Prices already know what you know

Malkiel's starting point is the efficient market hypothesis: at any moment, a stock's price reflects all publicly available information about it. If a piece of news suggests a company is undervalued, professional investors pounce on it within seconds, and the price adjusts before an ordinary investor can act. What's left over — the next day's move — is driven by new information nobody has yet, which by definition can't be predicted.

This is why he compares picking stocks to a random walk: like a drunk stumbling from a lamppost, each step is independent of the last, and looking at yesterday's steps tells you nothing about tomorrow's direction. Patterns that seem to appear on a price chart are, statistically, closer to the patterns you'd find in a sequence of coin flips.

The practical sting is direct: if prices already incorporate everything knowable, then reading one more analyst report or headline rarely gives you an edge, because everyone else read it too.

Takeaway: assume the price already reflects the news — your edge has to come from somewhere else, or not exist at all.