Full Idea
If the law [of induction] can be proved, it will be included amongst the theorems of mathematics; if it cannot, it will be included amongst the axioms.
Gist of Idea
If principles are provable, they are theorems; if not, they are axioms
Source
Gottlob Frege (Logic in Mathematics [1914], p.203)
Book Reference
Frege,Gottlob: 'Posthumous Writings', ed/tr. Hermes/Long/White etc [Blackwell 1979], p.203
A Reaction
This links Frege with the traditional Euclidean view of axioms. The question, then, is how do we know them, given that we can't prove them.