(A) If a and b are any two objects of the domain, there always exists a set {a, b} containing as elements a and b but no object x distinct from them bothThis is a rule (the ‘axiom of pairs’) that tells us that we can ‘construct’ a set {a, b} given the existence of its members a and b. We need this rule because we cannot infer the existence of a mathematical set, an individual object different from either of its two members, from the existence of its members alone. The consequent does not logically follow from the antecedent. In this, by contrast -
(B) If Peter preached in Jerusalem and Paul preached in Jerusalem, then Peter and Paul preached in Jerusalem.
we are not giving a rule for constructing any non-linguistic entity, nor are we making any existence assumptions beyond what is given in the antecedent. (B) simply gives a rule for constructing expressions: it tells us that the consequent means the same thing as the antecedent. Given the propositions ‘Is_F(a) and Is_F(b) and Is_F(c) and …’ the rule allows us to construct the proposition ‘are_F(a and b and c and …)’.
So my question remains. We assume the following
(1) At least one element exists
(2) One element is finite
(3) Any finite x’s and a single element are finite
(4) Any finite x’s are such that there is some y such that y is not one of the x’s.
This does not ‘construct’ anything. Rather, it asserts the existence of certain things. The only things it explicitly asserts are the existence of finite things. For example, it asserts the existence of one thing (the ‘first’ thing). It asserts (by inference) the existence of two things (the first thing plus some y which is not that thing), the existence of three things (the first two things and some other y), all of which are finite. The question is whether from statements 1-4 we can also implicitly infer the existence of infinite things (an infinite oset) in exactly the way that we can infer the existence of Peter and Paul from a statement about Peter and a statement about Paul. Can we construct an expression that refers to all of the elements of the domain? For if we can, it follows that all the elements of the domain exist – whether or not we actually constructed the expression. Peter and Paul exist whether or not we have an expression such as ‘Peter and Paul’. Do all the infinite elements of the domain exist, whether or not we construct the expression ‘all the elements of the domain’?
I hope this makes the problem clearer.