Explaining Camels

There are certain kinds of explanations, which when available, are particularly satisfying.  What makes them such is that they not only explain the facts in front of you, but do so in ways that make the facts inevitable.  How do they do this? Well, one way is by rendering the non-extant alternatives not merely false, but inconceivable.  A joke/riddle that I like to tell my students displays the quality I have in mind.

One physicist/mathematician asks another: why are there 1-humped camels and 2-humped camels, but no N-humped camels, N>2? Answer: Because camels are convex or concave; no other models available.

I love this answer. It’s perfect. How so? By shifting the relevant predicates (from natural numbers to simple curves) the range of possible camels is reduced to two, both of which are attested!  Concave/convex, exhaust the space of options.  And once you think of things in this way, it is clear why 1 and 2 are the only possible values for N.

Let me put this another way: one gets a truly satisfying explanation if one can embed it in concepts that obviate further why questions. How are they obviated? By exhausting the range of possibilities. Why are there no 3-humped camels? Because 3-humped camels are neither concave nor convex and these are the only shapes camels can come in.

The joke/riddle has a second useful attribute. I displays what I take to be a central aim of theoretical research: to redescribe the conceivable alternatives in such as way as to restrict the range of available alternatives to what one actually sees. The aim of theory is not merely to cover the data, but to explain why the data falls in the restricted range it does, and this requires carefully observing what doesn’t happen (what Paul Pietroski calls ‘negative’ facts (e.g. here)).

So, do we have any of these kinds of explanations within syntax? I think we do, or at least there have been attempts to provide such. Let me illustrate.

One current example is Chomsky’s proposed account for why grammatical operations are structure dependent. This is in Problems of Projection (which I would link to, but it is behind a Lingua paywall with an exorbitant price so I suggest that you just get a copy from someplace else). Here’s what we want to explain: given that rules that move T-to-C (as in Y/N questions in English) target the “highest” Ts and not the linearly closest Ts (i.e. leftmost), why must they target these Ts, (i.e. why can’t they target the linearly most proximate Ts)?

The answer that Chomsky gives is that grammatical operations cannot use notions like linear proximity, because linguistic objects are not linearly specified until Spell Out, i.e. the final rule of the syntax. So why can’t grammatical operations be linearly dependent (i.e. non-structure dependent)? Because the syntactic manipulanda (i.e. phrase markers) contain no linear (i.e. left-right order) information. Thus, if grammatical rules manipulate phrase markers and these don’t contain linear information then there is no way to state linearly-dependent rules over these objects.  In other words, why do such rules not appear to exist? Because they can’t be specified for the objects over which the grammatical rules operate, and that’s why linear dependent syntax rules don’t exist.

Or, put positively: why are all syntactic rules structure dependent? Because that’s the only way they can be. In other words, once the impossible options are eliminated all that’s left coincides with what we find.  In this way, the actual is explained via the possible and explanatory oomph is attained. Indeed, I suspect (believe!) that the best way to explain anything is by showing how the plausible alternatives are actually conceptually impossible when thought about in the right way.

Here’s another minimalist example. It comes from current conceptions of Spell Out and how they’ve been used to account for phase impenetrability (i.e. the prohibition against forming dependencies across a phase head).  Here’s the question: why are phases impenetrable? Answer: because Spell Out “sends” phase head complements to the interfaces thereby removing their contents from the purview of the syntax/computational system. In effect, dependencies across phase heads are not possible because complements of phase heads (and hence their contents) are not syntactically “there” to be related.

Note the similarity to the first account: just as linear information is not there to be exploited and hence only structure dependent operations are stateable, so too phase complement information is not there and so it cannot be exploited. In both cases, the “reason” the condition holds is that it cannot fail to hold. There really is only one option when properly conceptualized.

Here’s another example from an earlier era: one of the most interesting arguments in favor of dispensing with constructions as grammatical primitives came from considering a conundrum relating to examples like (1c).

