I start my class every year by saying, “These are models. And the reason we call them models is that they’re not 100 percent true. If they were, we would call them reality, not models. They’re simplifications.” But the acid test is, how good are the simplifications for your purposes? And for almost all purposes, market efficiency is a very good approximation.
Eugene Fama (in a Nov '07 interview)
Read the quote of the previous week.