At first approximation, the Definiteness Effect refers to the fact that low subjects in the expletive-associate construction allow only a subset of the determiners that are allowed in canonical subject position (the observation goes back to Milsark's 1974 dissertation):
(1) There was/were {a/some/several/*the/*every/*all} wolf/wolves in the garden.
Now, I must admit I'm not as familiar as I should be with the semantic literature on this topic. What I do know is that there is a long tradition (going back to Milsark himself) of attributing this effect in one way or another to "existential force." The idea is that sentences like (1) assert the existence of a wolf/wolves who satisfy the predicate in the garden, and that we should seek an explanation of the Definiteness Effect in terms of the (in)compatibility of the relevant determiners with this assertion of existence.
This is plainly wrong, as we will see shortly. But since this is an unusually narrow thing to be writing about on Norbert's blog, let me say a bit more about why I think this is an interesting/illuminating test case.
There's a persistent intuition, which has pervaded work within the Principles & Parameters / Government & Binding / Minimalist Program tradition, whereby syntax is a derivation "towards LF." In other words, insofar as syntax has a telos, that telos is assembling a structure to be handed off to (semantic) interpretation. The other interface, externalization to PF, is something of an "add-on." (See, for instance, this call for papers, which takes this (supposed) asymmetry between LF and PF as its point of departure.)
Now, the general claim that LF has a privileged role might be correct regardless of what we find out about the true nature of the Definiteness Effect in particular. But the only way I can envision reasoning about the general claim is by looking carefully at a series of test cases until a coherent picture emerges. With that in mind, it is interesting to consider the Definiteness Effect precisely because it looks, at first blush, like an instance where semantics is "driving the bus": a semantic property (the interaction of existential force with a certain class of determiners) dictates a syntactic property (where certain noun phrases can or cannot go). What I'd like to show you is that this is not actually how the Definiteness Effect works.
The crucial data come from Icelandic (I know, try to contain your shock). One important way in which Icelandic differs from English is that the element that will move to subject position, in the absence of an expletive or some other subject-position-filling element, is simply the structurally closest noun phrase – regardless of its case. To see why this matters, let's start with a sentence like (2). This sentence behaves the same in Icelandic as it does in English, but it forms the baseline for the critical case, later on.
(2) There seems to be {a/*the} wolf in the garden.
In ex. (2), the noun phrase [DET wolf] is part of the infinitival complement to seem, rather than in post-copular position as it is in (1). But the semantics-based explanation of the Definiteness Effect cannot afford to treat the similarity between (1) and (2) as a coincidence if it has any hope of remaining viable, and so whatever one says about "existential force" in (1) must extend to [DET wolf] in (2).
Now consider sentences of the form in (3), an English version of which is given in (4):
(3) EXPL seems [DET1 experiencer (dative)] to be [DET2 thing (nominative)] in the garden.
(4) There seems to the squirrels to be {a/*the} wolf in the garden.
The semantics-based explanation must now extend the same treatment to [DET2 wolf] in (4). But here's where things start to go awry. In Icelandic, it is not DET2 that is subject to the Definiteness Effect in a structure like (3); the restriction in Icelandic affects DET1, while DET2 can be whatever you want.
Here's some Icelandic data showing this, from Sigurðsson's (1989) dissertation:
In exx. (14a-b) we see that, in the absence of a dative experiencer, Icelandic behaves like English (cf. (2), above): the Definiteness Effect applies to the nominative subject of the embedded infinitive. In exx. (15a-b), however, we see that when there is a dative experiencer (mér me.DAT), it is the experiencer that is subject to the Definiteness Effect, whereas the nominative subject of the experiencer can now be definite (barnið child.the.NOM) even while remaining in its low position.
Where does this leave the semantics-based explanation? Insofar as "existential force" is responsible for the Definiteness Effect, it has to be the case that existential force shifts from the downstairs subject to the dative experiencer only when the experiencer is present (cf. (14b) vs. (15a)), and only in Icelandic (not in English; cf. (4) vs. (15a)). This seems to me like a reductio ad absurdum of the "existential force" approach.
