 The claim that English is not accepted by any finite-state automaton can only be supported by showing that every grammar for English has center- embedding to an unbounded depth (see Levelt 2008: 20–23 for an exposition and proof of the relevant theorem, originally from Chomsky 1959). But even depth-3 center-embedding of clauses (a clause interrupting a clause that itself interrupts a clause) is in practice extraordinarily hard to process. Hardly anyone can readily understand even semantically plausible sentences like Vehicles that engineers who car companies trust build crash every day. And such sentences virtually never occur, even in writing. Karlsson (2007) undertakes an extensive examination of available textual material, and concludes that depth-3 center-embeddings are vanishingly rare, and no genuine depth-4 center-embedding has ever occurred at all in naturally composed text. He proposes that there is no reason to regard center-embedding as grammatical beyond depth 3 (and for spoken language, depth 2). Karlsson is proposing a grammar that stays close to what performance data can confirm; the standard Essentialist view is that we should project massively from what is observed, and say that depth-n center-embedding is fully grammatical for all n.