Causal vs Lagging PMF Signals
Most PMF definitions are lagging indicators. The causal moment is conversion from cold landing page to first paid action, measured before anyone tells you you have PMF.
Rahul Vohra's line about PMF: by the time investment bankers are staking out your house, you already have product-market fit. The compliment is also the diagnosis — most PMF definitions are lagging indicators. Revenue growth, virality, retention curves: all real, all useful, all six to twelve months downstream of the actual causal moment.
The causal moment is when a specific customer, with a specific job-to-be-done, in a specific context, encounters a specific framing of your product and converts inside a single session. The leading signal isn't "how would you feel if you couldn't use this?" — it's the conversion rate from cold landing page to first paid action, measured before anyone tells you you have PMF.