Here are some (unorganised) quotes I like.

“The test of a first-rate intelligence s the ability to hold two opposing ideas in mind at the same time and still retain the ability to function. One should, for example, be able to see that things are hopeless yet be determined to make them otherwise.”

Fitzgerald

For a man to achieve all that is demanded of him, he must regard himself as greater than he is.

Goethe

“For ourselves,” the Athenians said, “we shall not trouble you with specious pretences … since you know as well as we do that right, as the world goes, is only in question between equals in power, while the strong do what they can and the weak suffer what they must.”

Not a fan of this one, but the Athenians said this before massacring the Melians - people might actually be like this.

All models are wrong, but some are useful.

George E.P. Box