Historical data shows no exceptions to the rule that an asset class reaching a two-standard-deviation (two sigma) valuation above its long-term trend will eventually return to that trend. This statistical certainty applies to stocks, bonds, commodities, and currencies, making severe drawdowns from such peaks inevitable.

Ahamed argues asset bubbles cannot be judged purely by high valuations, especially when structural housing shortages exist. Instead, the clear signals of an unsustainable bubble lie in excessive rates of price change and speculative accumulation across existing inventory.

High valuation metrics like the CAPE ratio seem alarming but are skewed by post-2008 currency printing. The "Price" in P/E ratios inflates due to debasement, while "Earnings" grow more slowly with GDP. This structural shift makes historical valuation averages an unreliable guide for today's market.

A practical definition of a bubble is when investor enthusiasm pulls all potential future cash flows and upside into the present-day price. This results in an asset that offers zero forecasted returns over a long period, making it a foolish investment.

A market enters a bubble when its price, in real terms, exceeds its long-term trend by two standard deviations. Historically, this signals a period of further gains, but these "in-bubble" profits are almost always given back in the subsequent crash, making it a predictable trap.

During bubbles, investors abandon traditional valuation metrics for unprofitable companies. They create and rationalize new KPIs like user growth or page views to justify sky-high prices, a clear sign of speculative denial.

In a late-stage bubble, investor expectations are so high that even flawless financial results, like Nvidia's record-breaking revenue, fail to boost the stock price. This disconnect signals that market sentiment is saturated and fragile, responding more to narrative than fundamentals.

Grant argues that modern economics, with its focus on metrics like CPI, misses the full picture of inflation. He revives an older view that excessive valuations and leverage in markets are a "species of inflation" in themselves, a concept modern central banking ignores.

Instead of a vague label, Cliff Asness uses a rigorous test for a bubble: can you make the math work? He takes a stock like Cisco in 2000, assumes unprecedented growth for a decade, and if the valuation *still* doesn't make sense, he considers it a bubble.