The concept of the Swiss Army knife also explains language learning without any need to invoke two phenomena required by the universal grammar theory. One is a series of algebraic rules for combining symbols — a so-called core grammar hardwired in the brain. The second is a lexicon — a list of exceptions that cover all of the other idioms and idiosyncrasies of natural languages that must be learned.

The problem with this dual-route approach is that some grammatical constructions are partially rule-based and also partially not — for example, “Him a presidential candidate?!” in which the subject “him” retains the form of a direct object but with the elements of the sentence not in the proper order. A native English speaker can generate an infinite variety of sentences using the same approach: “Her go to ballet?!” or “That guy a doctor?!” So the question becomes, are these utterances part of the core grammar or the list of exceptions? If they are not part of a core grammar, then they must be learned individually as separate items. But if children can learn these part-rule, part-exception utterances, then why can they not learn the rest of language the same way? In other words, why do they need universal grammar at all?