It is wrong for a man to say that he is certain of the objective truth of any proposition unless he can produce evidence which logically justifies that certainty.

Education is the instruction of the intellect in the laws of Nature, under which name I include not merely things and their forces, but people and their ways; and the fashioning of the affections and of the will into an earnest and loving desire to move in harmony with those laws.

The improver of natural knowledge absolutely refuses to acknowledge authority, as such. For him, skepticism is the highest of duties; blind faith the one unpardonable sin.

Though under-instruction is a bad thing, it is not impossible that over-instruction may be worse.

For every man the world is as fresh as it was at the first day, and as full of untold novelties for him who has the eyes to see them.

It is one of the most saddening things in life that, try as we may, we can never be certain of making people happy, whereas we can almost always be certain of making them unhappy.

The question of all questions for humanity, the problem which lies behind all others and is more interesting than any of them, is that of the determination of man's place in nature and his relation to the cosmos.