RichardKennaway comments on Why CFAR's Mission? - Less Wrong

38 Post author: AnnaSalamon 02 January 2016 11:23PM

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Comment author: ChristianKl 11 January 2016 01:19:54PM *  0 points [-]

I learned math with the Peano axioms and we considered the symbol 2 to refer to the 1+1, 3 to (1+1)+1 and so on. However even if you consider it to be more complicated it still stays an analytic statement and isn't a synthetic one.

If you define 2 differently what's the definition of 2?

Comment author: RichardKennaway 12 January 2016 01:26:14AM *  1 point [-]

If you define 2 differently what's the definition of 2?

One popular definition (at least, among that small class of people who need to define 2) is { { }, { { } } }.

Another, less used nowadays, is { z : ∃x,y. x∈z ∧ y∈z ∧ x ≠ y ∧ ∀w∈z.(w=x ∨ w=y) }.

In surreal numbers, 2 is { { { | } | } | }.