nailed it
A thesis from J.S. Mill that is criticized by Frege is the claim that the proposition that 1 + 2 = 3 is made true by our experience of finding that a group of objects that looks like OOO can be separated into two groups that look like O and OO. Frege jokes that in that case it is a good thing that all the objects in the world aren’t nailed down, otherwise we wouldn’t be able to separate them and thus it wouldn’t be true that 1 + 2 = 3.