Our friends show us what we can do; our enemies teach us what we must do.

Necessity is cruel, but it is the only test of inward strength. Every fool may live according to his own likings.

We can really respect a man only if he doesn't always look out for himself.

For just when ideas fail, a word comes in to save the situation.

Mathematics has the completely false reputation of yielding infallible conclusions. Its infallibility is nothing but identity. Two times two is not four, but it is just two times two, and that is what we call four for short. But four is nothing new at all. And thus it goes on and on in its conclusions, except that in the higher formulas the identity fades out of sight.

Thus one can observe that those who proclaim piety as their goal and purpose usually turn into hypocrites.

Nothing is so atrocious as fancy without taste.