The Principle of Tolerance, fixed once for all the realization that all knowledge is limited. It is an irony of history that at the very time when this was being worked out, there should rise, under Hitler in Germany and other tyrants elsewhere, a counter-conception: a principle of monstrous certainty. When the future looks back on the 1930's, it will think of them as a crucial confrontation of culture as I have been expounding it - the ascent of man against the throwback to the despots' belief that they have absolute certainty.

Science is a tribute to what we can know, although we are fallible.

Nations in their great ages have not been great in art or science, but in art and science.

The values by which we are to survive are not rules for just and unjust conduct, but are those deeper illuminations in whose light justice and injustice, good and evil, means and ends are seen in fearful sharpness of outline.

Knowledge is not a loose leaf notebook of facts.

We are all shot through with enough motives to make a massacre, any day of the week that we want to give them their head.

The world today is made, it is powered by science; and for any man to abdicate an interest in science is to walk with open eyes towards slavery.