The "wall of worry" is one of the oldest phrases in market commentary, but Fisher uses it with analytical precision rather than as a platitude. Bull markets, by definition, advance in the presence of bad news, unresolved uncertainty, and widespread investor skepticism. If investors waited for a clean, uncertainty-free signal to enter the market, that signal would almost never arrive — and every major bull market in history would have been missed by investors waiting for it.

Fisher's historical documentation is useful here: the early stages of virtually every major bull market have been accompanied by serious unresolved risks. The 2009 recovery began when unemployment was still rising, bank failures were ongoing, and economic forecasters were debating the depth of the depression. Investors who waited for "clarity" missed the most powerful portion of the subsequent decade-long advance.

The behavioral mechanism is that human beings are wired to associate the absence of visible threats with safety and the presence of visible threats with danger. Markets do not work this way. Visible, widely-discussed risks are priced into securities. It is the hidden or underweighted risks that tend to actually surprise markets. The widely feared scenarios are often the ones that don't materialize, precisely because everyone has prepared for them.

For practical investors, the implication is uncomfortable: the moments when investing feels most dangerous — when the news is worst and uncertainty is highest — are often the moments when forward returns are best. Fisher's data makes this counterintuitive claim hard to dismiss.