We are justified in calling numbers a free creation of the human mind.
"I see it, but I don't believe it."
For what I have accomplished and what I have become, I have to to thank my industry much more, my indefatigable working, rather than any outstanding talent.
That which is provable, ought not to be believed in science without proof.
As professor in the Polytechnic School in Zürich I found myself for the first time obliged to lecture upon the elements of the differential calculus and felt more keenly than ever before the lack of a really scientific foundation for arithmetic.