Liberty is the right to do what I like; licence, the right to do what you like.

What science cannot discover, mankind cannot know.

I do not pretend to start with precise questions. I do not think you can start with anything precise. You have to achieve such precision as you can, as you go along.

It is permissible with certain precautions to speak in print of coitus, but it is not permissible to employ the monosyllabic synonym for this word.

Mathematics, rightly viewed, possesses not only truth, but supreme beauty-a beauty cold and austere ... yet sublimely pure and capable of stern perfection such as only the greatest art can show.

Three passions, simple but overwhelmingly strong, have governed my life: the longing for love, the search for knowledge, and unbearable pity for the suffering of mankind.

[Regarding] the convention that clergymen are more virtuous than other men. Any average selection of mankind, set apart and told that it excels the rest in virtue, must tend to sink below the average.