Never try to have the last word. You might get it.

It seems to me that the secret of true happiness in life is to know what you are and then be content to be that, in style, head up and proud, and not yearn to be something else.

There are two ways of forming an opinion. One is the scientific method; the other, the scholastic. To the scientific mind, experimental proof is all-important, and theory is merely a convenience in description, to be junked when it no longer fits. To the academic mind, authority is everything, and facts are junked when they do not fit theory.

Spacemen - men who work in space, pilots and jetmen and astrogators and such - are men who like a few million miles of elbow room.

Butterflies are not insects,' Captain John Sterling said soberly. 'They are self-propelled flowers.

Secrecy is the keystone to all tyranny. Not force, but secrecy and censorship. When any government or church for that matter, undertakes to say to its subjects, "This you may not read, this you must not know," the end result is tyranny and oppression, no matter how holy the motives. Mighty little force is needed to control a man who has been hoodwinked in this fashion; contrariwise, no amount of force can control a free man, whose mind is free. No, not the rack nor the atomic bomb, not anything. You can't conquer a free man; the most you can do is kill him.

His older self had taught his younger self a language which the older self knew because the younger self, after being taught, grew up to be the older self and was, therefore, capable of teaching.