You can't use an old map to explore a new world.

A theory can be proved by experiment; but no path leads from experiment to the birth of a theory.

Every day I remind myself that my inner and outer life are based on the labors of other men, living and dead, and that I must exert myself in order to give in the same measure as I have received and am still receiving.

In the midst of difficulty lies opportunity.

Whether or not you can observe a thing depends upon the theory you use. It is the theory which decides what can be observed.

I am not a Jew in the sense that I would demand the preservation of the Jewish or any other nationality as an end in itself. Rather, I see Jewish nationality as a fact and I believe that every Jew must draw the consequences from this fact.

A life directed chiefly toward the fulfillment of personal desires will sooner or later always lead to bitter disappointment.