Re: Is X a Y?

Hal Finney (
Mon, 28 Jul 1997 08:28:38 -0700

Nicholas Bostrom, <>, writes:
> No, that is not the question. Not many would say that
> consistency guarantees existence (apart from David Lewis). The
> snowman is a logically consistent notion but that does not mean that
> the snowman exists.
> Now, obviously, if something is asserted to exist by a logically true
> proposition, then it exists. So the questions are (1) Is the
> proposition that the Mandelbrot exists logically true? (which boils
> down to whether the notion of "logical truth" in the traditional
> way really makes sense), and (2) Is this kind of existence somehow
> fundamentally different from the existence of objects which cannot be
> claimed to exist by any logically true proposition.

Or as another example, with a definition of "reality" which classes the
Mandelbrot set and the Virgo cluster as the same, can we reject any
logically consistent but "actually" non-existent phenomena as "real"? Must
we say that alien spaceships "really" are visiting the earth today, simply
because there is no logical contradiction in doing so? Is the reality
of such alien visitations any less than the reality of the Mandelbrot
set? Presumably we could create a computer simulation which would let
us view the former just as we view the latter.