From Wikiquote, after reading Scott Berkun. Not funny (as some of these), but thoughful (who’d have expected that? ;-)
Whenever a theory appears to you as the only possible one, take this as a sign that you have neither understood the theory nor the problem which it was intended to solve.
If we are uncritical we shall always find what we want: we shall look for, and find, confirmations, and we shall look away from, and not see, whatever might be dangerous to our pet theories.
Science may be described as the art of systematic over-simplification — the art of discerning what we may with advantage omit.
In his book Making Things Happen, the above-mentioned Scott Berkun summarizes Karl Popper as saying that there are only two kinds of theories: those that are wrong and those that are incomplete.
Notes on Disordered Matter 