You apply it one way that you call “fusion in the case of Emptiness or Fullness” and another you call “collection in the case of A as a member of B”.
This is exactly what the ZF axiom of the empty set does.
It defines the empty set by the absence of
collections where "absence of
collections" is exactly the
fusion known as Emptiness.
Furthermore, the axiom of the empty set does not determine the existence of the empty set because by the ZF axiom of the empty set:
"
There exists set A such that any set (including A, or in other words, A exists independently of the axiom's description) is not a member of A"
Pay attention to the fact that if
A exists because of the axiom of the empty set, then we are using a circular reasoning, where
A existence depends on
A existence (also please pay attention to the fact that if
A is not one the things that are not members of
A, then
A is not empty).
In order to avoid this circularity,
A must exist independently of the axiom, which only describes its properties and does not determine its existence.
So "
There exists set A" is a fact of existence that does not depend on the description of the ZF axiom of the empty set, and
A existence is used to measure "the absence of
collections", which is exactly the
fusion known as Emptiness.
By tha same reasoning, the existing
collection B is used to measure the existence of
collection A.
EDIT:
In other words
collections are used to measure the existence of both
collections or
fusions.
On the contrary,
fusions cannot be used to measure the existence of
collections or
fusions, because
fusions' existence can be also too weak (in the case of Emptiness) or too strong (in the case of Fullness).
This is exactly the reason of why mathematicians using
collection (and not
fusion) as a measurment tool.
jsfisher said:
you cannot simply assume the existence of numbers when attempting to explore the foundations of Mathematics (even in a philosophic sense)
Since cardinality measures the existence of things, it is used here to also measure their absence (as clearly shown above by using the independent existence of the ZF axiom of the empty set from the description of the axiom).
In that case Cardinality as a measurement unit of existence, which can be measured only if there exists a measurement tool.
So as you see nothing is measurable unless there exists at least a measurement tool, and cardinal 1 is simply a measurement unit of such an existence, and not determines its existence.
Number 1 does not determine the existence of a collection exactly as the ZF axiom of the empty set does not determine the existence of the empty set.
Both number 1 or the ZF axiom of the empty set generally do not determine the existence of the things that they describe.
In other words, nothing is circular here.