So, exactly what do we mean by "immortal," anyway?

Seems a simple enough question: immortal means you don't die. But it
turns out that different people mean different things when they use
the "I" word. When we talk to each other about ?immortality,? it is
important that we be sure that we are all talking about the same
thing. In other words, perhaps we need to decide on what we mean by
immortal. If we just mean "you never die of old age", that's one
thing. If we just mean "you never die for any reason at all, ever,
even if you lie naked on top of an H-bomb as it goes off", then that
is something else again.

I would like to suggest that we start talking about several types of
immortality, so that we know at once if we are arguing over ?never get
old? or ?naked on the H-bomb? immortality. To that end, I propose:

TYPE I Immortality: This is simply the quality of never dying from old
age, but still short of what most people think of when they use the
?I? word. Type I immortals can be injured, can be killed (accident or
murder), can even get sick and die. Also, they continue to age.
Tithonus, the man out of Greek mythology who was granted eternal life
from the Gods, but forgot to ask for eternal youth, would be an
example of a Type I immortal, as would be somebody who just managed to
hit the Breakeven Point.
A Type I makes for an interesting fictional character, but is not what
most of us would wish for ourselves or our loved ones. Who wants to
spend forever getting older and sicker and sicker and older? Even if
we assume a dramatic slowing of the aging process, we are still
talking Type I (call it the ?high end? of Type I immortality). Think
about it: even if aging is slowed by a factor of a hundred, and you
become Type I at the age of twenty, you will still get old and feeble.
Sure, when your chronological age is 120, your biological age will be
21, but when your chronological age is 10,000, your biological age
will be 100. Probably, most people would decide that a hundred years
of being 85 is enough (followed by a hundred years of being 86,
followed by?), and take themselves out.
In truth, Type I immortality is probably impossible, because let?s
face it, your body can only take so much aging before it kills you.
Thus, Type I isn?t really immortality at all, but longevity. Really,
there is only one reason to put up with it, and that is because it
allows you to live long enough to become a Type II immortal.

TYPE II Immortality: Now this is more like it. Type II is what most
people mean when they talk about ?scientists discovering immortality.?
A Type II immortal not only doesn?t die of old age, he simply doesn?t
get old. The problem of ?your body can only take so much aging before
it kills you? is resolved because your body doesn?t have to take any
aging at all. The ?high end? in this case would be that aging could
even be reversed, if one happens to already be old when Type II
immortality becomes available.
One can still die if one is hit by a bus, or gets shot betwixt the
eyes, or hangs oneself, and so forth. But aging is banished, and so
perhaps is disease (even more ?high end?). This is actually what most
?immortal heroes? in fantasy and science fiction enjoy (or suffer
through, depending on the bent of the author). They can always come
back in the next book, never getting too old to go adventuring about,
but still susceptible to a well-placed swording. Some of them have
Type III.

TYPE III Immortality: This Type of immortal not only doesn?t grow old,
but doesn?t get sick and, this is what makes it Type III, recovers
from any injury that does not flat-out kill him. If his finger gets
cut off, he can either put it back on, click on a spare, or grow a new
one. At the very least, he can go to the hospital and get a new one
(or get the re-growing process started). Same with a leg, or a
testicle, or an eye. He could die by drowning, same as I could.
However, I could be rescued at the last minute, CPR?ed, and live the
rest of my life with irreparable brain damage. With this guy, nothing
short of actual death is irreparable.
What makes this Type III instead of just really, really high end Type
II is that we have now gone beyond long-lived, even infinitely
long-lived, and started moving into the area of indestructibility. We
are now somewhere between ?never get old? and ?naked on the H-bomb.?
The high end of Type III would be instant healing/regeneration. There
would be no risk of bleeding to death, nor of spending large amounts
of time with a missing arm, a damaged brain, etc. If you ain?t dead
in a split second, you will recover, AND you never get old! Or sick!
This is what we see in the ?Highlander? movies. Perhaps they should
be called the ?Highender? movies instead. ;)

TYPE IV Immortality: This is it. Give that H-bomb a big, friendly,
naked hug! Who cares if it goes off: you?re unkillable! The combined
arsenals, natural disasters, and bad luck of the universe can?t end
your life.
This would be accomplished by simply being indestructible (like
Superman, but without any of that nasty Kryptonite around), or by high
end Type III healing/regeneration combined with resurrection on
demand. Having back-ups of yourself that are constantly updated, so
that after you are vaporized you walk out of the storage room with
full memories of everything up to the point that you ?died,? would be
a form of resurrection on demand.