Michael Shermer writes in The American Scientist:
According to Greek legend, Poseidon’s son Theseus sailed to Crete to slay the monster Minotaur. After his triumphant return to Athens, his ship was preserved as a memorial. As the vessel aged, decaying planks were replaced with new ones; eventually, all the original timber was replaced. Philosophers know the story of Theseus’s ship as a classic example of the problem of identity. What was the true identity of the ship, the shape or the wood? A more contemporary example may be found in the form of my first car, a 1966 Ford Mustang with a 289-cubic-inch engine and a speedometer that pegged at 140 m.p.h. As a young man high in testosterone but low in self-control, by the time I sold the car 15 years later there was hardly an original part on it. Nevertheless, my “1966” Mustang was now considered a classic, and I netted a tidy profit. Like Theseus’s ship, its essence—its “Mustangness”—was intact.
The analogy holds for human identity. The atoms in my brain and body today are not the same ones I had when I was born. Nevertheless, the patterns of information coded in my DNA and in my neural memories are still those of Michael Shermer. The human essence, the soul, is more than a pile of parts—it is a pattern of information.
As far as we know, there is no way for that pattern to last longer than several decades, a century or so at most. So until a technology can copy a human pattern into a more durable medium (silicon chips perhaps?), it appears that when we die our pattern is lost. Scientific skepticism suggests that there is no afterlife, and religion requires a leap of faith greater than many of us wish to make.
Whether there is an afterlife or not, we must live as if this is all there is. Our lives, our families, our friends, our communities (and how we treat others) are more meaningful when every day, every moment, every relationship and every person counts. Rather than meaningless forms before an eternal tomorrow, these entities have value in the here-and-now because of the purpose we create.
Humans have an evolved sense of purpose—a psychological desire to accomplish goals—that developed out of behaviors that were selected for because they were good for the individual or the group. The desire to behave in purposeful ways is an evolved trait; purpose is in our nature. And with brains big enough to discover and define purpose in symbolic ways that are inconceivable to millions of preceding and coexisting species, we humans are unique.
Read more here.