Counting
Klyver Markus on FB.
somebody who math, how come it isn't just {{{{}}}}?
It's counting how many distinct elements are inside the (outmost) brackets. {{{{}}}} contains only one element: {{{}}}.
It’s standard to model the natural numbers such with 0 as the empty set and each other natural number being the set of all preceding ones. So here we see 4 as {0, 1, 2, 3}.
I don’t know why this is standard. It has the nice property that the cardinality of the set you use for a number is equal to that number, and I think it makes some things with infinities nicer, but I’m not sure if those are the actual historical reasons. Certainly your version, or “the successor of a number is its powerset”, or a bunch of other options, are all possible models.
Because multiple models work, I would say that isn’t 4. It’s just one way to represent it. Similarly, the word “bird” is not a bird.
To be precise and reiterate daniel-r-h's point, these constructions aren't really how the naturals are "defined". The naturals are defined up to isomorphism by whatever number axioms you're using, of which both the simple nesting sequence implied by Shieldfoss's question and the one in the OP are models. The simple nesting is the Zermelo numerals, and the more complex one is the von Neumann construction of the ordinals.
The names spoil why the von Neumann construction is preferred. Both model the naturals, but the von Neumann construction is really a model of the ordinals, of which the naturals are an initial segment. This is because while a Zermelo numeral is a set containing only its predecessor, a von Neumann ordinal is the set containing all of its predecessors. This gives the nice property that in this model < is ∈, and ≤ is ⊆. For example, 4 is the set {0, 1, 2, 3} (shown above), ω is the set ℕ, and ε₀ = ⋃(ω, ω^ω, ω^ω^ω, … ).
In contrast, the Zermelo numerals "stall at the first limit", and cannot be extended in this way. Since a Zermelo numeral contains only its immediate predecessor, it cannot express limit ordinals which have none.


















