# Confused about models of ZFC and passage of book

I have a question about a passage of Just/Weese. In the following, let $\mathcal M = \langle M, E \rangle$ be a model of ZFC. Here is the first half of what I'm about to ask a question:

So I asked myself the question of whether the other way around is also true: does every element $a \in M$ correspond to a subset $A \subset M$? Given that the argument for the other direction is purely based on the size of the sets one would expect that yes. But since I constructed a counter example, I think the answer is no (unless my counter example is incorrect (please correct me)):

Let $M = \{ \{\varnothing\}, \{\varnothing, \{\varnothing\}\}, \{\{\varnothing\}\} \}$, $E = \langle \{\varnothing\}, \{\varnothing, \{\varnothing\}\}\rangle, \langle \{\{\varnothing\}\}, \{\varnothing, \{\varnothing\}\}\rangle \}$.

Then $A = \{ \{\varnothing\}, \{\{\varnothing\}\} \}$ corresponds to $a = \{\varnothing, \{\varnothing\}\}$ But the element $a=\{\varnothing\}$ does not correspond to any subset $A \subset M$.

Now look at this:

In particular, "...Let $A$ be the subset of $M$ that corresponds to $a$. ...". But there doesn't necessarily have to be such an $A$, so this passage is not right.

What am I missing? Thanks for your help.

Corresponds is given as follows:

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Each $a \in M$ will correspond to $\{ b \in M : b \mathrel{E} a \} \subseteq M$. (All that is required here is that the relation $E$ is extensional, so that different elements of $M$ will have different sets of elements $E$-below them.)

Note, also, that the model you have constructed does not satisfy extensionality. Indeed we have that $$\begin{gather} \{ \varnothing \} \mathrel{E} \{ \varnothing ,\{ \varnothing \} \} \\ \{ \{ \varnothing \} \} \mathrel{E} \{ \varnothing , \{ \varnothing \} \} \end{gather}$$ and there are no other relations, so that $\{ \varnothing \}$ and $\{ \{ \varnothing \} \}$ cannot be differentiated by what is $E$-below them.

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Exactly. Thank you! Hence unless we assume $E$ to be extensional, the passage is not necessarily true. –  Rudy the Reindeer Nov 20 '12 at 16:34
@Matt: Yes, but since we are starting with a model of ZFC, the $E$-relation must be extensional! –  Arthur Fischer Nov 20 '12 at 16:35
Oooh! Perhaps we also have to assume transitivity? –  Rudy the Reindeer Nov 20 '12 at 16:40
@Matt: Suppose $( M , E )$ is any structure such that $E$ is extensional. Taking any object/set $\star$ define $M^\star = \{ \langle x , \star \rangle : x \in M \}$ and $E^\star$ so that $\langle x , \star \rangle \mathrel{E^\star} \langle y , \star \rangle$ iff $x \mathrel{E} y$. Then $( M , E ) \cong ( M^\star , E^\star )$, so $E^\star$ is also extensional. But $M^\star$ is surely not a transitive set! (cont...) –  Arthur Fischer Nov 20 '12 at 16:58
(...inued) You are sort of correct that we would prefer our models of ZFC to be of the form $( M , E )$ where $M$ is a transitive set and $E$ is the real $\in$ relation, but unfortunately this is a very strong assumption to make. (Even assuming the consistency of ZFC, there still may not be a transitive standard model.) To get used to these ideas you really have to work through things slowly, quite formally, and very meticulously until you get a feel for what is going on. Just as there is no royal road to geometry, there is no royal road to axiomatic set theory. –  Arthur Fischer Nov 20 '12 at 17:02

First we need to figure out what "corresponds" means.

It means that $A=\{b\in M\mid b\mathrel{E}a\}$. Indeed there are only a few of those. One example is that if $\langle M,E\rangle$ has non-standard $\omega$, then there is an $A$ which corresponds to the "true" $\omega$, but that $A$ is definitely not going to be any element of $M$, as we discussed in a previous thread.

As for the second question, assume that there is such $A$. For example, if $a$ is the set $\langle M,E\rangle$ sees as the real numbers, $A$ itself might be countable, but $a$ is uncountable in $M$.

(You may be interested in the questions of pichael and the answers I have provided to some of them about arguing internal vs. external points of view)

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The definition of corresponds is given in the book, sorry for not including it in the question. Will do so now. –  Rudy the Reindeer Nov 20 '12 at 16:31
@Matt: Yes, that is what I wrote. –  Asaf Karagila Nov 20 '12 at 16:59