There is a much simpler, syntax-based alternative to all of this. The Definiteness Effect can be accounted for if any DP headed by a strong determiner must attempt to move to subject position (even 'the garden' in (1-2, 4) must attempt to do so!). The rules on what can actually successfully move to subject position in different languages are different, and, consequently, the question of which noun phrases can and cannot be definite in-situ will have different answers in different languages, too.
Another thing to note is that the expletive plays no role, here. The ungrammatical variants of (1-2, 4, 14, 15), showing the Definiteness Effect, all have expletives in them. But if your language happens to allow other things – e.g. an adjunct like 'today' – to occupy the preverbal subject position, you can get the same effect with no expletive at all. Here's some more Icelandic data, this time from Thráinsson (2007), showing this:
On the syntactic approach to the Definiteness Effect sketched above, what these data mean is that adjuncts get to move to preverbal subject position only if no nominal has done so; meaning if there is a nominal that must attempt movement to subject position (as all nominals headed by strong determiners must do), and which is in a position where such an attempt would be successful (e.g. allir kettirnir all cats.the.NOM in (6.52d)) – it preempts any adjunct from being able to move there. Definiteness Effect sans expletives.
So the narrow take-home message is that the Definiteness Effect has nothing to do with "existential force." What does this mean for the relation between syntax and semantics? Obviously, weak and strong determiners do differ semantically; that is a truism. But a noun phrase will not exhibit the Definiteness Effect unless it is a position where it is a candidate for movement-to-subject. And which positions these are is a matter that is subject to morphosyntactic variation of a kind that has nothing to do with semantics. Basically, some noun phrases bear a diacritic that forces them to try to move to subject position; whether they bear this diacritic or not seems to be grounded in an interpretive property (strong vs. weak determiners); but whether they succeed in moving or not seems to have no effect on their interpretation (see, e.g., barnið child.the.NOM in (15a), happily interpreted as definite in its low position). The Definiteness Effect, then, is not about semantics except insofar as the presence of the diacritic [+must try to move to subject position] is semantically grounded. Hardly a derivation "towards LF"; the diacritic must be present on the relevant determiners to begin with, before (the relevant part of) the derivation even starts.
What are the consequences of this for the broader question concerning LF as the telos of the derivation? In one sense, not much: this is but one case study; so it turns out that the Definiteness Effect does not match the relevant profile of LF-as-telos. It just means one less entry in the relevant column. Not exactly earth-shattering, there. But in another sense, I think the profile of the Definiteness Effect is the norm, not the exception: syntax pays attention to some (interestingly, not all) semantic distinctions, but it in no way "serves" those distinctions. Definite noun phrases don't move "in order to achieve a definite interpretation" – they just move or don't move (perhaps in a way that depends on their definiteness and the morphosyntactic properties of the language in question), and then they are interpreted however they are interpreted, regardless of where they ended up. I've argued that the exact same thing is true of the relation between specificity and Object Shift. And I suspect that the same is true of almost every single case in which it looks like syntax "serves" interpretation: it is an illusion. There are certain syntactic features that are interpretively grounded (definiteness, specificity, plurality, person features, etc.); and these features can drive certain syntactic operations. But what the syntactic derivation is doing is not constructing a representation that more closely matches the target interpretation. It's doing its own thing. Sometimes this will line up with semantic properties of the target interpretation – like "existential force" – but in those narrow instances where it does, it's really something of an accident. The next time someone tells you that syntax is about constructing "meaning with sound," take it with a boulder of salt.
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UPDATE: As some commenters (esp. Ethan Poole over on facebook, and David Basilico down here in the comments) have pointed out, the data in (14-15) are confounded in some non-trivial ways. I was attempting to be cute and show that the essential observations have been around for close to 30 years – which I still believe to be true – but I now see that it would also have been helpful to include some less confounded data. In service of this, here is some data from my own 2014 monograph that hopefully demonstrates the same points more clearly:
