That which is provable, ought not to be believed in science without proof.
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.
"I see it, but I don't believe it."
We are justified in calling numbers a free creation of the human mind.
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.