[FOM] Re: "Leibniz's Law"

[William Tait]

On Thursday, June 12, 2003, at 03:54  AM, James Fordyce wrote:

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>
>
> >From: William Tait
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> >To: fom
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> >Subject: Re: [FOM] Re: "Leibniz's Law"
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> >Date: Sun, 8 Jun 2003 10:15:13 -0500
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> >
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> >
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> >I have just looked through a number of postings whicg refer to
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> >``Leibniz's Law''. Forgive me if I'm missing something; but these
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> >postings seem to be taking this law to be stating that identical
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> >things have the same properties (and more surprisingly, seem to be
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> >arguing about its validity). But Leibniz's law---or at least the
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> >striking part of his assertion about identity---is in fact the
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> >converse statement that things with the same properties are
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> >identical (which makes some kind of sense in the context of his
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> >metaphysics, if not [pace Frege] otherwise).
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> >
>
> >Bill Tait
>
> The following inference seems doubtful to me:
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> 1) names("Hesperus", Hesperus)
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> 2) Hesperus = Phosphorus
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> 3) Ergo: names("Hesperus", Phosphorus)
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> If this inference doesn't preserve truth, what's the bearing on the
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> indiscernibility of identicals?


I assume this is in response to my expression of surprise at the 
extended discussion of the indiscernibility of identicals.  My short 
answer to this is that FOM is supposed to be concerned with fom and, in 
mathematics, there are no contexts in which this principle fails. 
(Maybe a discussion of why that is so would be interesting.)

I suppose that I was giving expression to a concern, which I think I 
share with others, that FOM is in danger of being flooded with postings 
that have more to do with what might loosely be called the logic of 
natural language than with fom.

Bill Tait