Derivational Order in Syntax : Evidence and Architectural Consequences *

generative grammars have traditionally been studied under the assumption that they are implementation independent. It is assumed, for example, that eventually we will be able to write software or build hardware to implement the human language system in computers that differ from human ‘hardware’ in many respects. As such, this is taken as evidence for implementation independence. This, however, is clearly an empirical question. It is a real and attractive possibility that computational modelers of the future might succeed in that effort, but it is far from having been accomplished at this time. The fact that some grammars that approximate fragments of human language have been digitally implemented does not settle the question. A relevant model should minimally be completely descriptively adequate, and one might reasonably want the model to approximate human linguistic abilities in more ways than simply classifying (un)acceptable sentences. Such models currently inhabit the realm of thought experiments. Here we disagree with Neeleman & van de Koot (2010), who agree with us on the importance of implementation (in)dependence, but take it as quite significant that some natural language grammars could be implemented in different artificial devices, concluding that human language is implementation independent. In the meantime, we argue that the emphasis on the implementation independence of generative grammars is misplaced if the purpose of the endeavor is to understand the nature of the human system. If a research community makes the choice to carve out one sub-area of the study of human language abilities, e.g., classifying (un)acceptable sentences, and then finds that this sub-area can be described in implementation independent terms, then this surely does not entail that the sub-area corresponds to a privileged, implementation independent sub-area of the human language faculty. There is no doubt that it is interesting and useful to develop explicit computational models of human language, but such models cannot show whether human grammatical abilities are, in fact, independent of their cognitive or neural implementation. Regardless of what one’s personal hunch is about the likelihood of successful non-human implementation of language, a more interesting question for our purposes is whether it is implementation independent within humans. According to the standard formulation of the principled extensionalist view, speakers of a language have knowledge of the (un)acceptable sentences of their language, and this knowledge can be described at an abstract level by a generative grammar. Importantly, it is assumed that this abstract knowledge is implementation independent, and that speakers can put their knowledge to use in different ways in activities such as speaking, understanding, and internally generated speech. This means that for any given well-formed sentence structure defined by the grammar a speaker may have a number of different ways of mentally assembling the structure. If this assumption is supported, then the principled extensionalist is justified in separating speakers' knowledge of what is well formed from their knowledge of how to assemble wellformed structures. But if speakers do not, in fact, have multiple ways of constructing 2 Neeleman and van de Koot describe a thought experiment in which a pair of computers with radically different underlying architectures are both able to pass a kind of Turing Test for human language, such that their performance is indistinguishable from human native speakers of the language. They argue that one would want to say that both computers speak the relevant language, and conclude that this is because the computers and the humans would share an abstract, implementation-independent grammar. We agree with the intuition behind this thought experiment, but it seems to reveal more about the common usage of predicates like “speaks French” than about the nature of the human language faculty. Despite this misgiving, we find much to like about Neeleman and van de Koot’s discussion of the challenges involved in understanding grammatical theories at different levels of description. Derivational order in syntax Colin Phillips & Shevaun Lewis 16 the same representations, then human language appears to be more implementation dependent. The evidence on whether speakers have multiple ways of constructing the same sentence representation is not extensive, but there are a number of reasons to think that speakers have one and only one way of assembling any individual representation. There is almost no evidence for the alternative view that speakers have multiple ways of building the same representation. First, speaking and understanding proceed in the same (roughly) left-to-right fashion. Although they have different goals, they have a great deal in common, appear to construct the same representations, and plausibly do so in the same order, although this topic has not been investigated in great detail. Second, in comprehension and production there is much evidence that speakers build structures and interpretations incrementally, in roughly the order in which words are presented. We are unaware of evidence that speakers are able to construct the same interpretation in different orders. For example, reading backwards is a task that lies somewhere in the difficult-to-impossible range, despite its correspondence with the derivational order assumed in many generative grammars. Third, in cases of reanalysis in sentence understanding, where comprehenders are led into a ‘garden path’ and must then reorganize their initial parse, some evidence suggests that speakers repair by returning to the error point and simply re-parsing the sentence in exactly the same order that it was presented (Inoue & Fodor, 1995; Grodner et al., 2003). We therefore adopt the working hypothesis that natural language grammars are implementation dependent with respect to how sentences are assembled: there is only one algorithm for structure-building across all human speakers (of the same language). We would certainly welcome more systematic evidence, but currently the evidence for the alternative implementation independent position is practically non-existent. The implementation dependent position is the simpler and more falsifiable hypothesis, and hence should be preferred until proven otherwise. Consequently, we think that the principled extensionalist position is unwarranted, and that the motivations for developing abstract generative grammars are more pragmatic than principled. We have no problems with pragmatic motivations, and we recognize the value of focusing on characterizing good/bad sentences as a way of making headway in describing human language. But this is very different from the position that the characterization of good/bad sentences is a fundamentally separate enterprise from understanding realtime language processes. Ultimately we seek theories that capture how sentences are put together, and not just what their final form is. In discussions of these issues we sometimes encounter an objection in the form of a 'slippery slope' argument. If human language is better described in terms of real time cognitive processes rather than abstract functions, so the argument runs, then why stop there – why not continue all the way down to the level of neurochemical processes in brain cells or beyond? As with other slippery slope arguments, the expectation is that we should find this consequence appalling, and hence should drop the entire argument. We acknowledge the concern, but disagree with this argument. First, we are not arguing against the usefulness of abstract descriptions in the study of language. We find them exceedingly useful. We are simply arguing that there is no privileged level of abstraction – the level occupied by most current generative grammars – that is 3 Although we have focused our attention here on the interpretation of derivational grammatical theories, the issue of implementation (in)dependence is equally relevant to interpreting grammatical theories that assume no derivations. Non-derivational syntactic theories, which are often presented as preferable due to their order-neutrality, imply implementation independence. If syntactic representations are not, in fact, implementation independent, then that should count against nonderivational theories, just as it counts against bottom-to-top derivations. STiL – Studies in Linguistics Vol. 6 17 exempt from considerations of psychological implementation. Second, it should be emphasized that implementation (in)dependence is not an all-or-nothing property of an abstract system. Rather, a system's implementation (in)dependence must be evaluated at each successive degree of abstraction. Our focus in the current discussion is on the relation between symbolic descriptions of the structure of sentences and symbolic descriptions of the procedures for assembling sentences in real time. This leaves open the possibility that human language is implementation independent at a lower level. For example, there are interesting arguments that the basic notion of hierarchical structure in language must be implementation independent with respect to its neural encoding, because of the very different demands of immediate vs. long-term encoding of sentences in memory (Jackendoff, 2002; van der Velde & de Kamps, 2006). Briefly, the most plausible method for long-term information storage in the brain – through changes in synaptic connectivity – is too slow for the millisecondscale processes needed for real-time language use. The need for dual encodings of the same structures suggests that structured representations are not always neurally encoded in the same fashion, and hence that they are implementation independent. It is far from certain that this argument goes through, because it is not clear that immediate and long-term encodings of sentences in memory are genuinely iso