The market's return is fixed; the investment industry's cut isn't

Bogle's foundational arithmetic is disarmingly simple: the stock market generates a certain gross return in any given period, and every investor's net return equals that gross return minus whatever costs they paid to participate. Since all investors collectively own the entire market, they collectively earn the market's return before costs — but after costs, the investing public as a whole necessarily earns less than the market, with the difference flowing straight to brokers, fund managers, and advisors.

He calls this the "cost matters hypothesis" and treats it as more reliable than any theory about market efficiency, because it doesn't depend on whether markets are perfectly rational — it's just subtraction. Whatever the market returns, costs reduce it, full stop, for every single investor who pays them.

This is Bogle's rhetorical judo: he doesn't need to convince you markets are unbeatable in principle to make his case — he only needs you to accept that fees are a real, certain drag on an uncertain, hoped-for return.

Takeaway: before asking what a fund might earn you, ask what it guarantees to take from you.