(1)  a. John is likely to have kissed Mary

b. John was seen/believed by Mary

c. John is seen/believed to have kissed Mary

The puzzle is the following in the context of a construction-based conception of grammatical operations (i.e. a view of FL in which the basic operations are construction based rules like Passive, Raising etc).[1] In (1a), John moves from the post verbal position to the subject position via Raising. In (1b) the operation that moves John to the top is Passivization. Question: what is the rule that moves John in (1c)?  Is this Passive or Raising?  There is no determinate answer.

Eliminating constructions gives a simple answer to the otherwise unanswerable question: it’s neither, as these kinds of rules don’t exist. ‘Move alpha’ is the sole transformation and it applies in producing both Raising and Passive constructions. Of course, if this is the only (movement) transformation, then the question is it Raising or Passive dissolves. It’s move alpha and only move alpha. As the earlier question (Raising or Passive?) had no good answer, a conception of grammar where the question dissolves has its charms.

One last example, this one from an undergrad research thesis by Noah Smith (of CMU fame; yes he was once a joint ling/CS student). He wrote when Jason Merchant’s work on ellipsis was first emerging and he asked the following question: Given that Merchant has shown that ellipsis is deletion and not interpretation, why can’t it be the latter?  He rightly (in my view) surmised that this could not be a data driven fact as the relevant data for determining this was very subtle. For Jason it amounted to some case and preposition stranding correlations in sluiced and non-sluiced constructions. On the reasonable assumption that these fall outside the PLD, the fact that ellipsis was deletion could not have been a data driven outcome. Say this is correct. Noah asked why it had to be correct; why ellipsis had to be deletion and could not be interpretation a la Edwin Williams (i.e. ellipsis amounted to filling in the contents of null phrase markers with null terminals at LF).[2] Noah’s answer? Bare Phrase Structure (BPS). BPS replaces the earlier combo of phrase structure + lexical insertion rules. This has the effect of eliminating the distinction between the content and position of a lexical item. As such, he argued, the structures that the interpretive theory of ellipsis presupposed (phrases with no lexical terminals) were conceptually unavailable and this leaves the deletion analysis as the only viable option. So why is ellipsis deletion rather than interpretation? Because the interpretive theories required structures that BPS rendered impossible. I confess to always liking this story.

One caveat before concluding: I am not here proposing that the proposed explanations above are correct. I have some questions regarding the Spell Out explanation of the PIC for example and there are empirical challenges to Merchant’s evidence in favor of deletion analyses of ellipsis. However, the kinds of proposals mentioned above are interesting and important for, if correct, they explain (rather than describe) what we find. And explanation is (or should be) what scientific inquiry aims for.

To end: one of the aims of theoretical work is to find ways of framing questions in such a way that all and only the conceptually possible answers are actualized.  This requires finding a vocabulary that not only accommodates/describes the actual, but renders the non-attested impossible, i.e. unstateable. This makes the consideration of systematic absences (viz. negative data) central to the theoretician’s task. Explanation lies with the dogs that don’t bark, the things that though logically possible, don’t occur. Theories explain what happens in terms of what can happen. This means keeping one’s eyes firmly focused on what we don’t find, the actual simply being the residue once the impossible has been pared away.

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[1] This is not the place to go into this, but note that rejecting constructions as grammatical primitives does not imply that constructions might not be derived objects of possible psycholinguistic interest.  I discuss this a bit (here) in the last chapter.

[2] There was an interesting and animated debate about ellipsis between Edwin Williams and Ivan Sag, the former defending an interpretive conception (trees with null terminals filled in at LF) vs the latters deletion analysis (similar to Merchant’s contemporary approach).

Poverty of Stimulus Redux

 This paper by Berwick, Pietroski, Yankama and Chomsky (BPYC) offers an excellent succinct review of the logic of the Poverty of Stimulus argument (POS). In addition it provides an up to date critical survey of putative “refutations.”  Let me highlight (and maybe slightly elaborate) on some points that I found particularly useful.

