Full Idea
Even the fundamental theorems about limits could not [at first] be proved because the reals themselves were not well understood.
Gist of Idea
Theorems about limits could only be proved once the real numbers were understood
Source
Penelope Maddy (Naturalism in Mathematics [1997], I.2)
Book Reference
Maddy,Penelope: 'Naturalism in Mathematics' [OUP 2000], p.27
A Reaction
This refers to the period of about 1850 (Weierstrass) to 1880 (Dedekind and Cantor).