Only two things are infinite, the universe and human stupidity, and I’m not sure about the former. (Albert Einstein)

The optimist proclaims that we live in the best of all possible worlds, and the pessimist fears this is true. (James Branch Cabell)

I don’t know why we are here, but I’m pretty sure that it is not in order to enjoy ourselves. (Ludwig Wittgenstein)

The power of accurate observation is frequently called cynicism by those who don’t have it. (George Bernard Shaw)

Each problem that I solved became a rule which served afterwards to solve other problems. (Rene Descartes, Discours de la Methode)