Re: Is X a Y?

Nicholas Bostrom (
Tue, 29 Jul 1997 14:38:53 +0000

John K Clark <> wrote:

> I talked about consistency because that's as precise and objective
a concept
> as any known, truth on the other hand is as murky as reality.

And yet the standard definition of "consistency" is in terms of
"truth"! (P is consistent iff there is a model in which P is true.) I
Suspect you find the notion of model-relative truth unproblematic as
opposed to the notion of truth, simpiciter.

>I also find
> there is very often a disturbing circular element. What is truth? That which
> is (exists). What is existence? That which is true.
> I can't explain what truth is without using the idea of consistency

How does the Tarski definition of truth involve the idea of
consitency? (For those who don't know, the basic idea behind Tarski's
celebrated definition of truth is that:

The sentence "Snow is white." is true if and only if snow is white.

Ingenious! Before: A sentence including the word "truth". After: a
sentence not including the word "truth"(or true) but with exactly
the same meaning as the former sentence. Result: a successful
contextual definition of the word "truth" has been given.)
Nicholas Bostrom

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