“A man should look for what is, and not for what he thinks should be.”

“The value of a man should be seen in what he gives and not in what he is able to receive.”

“Knowledge of what is does not open the door directly to what should be.”

“A man's ethical behaviour should be based effectually on sympathy, education, and social ties and needs; no religious basis is necessary. Man would indeed be in a poor way if he had to be restrained by fear of punishment and hope of reward after death.”

“One should not pursue goals that are easily achieved. One must develop an instinct for what one can just barely achieve through one’s greatest efforts.”

“The tragedy of life is what dies inside a man while he lives.”

“In the matter of physics, the first lessons should contain nothing but what is experimental and interesting to see. A pretty experiment is in itself often more valuable than twenty formulae extracted from our minds.”