>>587
つづき

My unofficial rationale may be more philosophically appealing. Reverse
mathematics shows us that you can do an awful lot of math using only
the natural numbers and sets thereof. It *also* shows us that you can
do a lot of math using weaker inductive assumptions; it highlights five
interesting "levels" of induction (the celebrated RCA, WKL, ACA, ATR,
and Pi11CA theories) and Simpson's book hints that a few weaker systems
like EFA might turn out to be of similar interest. So I personally see
induction over the ordinals in ZFC as one of many points on a spectrum
of induction that's bounded below by RCA (or maybe EFA) and unbounded
above by Godel's incompleteness theorem.

つづく