order for the consciousness to survive, it must either hit the "eject" button prior to death, or it must originate from somewhere else.
Now, we get into what constitutes life. If humans have something special about them that no inanimate matter posesses, it follows that animals do too. We all have the consciousness needed to constitute life. Now, where do you draw the line? Can you even draw a line without contradicting what you set out to prove? Are plants alive in the same way we are? Yes. They evolve, reproduce, spread, and adapt. The only difference is complexity. Now, what would life after death be like for a plant, exactly? What is consciousness like for a plant? Does a plant even experience consciousness? Well, let's try something more basic. Would a bacterium experience an afterlife? Would such things even matter to the bacteria, with no method of thought whatsoever and with only a few thousand genes? If you say "Well, a bacterium wouldn't get an afterlife because it's so basic and couldn't even appreciate it," I say this: a bacterium posesses all of the qualities that constitute life. If you seperate the bacterium, the lichen, or the algae from the creatures with souls, you neccesarily seperate yourself. All manner of life is more or less the same in its properties. But the further back you go, the more you find that life becomes increasingly similar to inorganic matter: for example, the virus. So where do you draw the line, and if you don't, what do you think that implies?
Some people will say that our consciousness now is merely a shadow of our true cognitive capabilities, and that all souls are the same: it's just their physical representations that inhibit their true selves. I think Paul said this to me once. So, in this case, where are all these souls coming from? If you clone a human, would that human have a soul? If you say yes, then where is this seemingly infinite source of souls? As long as humans keep reproducing, more and more souls are produced. So does our conception "create" a soul or was it there all the time? If it was always there, does this mean we'll eventually "run out" of souls, once every soul in the spirit realm is accounted for? Does this make sense to you? If our conception creates a soul, you have just more or less proven that souls are bound to us, not the other way around. This is a contradiction in most people's eyes. And if you say that the souls are somewhere else, waiting in line to come here on a vacation, then eventually we will theoretically run out of them. Right? What happens then?
Now, if you say "No, a clone will not have a soul," you have just invalidated your own argument. A clone would have the same physical properties as any other human, the same feelings, emotions, and characteristics as everyone else. If that being would not have a soul, then souls are irrelevant; we can function without them, and if a conscious being can exist without a soul, then the entire argument for their existence crumbles to dust.
These random thoughts probably don't make any sense to anyone, but this is the line of thinking that led me to the conclusion that the existence of souls is much less probable than the permanency of death.