Only a few know, how much one must know to know how little one knows.

It will never be possible by pure reason to arrive at some absolute truth.

The exact sciences also start from the assumption that in the end it will always be possible to understand nature, even in every new field of experience, but that we may make no a priori assumptions about the meaning of the word understand.

The existing scientific concepts cover always only a very limited part of reality, and the other part that has not yet been understood is infinite.

Modern physics has changed nothing in the great classical disciplines of, for instance, mechanics, optics, and heat. Only the conception of hitherto unexplored regions, formed prematurely from a knowledge of only certain parts of the world, has undergone a decisive transformation. This conception, however, is always decisive for the future course of research.

Whenever we proceed from the known to the unkown we may hope to understand, but we may have to learn at the same time a new meaning of the word 'understanding'

You may object that by speaking of simplicity and beauty I am introducing aesthetic criteria of truth, and I frankly admit that I am strongly attracted by the simplicity and beauty of mathematical schemes which nature presents us. You must have felt this too: the almost frightening simplicity and wholeness of the relationship, which nature suddenly spreads out before us.