It is a habit of mankind to entrust to careless hope what they long for, and to use sovereign reason to thrust aside what they do not desire.
-- Thucydides
If I may, I prefer the fuller version:
"...their judgment was based more upon blind wishing than upon any sound prevision; for it is a habit of mankind to entrust to careless hope what they long for, and to use sovereign reason to thrust aside what they do not fancy."
Also, dupe: http://lesswrong.com/lw/2ev/rationality_quotes_july_2010/28gb?c=1
Post quotes.