[FOM] The Minimal Model of ZF

joeshipman@aol.com joeshipman at aol.com
Tue Jan 1 00:49:43 EST 2008

The following paragraph is adapted from Cohen's book "Set Theory and 
the Continuum Hypothesis".

If ZF has a standard model (that is, a model which, when viewed 
externally, has no infinite descending membership chains), then there 
is a minimal such model M. M is countable, and satisfies V=L (and M 
does NOT satisfy "ZF has a standard model", though it satisfies all 
true statements of arithmetic including Con(ZF)). M is the intersection 
of all standard models, and for every element x in M there is a formula 
A(y) in the language of set theory such that x is the unique element of 
M satisfying A. Thus in M every element can be "named".

M (which was first studied by J.C. Shepherdson in 1951) can also be 
defined by a transfinite recursion in the same way L is, except that 
the operations are simpler: start with the empty set and omega  and 
close transfinitely under pairing, sumset, internal powerset (for X the 
current partial universe and x in X, the set of y in X which are 
subsets of x), and taking ranges of functions. If there is a standard 
model, this construction stops adding new sets after a certain 
countable stage; otherwise, it goes on forever and simply gives L. 
Thus, we can treat M as a class and avoid the assumption that a 
standard model exists.

The usual proof of the consistency of V=L, GCH, and AC can be carried 
out in this setting. But what I am interested in is what happens if we 
strengthen V=L to a new axiom "V=M" (this is equivalent to the 
conjunction of V=L and "there is no standard model").  V=M means that 
not only is there no SET that can serve as a standard model, the only 
CLASS that can do so is V; in other words, the only sets that exist are 
the ones that MUST exist. Once you start building V, there will never 
be any place you can stop with a model of ZF until you have got 

This is as parsimonious as a set theory can be. But there is a 
confusing issue. When there IS a standard model, so that M is a set, 
every element of M has a name. If there is no standard model, M is a 
proper class, so there are way too many things in M to correspond to 
countably many formulas. I'd like to ask what is the first set in M 
which does not have a name, but this runs into the usual paradoxes. On 
the other hand, if M is a set, then internally M satisfies V=M, and 
there IS a first set in M which, externally, we can name, but which M 
does not "know" has a name.

What else can be said about this set? And can anyone clarify the 
situation philosophically? It seems to be a more vicious form of 
Skolem's paradox.

-- JS
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