What is a rule of grammar?
There can be little doubt, as Amaral and Roeper (this issue; henceforth A&R) observe, that second language (L2) grammars are affected by and interact with prior-language knowledge at early, subsequent, and even near-native, stable stages of proficiency, and that a prior-language grammar may in turn be affected in some ways by the L2 grammar. As A&R readily acknowledge, this is not a novel claim. Rather, the novelty of their approach lies in their extension to second language acquisition (SLA) of Roeper’s (1999) Multiple Grammars (MG) framework, according to which the rules of grammar ‘must be simple’ (p. 55; italics in original), with no optionality, subcategories, exceptions or dependencies. Moreover, rules are not modified or replaced in the course of acquisition; instead, new rules are simply added whose properties may be contradictory or incompatible with other rules. As each rule is added, it must be classified according to its productivity: as [+/–productive] (e.g. A&R’s example (11) on p. 20). Rules that are unproductive or part of idiosyncratic constructions are for the most part lexically triggered (p. 3). For L2 or bilingual acquisition, of course, each rule or property along with its productivity status must additionally be classified according to the source language that serves as evidence for the rule (p. 19). Given A&R’s emphasis on keeping each rule simple and non-optional, certain foundational questions arise: How should we more precisely define the terms ‘rule’, ‘simple’, and ‘productive’? Do the very same rules exist cross-linguistically, differentiated only by degree of productivity? What counts as ‘the same’? Let us consider A&R’s formulation of their V2 (verb second) rule, which they observe is productively applied in German but limited to direct quotes in English, as in ‘Nothing,’ said Bill. This restriction is achieved in English by attaching a feature to verbs of quotation that links them to ‘a simple rule of V2 languages ... Now, in effect, we still have exactly the German rule that allows the main verb to move to the CP, but it is linked as a diacritic to a lexical class’ (p. 9; italics added). A&R hypothesize that a native English