The heuristic, offered mid-conversation and attributed to Chris Dixon, is a single question: “a good measure of how interesting the thing is — can you find the edge? And for the most interesting things, you typically can’t. Even if you find the edge of the conversation, you can’t find the intellectual edge. Which I think is a very good sign.” The two edges are distinct and the distinction carries the test. Running out of things people have said about a subject is common and means little; running out of things that can be thought about it is the real signal, and its absence is what marks a live problem.
It pairs with the surrounding argument that the valuable parts of an idea maze are “the weird fringe parts” and that the recurring Silicon Valley pattern is a thing that first looks like a toy — the personal computer’s originators described in the conversation as “fringe weirdo punks” whose Homebrew Computer Club conferred no status at all.
This is the entry test that complements the exit test in Your first subfield is an accident of timing — wander across several before you settle, because breadth is insurance: that insight says subfields saturate and breadth carries you through the transition, while this one asks whether a subfield is worth entering at all. It is also the observable proxy for Reason backward from an outcome you want to exist — it manufactures originality that absorbed problems can't — an outcome nobody has reasoned toward will not have a findable edge — and it gives Taste is a muscle, not a gift — train it by forecasting every result before you see it something concrete to forecast against, since a field whose edge you can see is one where your predictions will mostly be confirmations.