Immortality in Fiction:
I write a lot of Science Fiction and Fantasy, so the subject of immortality comes up on occasion. I've never been a fan of the truly invincible immortal because, really, that kind of takes all the fun out of conflict, doesn't it. If you want to build real tension, you have to give them some kind of weakness (say Kryptonite or a wooden stake through the heart), but then, technically, didn't you just prove that they weren't invincible.
I much prefer the 'immune to disease and the effects of age, but if you take a fatal injury you’re pushing up daisies with the rest of us' brand of immortal because that provides a lot more tension. However, that's not immortality, at least not according to the first definition at Dictionary.com that defines it as: not mortal; not liable or subject to death; undying.
I have a race in my current novel that is more of the elven extremely long-lived if they don't get themselves killed type. I like this option because it does allow for more tension and gives the characters the brackets of a life expectancy, even if it is a very long one, in which to live, love, and experience their world(s). It takes more courage to face down a foe when you have the promise of a long life that can be taken away.
Immortality and Reality:
Let’s take this hypothetical invincible immortality and apply it to real life. You are going to live forever. That's pretty cool. Remember all those things you wanted to do and see and learn, but didn't think you’d have time for? Now you have all the time in the world.
Only, there are some drawbacks. We'll assume that you can't die from starvation. Great! No need to go grocery shopping. You'll probably want clothing though, and a place or two to live, and transportation and...
Well, that settles that. Unless you're one heck of a smooth talker, you need a job. If you never die, you probably can't afford to retire because you'll need to maintain that car and shelter and replace clothing when it wears out. If your friends and family don’t share your condition, you'll have to watch them die. After three or four hundred years, the whole situation might get depressing and perhaps a touch tedious.
Bring in option number two, immortal with caveats. This might make a story more exciting, but will all the problems of the invincible immortal and the ability to be injured/killed there isn't much appeal to this option in real life.
Personally, I'd be all for a few hundred years in which to experience life. This would be especially nice if aging either stopped at the right time or was drastically slowed. Imagine how many books I could write. :)
Alas, this isn't something I'm counting on becoming an option in the next few days. Don’t give up hope though. Many things can help you live longer. With the obvious healthy living options aside, I’ve heard it said that cats help you live longer. I have three. I'm wondering if this is an all of the time benefit of cats or if it only works when they are purring on your lap. Are they still helping you live longer when they puke on the floor, leave a rodent corpse in your shoe, or need an emergency trip to the vet? And exactly who decided that they helped you live longer anyway?
So now, who wants to live forever?
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