First, they offer an important perspective, one in which the POS is one step in a more comprehensive enterprise. What’s the utility in identifying “innate domain specific factors” of linguistic cognition? It is “part of a larger attempt to…isolate the role of other factors [my emphasis](1209).”  In other words, the larger goal is to resolve linguistic cognition into four possible factors; (i) innate domain specific, (ii) innate domain general, (iii) external stimuli, and (iv) effects of “natural law.” Understanding (i) is a critical first step in better specifying the roles of the other three factors and how they combine to produce linguistic competence. As they put it:

The point of a POS argument is not to replace “learning” with appeals to “innate principles” of Universal Grammar (UG). The goal is to identify factor (i) contributions to linguistic knowledge, in a way that helps characterize those contributions. One hopes for subsequent revision and reduction of the initial characterization, so that 50 years later, the posited UG seems better grounded (1210).

I have the sense that many of those that get antsy with UG and POS do so because they see it as smug explanatory complacency: all linguists do is shove all sorts of stuff into UG and declare the problem of linguistic competence solved!  Wrong. What linguists have done is create a body of knowledge outlining some non-trivial properties of UG. In this light, for example, GB’s principles and parameters can be understood as identifying a dozen or so “laws of grammar,” which can now themselves become the object of further investigation, e.g. Are these laws basic or derived from more basic cognitive and physical factors? (minimalists believe the latter), What more basic factors are these? (Chomsky thinks Merge is the heart of the system and speculation abounds about whether it is domain general or linguistically specific) and so forth.  The principles/laws, in other words, become potential explananda in their own right. There is nothing wrong with reducing proposed innate principles of UG to other factors (in fact, this is now the parlor game of choice in contemporary minimalist syntax). However, to be worthwhile it helps a lot to start with a relatively decent description of the facts that need explaining and the POS has been a vital tool in establishing these facts. Most critiques of the POS fail to appreciate how useful it is for identifying what needs to be explained.

The second section of the BPYC paper recaps the parade case for POS, Aux to Comp movement in Y/N questions. It provides an excellent and improved description of the main facts that need explanation.  It identifies the target of inquiry to be the phenomenon of “constrained homophony,” i.e. humans given a word-string will understand it to have a subset of the possible interpretations logically attributable to it.  Importantly, native speakers find both that strings have the meanings they do and don’t have the meanings they don’t. The core phenomenon is “(un)acceptability under an interpretation." Simple unacceptability (due to pure ungrammaticality with no interpreations) is the special case of having zero readings. Thus what needs explanation is how given ordinary experience native speakers develop a capacity to identify both the possible and impossible interpretations and thus:

…language acquisition is not merely a matter of acquiring a capacity to associate word strings with interpretations.  Much less is it a mere process of acquiring a (weak generative) capacity to produce just the valid word strings of the language (1212).

As BPYC go on to show in §4, the main problem with most of the non-generative counter proposals is that they simply misidentify what needs to be explained. None of the three proposals on offer even discuss the problem of  constrained homophony, let alone account for it. BPYC emphasize this in their discussion of string-based approaches in §4.1. However, the same problem extends to the Reali and Christensen bi-gram/tri-gram/recurrent network models discussed in §4.3 and the Perfors, Tanenbaum and Regier paper in §4.2, though BPYC don’t emphasize this in their discussion of the latter two. 

The take home message from §2 and §4.1 and §4.2 is that even for this relatively simple case of Y/N question formation, critics of the POS have “solved” the wrong problem (often poorly, as BPYC demonstrate).

Sociologically, the most important section of the BPYC paper is §4.2.  Here they review a paper by Perfors, Tenenbaum and Regier (PTR) that has generated a lot of buzz. They effectively show that where PTR is not misleading, it fails to illuminate matters much. The interested reader should take a careful look. I want to highlight two points.

