If you go back to the late '50s, there really was nothing called "academic finance." Well, there was something being taught in business schools as finance, but it really had no strong research underpinnings.

The problem that people don't understand is that active managers, almost by definition, have to be poorly diversified. Otherwise, they're not really active. They have to make bets. What that means is there's a huge dispersion of outcomes that are totally consistent with just chance. There's no skill involved it. It's just good luck or bad luck.

The proposition is that prices reflect all available information, which in simple terms means since prices reflect all available information, there's no way to beat the market.

Eugene Fama Quotes