On two previous occasions, I posted some cryptic hieroglyphics suggesting they may be useful sometime later. Now that the thread's star celeb is working on his monologue, re-presenting past nonsense, I will close out my participation in this arc of the thread explaining those two posts for anyone who really cares.
First, a reminder. This latest "infinity bad" arc had at its start "proof" based in the cardinality of infinite sets. Since seemingly standard terms and phrases have been used in such novel ways in this thread, it was only reasonable to demand my interlocutor define what he meant by cardinality. This proved impossible, but did show the need to build the desired conclusion on cardinality into the definition of cardinality, creating a natural loop.
Ok, set theory has the Axiom of Infinity. It is necessary in an axiomatic set theory; otherwise you cannot construct an infinite set. Here is the axiom expressed in first order predicate calculus:
∃I (Ø∈I ∧ ∀x (x∈I ⇒ x ⋃ {x} ∈ I))
Looks ugly, but it expresses a simple idea: There exists a set we will call I (∃I) which contains the empty set as a member (Ø∈I) and (∧) for any set x (∀x) if x is a member of I (x∈I) then (⇒) the "successor" of x is also a member of I (x ⋃ {x} ∈ I).
The Axiom of Infinity guarantees some set, I, exists that has two properties:
Ø ∈ I
∀a (a∈I ⇒ a ⋃ {a} ∈ I)
We can even give it a name. Any set that has these two properties is an "inductive set." Notice, by the way, that the two properties tell us somethings about what must be in the set, but say nothing about what cannot be in the set. The Axiom of Infinity guarantees one such set, but there could be many.
One way to look at inductive sets is that they all must contain Ø and they must contain an infinite sequence of successors arising from Ø as the starting point. Such a set could also contain other starting points, other than Ø, and the infinite sequence of successors arising from those other starting points.
So many, many inductive sets could exist in set theory, the Axiom of Infinity guarantees there is at least one, but is there a "minimum" such set, one that has only Ø and its sequence of successors and not others? Formally, is there a set which I will call N with this property:
∀a (a∈N ⇒ (a = Ø ∨ ∃b (b∈N ∧ (a = b ⋃ {b}))))
What these chicken-scratches express is that every member of N is either the empty set or the successor of some other member of N. I'll leave that as an exercise for the reader to decipher.
So, how can we show that such a set must exist? Well, more chicken-scratches:
N = {x∈I : ∀y ((Ø∈y ∧ ∀z ((z∈y ⇒ z ⋃ {z} ∈ y))) ⇒ x∈y)}
It certainly doesn't look like it, but all this is stating is that N (my minimal set) is the intersection of all the inductive sets. Obvious, now, right?
Well, the intersection of two sets, A ∩ B, is simply all the members that are common to both sets. More formally,
A ∩ B = {x∈A : x∈B}
The gibberish, N = {x∈I : ∀y ((Ø∈y ∧ ∀z ((z∈y ⇒ z ⋃ {z} ∈ y))) ⇒ x∈y)}, is just a generalized version of that. The ∀y ((Ø∈y ∧ ∀z ((z∈y ⇒ z ⋃ {z} ∈ y))) part is simply identifying all the inductive sets and intersecting them with the set I.
For completeness, we'd need to prove this set N has that additional property. I won't bore you further, but I will outline the proof: N is the intersection of all inductive sets. If N doesn't satisfy the additional property, then there must be some member besides the empty set in N that isn't the successor of some other member. We can then define a set N' that is N without that other member. N' must be an inductive set, and N ∩ N' = N'. Since N ≠ N', we have a contradiction of N being the intersection of all inductive sets, so N must satisfy the additional property.
Whew.