First, contrary to what one might expect given the opening paragraphs PTR do not engage with the original problem that Aux-inversion generated; whether UG requires that grammatical (transformational) rules be structure dependent. PTR address another question: whether the primary linguistic data contains information that would force an ideal learner to choose a Phrase Structure Grammar (PSG) over either a finite list or a right regular grammar (generates only right branching structures).  PTR conclude that given these three options there is sufficient information in the Primary Linguistic Data (PLD) for an ideal learner to choose a PSG type grammar over the other two. Whatever the interest of this substitute question, it is completely unrelated to the original one Chomsky posed. Whether grammars have PSG structure says nothing about whether transformations are structure or linearly dependent processes.  This point was made as soon as the PTR paper saw the light of day (I heard Lasnik and Uriagereka make it at a conference at MIT many years ago where the paper was presented) and it is surprising that the published PTR version did not clearly point out that the authors were going to discuss a completely different problem. I assume it’s because the POS problem is sexier than the one that PTR actually address. Nothing like a little bait and switch to goose interest in one’s work.

Putting this important matter aside, it’s worth asking what PTR’s paper shows on its own terms. Curiously, it does not show that kids actually use PLD to choose among the three competing possibilities.  They cannot show this for PTR is exploring the behavior of ideal learners, not actual ones.  How close kids are to ideal learners is an open question.  And similar to one Chomsky long ago considered. Chomsky’s reasons for moving from evaluation measure models (as in Aspects) to parameter setting ones (as in LGB) revolved around the computational feasibility of providing an ordering of alternative grammars necessary for having usable evaluation metrics.  Chomsky thought (and still thinks) that this is a very tall computational order, one unlikely to be realizable.  The same kind of feasibility issues affects PTR’s idealization. How computationally feasible is to assume that are able to order, compare and decide among the many grammars compatible with the data that are in the hypothesis space? The larger the space of options available for comparison the more demanding the problem typically is. When looking for needles, choose small haystacks. In the end, what PTR shows is that there is usable information in the PLD that were it used could choose PSGs over the two other alternatives. It does not show that kids do or can use this information effectively.

The results are IMHO more modest still. The POS argument has been used to make the rationalist point that linguistically capable minds come stocked full of linguistically relevant information. PTR’s model agrees. The ideal mind comes with the three possible options pre-coded in the hypothesis space. What they show is that given such a specification of the hypothesis space, the PLD could be used to choose among them. So, as presented, the three grammatical options (note the domain specificity: it's grammars in the hypothesis space) are given (i.e. innately specified). What's “learned” (i.e. data driven) is not the range of options but the particular option selected. What pposition does PTR argue against? It appears to be the following position: only by delimiting the hypothesis space of possible grammars so as to exclude all but PSGs can we explain why the grammars attained are PSGs (which of course they are not, as we have known for a long while, but ignore that here). PTR's proposal is that it's ok to widen the options open for consideration to include non-PSG grammars because the PLD suffices to single out PSGs in this expanded space. 

I can’t personally identify any generativist who has held the position PTR targets. There are two dimensions in a Bayesian scenario: (i) the possible options the hypothesis space delimits, (ii) a possible weighting of the given options giving some higher priors than others (making them less marked in linguistic terms).  These dimensions are also part of Chomsky’s abstract description of the options in Aspects chapter 1 and crops up in current work on questions of whether some parameter value is marked.  So far as I can tell, the rationalist ambitions the POS serves are equally well met by theories that limit the size of the hypothesis space and those that widen it but make some options more desirable than others via various kinds of (markedness) measures (viz. priors). Thus, even disregarding the fact that the issue PTR discuss is not what generativists mean by structure dependence, it is not clear how revelatory their conclusions are as their learning scenario assumes exactly the kind of richly structured domain specific innate hypothesis space the POS generally aims to establish. So, if you are thinking that PTR gets you out from under rich domain specific innate structures, think again.  Indeed if anything, PTR pack more into the innate hypothesis space than generativists typically do.

Rabbinic law requires that every Jew rehash the story of the exodus every year on Passover. Why? The rabbis answer: it’s important for everyone in every generation to personally understand the whys and wherefores of the exodus, to feel as if s/he too were personally liberated, lest it’s forgotten how much was gained.  The seductions of empiricism are almost alluring as “the fleshpots of Egypt.” Reading BPYC responsively, with friends, is excellent antidote, lest we forget!