Re: "Elementary" elements in ZFC.

Article View: sci.logic

Article #1076

#1076

From: John
Date: Mon, 19 Nov 2001 01:35

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torkel@scheutz.sics.se (Torkel Franz?) wrote in message news:<ul0bsi3cmx9.fsf@scheutz.sics.se>...
> john_correy@excite.ca (John) writes:
>>
>   Right, and if you add a predicate "x is a set" to the language of
> set theory, you can make "if x and y are sets" explicit.

You had written, "If we add a predicate 'x is a set' to ZF, we will of
course restrict extensionality to sets."
(news:<ul0n120jq03.fsf@scheutz.sics.se>)

Unfortunately, the meaning of "we will restrict extensionality to
sets" is hardly clear.  It could mean equi-membered things have to be
equal IF they are sets; it could mean equi-membered things have to be
equal ONLY IF they are sets; and it could mean equi-membered things
have to be equal IF AND ONLY IF they are sets.

If a student had written this, you would have requested clarification.
And you wouldn't have been satisfied with: "If you add a predicate 'x
is a set' to the language of set theory, you can make 'if x and y are
sets' explicit."

Shouldn't you conduct yourself on sci.logic in a manner you would be
pleased for your own students to emulate?

John

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From: john_correy@excite.ca (John)
Newsgroups: sci.logic,sci.math,fa.analytic-philosophy
Subject: Re: "Elementary" elements in ZFC.
Date: 18 Nov 2001 17:35:42 -0800
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