'Navie set theory': why when S(x) is (x = x), the specified x's do not constitue a set?

I"m reading 'Naive Set Theory' and the end of Section 3 there is a
following sentence:

[quote]
In case S(x) is (x \notin x), or in case S(x) is (x = x), the specified
x's do not constitute a set.
[quote]

The first one, (x \notin x) is Russell's Paradox, that one I
understand. I"m not clear what's wrong with the second one, (x = x).
Seems like this is tautology, always true. So if I have something like

B = { x \in A : x = x }

then B = A, since (x = x) is valid for all x's in A.

I assume I'm missing something, but what?

TIA.

.

Relevant Pages

  • Re: Why so little info about Sumerians?
    ... my reading was that you were saying no papyri survive from before ... include a list of Middle Kingdom belles lettres in his <The Darker Side ... Lichtheim, I'll quote below what Lichtheim has to say; ... > Actually I am quite surprised that the Old Kingdom using papyrus at ...
    (soc.history.ancient)
  • Re: Schechner - This is for you Farrow
    ... So he happens to be reading for his anthropology class and he comes across the following quote from Schechner, ... when Mr. Farrow was telling me I should read some Schechner in order to understand his argument for not messing with "classics" I wonder what he meant? ... So you're saying on the one hand that I dont know enough about the subject because I havent read enough, and then on the other saying that the writing that there is is substandard? ... I'm saying that it's dangerous to try and reinforce an argument by using a single, simple sentence excerpted from a much longer, much more complex theoretical work, and that it's dangerous - and suggestive of a lack of critical thinking on your part - to quote anything by a single academic theorist as an unassailable truth. ...
    (rec.arts.theatre.musicals)
  • Re: OLTL/Uncasting rumor
    ... only as a gentle reminder to quote back. ... One should not have to do research on a post to find out intent. ... If anyone out there is reading these posts, ... I would only suggest not regularly calling others stupid ...
    (rec.arts.tv.soaps.abc)
  • Re: Schechner - This is for you Farrow
    ... So he happens to be reading for his anthropology class and he comes across the following quote from Schechner, ... when Mr. Farrow was telling me I should read some Schechner in order to understand his argument for not messing with "classics" I wonder what he meant? ... So you're saying on the one hand that I dont know enough about the subject because I havent read enough, and then on the other saying that the writing that there is is substandard? ... I'm saying that it's dangerous to try and reinforce an argument by using a single, simple sentence excerpted from a much longer, much more complex theoretical work, and that it's dangerous - and suggestive of a lack of critical thinking on your part - to quote anything by a single academic theorist as an unassailable truth. ...
    (rec.arts.theatre.musicals)
  • Re: old pentium
    ... But myself and eveyone I've watched reading usenet will read ... >> I therefore expect them to skip the quote. ... I prefer to scroll. ...
    (alt.os.linux)