Burke, Edmund (attributed):

"The only thing necessary for the triumph of evil is for good men to do nothing." -18th century (See Bartleby.com)

Dick, Philip K.

"Reality is that which, when you stop believing in it, doesn't go away." -1972

Erasmus, Desiderius

"I doubt if a single individual could be found from the whole of mankind free from some form of insanity. The only difference is one of degree. A man who sees a gourd and takes it for his wife is called insane because this happens to very few people." -1509

Franklin, Benjamin (attributed)

"Beer is (living) proof that God loves us and wants us to be happy."

(Note, it doesn't seem as if Franklin ever actually said or wrote this, no matter how much I might like the quote. However, this site shows wherefrom the quote might actually be derived: a letter from Franklin to André Morellet, about wine, not beer. Which is, actually, a quote I'd be about as happy with as one about beer, even if I drink beer much more frequently than wine. Besides which, there's some interesting stuff in the letter concerning the word "divine," as well as the phrase "in vino veritas"....)

Niemöller, Pastor Martin (attributed)

"First they came for the Jews
and I did not speak out -
because I was not a Jew.

Then they came for the communists
and I did not speak out -
because I was not a communist.

Then they came for the trade unionists
and I did not speak out -
because I was not a trade unionist.

Then they came for me -
and by then there was no one left
to speak out for me." -1968

(Note, there are several different wordings of this quote/poem, it isn't clear which is the original.)

Sartre, Jean-Paul

"Hell is other people." -1944, "No Exit"

Schurz, Senator Carl

"The Senator from Wisconsin cannot frighten me by exclaiming, 'My country, right or wrong.' In one sense I say so too. My country; and my country is the great American Republic. My country, right or wrong; if right, to be kept right; and if wrong, to be set right." -1872

(Note, this is related to an earlier quotation by Navy Commissioner Stephen Decatur: "Our Country! In her intercourse with foreign nations may she always be in the right; but right or wrong, our country!" which is the genesis of the quote "My country, right or wrong." -1816. Obviously, my opinion sides with Schurz.)

Shaw, George Bernard

"The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man." -1903

Perhaps you would care to recommend a quote?