In this episode, we took a deep dive into the semantics of modal verbs, but we didn’t talk much about how they fit into the structures of sentences, and this seems to leave open some important questions. For starters, we made the claim that — in terms of their meanings — modal verbs combine with whole sentences, and not just the verb phrase that follows them. After all, the meaning of the sentence in (1a) seems to correspond to (1b).

     (1a)    The Observers must report to their commander.

     (1b)    It must be that the Observers report to their commander.

On the face of it, it seems weird that subjects in modal sentences appear separate from the main verb phrase, as in (1a), while being interpreted as though they were right next to them, as in (1b). It looks like this could be a big problem for our overall theory.

Thankfully, when we take into account some of the important discoveries we’ve talked about in past episodes, like the VP-Internal Subject Hypothesis, this problem goes away pretty quick. If it’s true that, in general, subjects start off somewhere inside the verb phrase, and only later move to a spot that's higher up (and more to the left, at least in English), we can suppose that the meanings of sentences — modal and all — are simply computed before the subject starts moving around, instead of after.

But, this still leaves us wondering what part of the tree modal verbs typically call home. In our old system, we might’ve been happy dumping all our modal verbs into the bucket labelled “inflection.” And this seems reasonable at first, since like other kinds of inflection (e.g., tense, aspect, voice), modality sometimes appears as an affix on the verb.

     (2)    Turkish:    gel-me-meli-siniz


                               ‘You ought not to come.’

But this probably isn’t the most sophisticated picture of how sentences get put together, given that concepts like tense and modality are pretty different from each other. Moreover, if they really were members of the exact same category, you might not expect to find them showing up in the same place at the same time, like how a coin can’t land both heads and tails; in fact, most modal verbs have both present tense (may, can, shall, will) and past tense (might, could, should, would) forms.

So, it probably makes more sense to think of modal verbs as appearing either somewhere just below, or somewhere just above, a dedicated tense phrase (TP).

It might even be both, given that modal verbs seem to interact with tense in subtly different ways, depending on their flavour:

     (3a)    Olivia could have used her powers, but she didn’t want to.

     (3b)    Olivia could have used her powers, but I haven’t found out yet.

After all, the sentence in (3a), which is saying something about Olivia’s abilities, seems to be about what was possible in the past (i.e., circumstances might be different now), whereas the sentence in (3b) says something about the speaker’s present state of knowledge. In other words, it looks as if the same modal verb either falls under the influence of the past tense, or else manages to escape it, depending on how it’s interpreted. This suggests there might be modal phrases both below and above the tense phrase.

Lastly, it’s important to say something about how modal phrases actually get their meanings, and what that says about their internal structure. In the episode, we gave the modal verb “must” a meaning that looked like this:

     (4)    “must” = λ B . λ p . B ⊆ p

Reading it from left to right, it says that a modal verb first combines with some contextually defined modal base B — whose job it is to give the flavour, like whether it’s about belief or ability — and then goes on to combine with the sentence p, saying of the two that the set of worlds described by the base is a subset of the set of worlds described by the sentence. The modal base, then, is kind of like a pronoun; it’s like “they” or “them,” since it picks up its meaning from the context of the conversation. Whether “must” says something about someone’s knowledge, or about some set of rules to be followed, depends on the content of B. And if all this is right, it means that the structures of modal phrases — at a minimum — look something like this:

In reality, a bit more needs to be said about this, since a modal base by itself (as we’ve been thinking of it) can’t completely determine the meaning of the sentence. That’s because all the context can really provide is a general description of what’s being talked about — like whether what’s being discussed includes beliefs, rules, goals, abilities, et cetera. But, it can’t take the extra step of supplying those beliefs or rules or goals. To get a sense of why this makes a difference, take a look at the following sentence.

     (5)    I must report to the Colonel.

Imagine that this sentence is said by someone who mistakenly believes that the Colonel asked to see them. In this case, unbeknownst to the speaker, the sentence is false. And the reason it’s false is because of what the actual requirements are, and not simply what the speaker might suppose they are. So, whether or not this kind of sentence ends up true or false depends on the way the world actually is, and not the way the speaker thinks it is. In other words, speakers can be uninformed about the content of the modal base, in a way that can’t be handled by context alone; that is, they can be mistaken about what the relevant rules or beliefs really consist of. And all of this means that the modal base must really be more like a function that’s meant to take some world as its input (the world in which the sentence is spoken), and then produce a set of worlds out of that, for the modal verb to work with — one that accurately captures whatever’s being talked about.

So, a more complete picture of what the internal structure of a modal phrase looks like is this:

What we see here is a function (sometimes called an accessibility relation) combining with a special kind of world pronoun. This relation R is what’s actually provided by the context of the conversation; it’s what determines the overall flavour. That w* symbol is what does the job of filling in the content of whatever’s being talked about, by providing the full set of rules or abilities under discussion. And it’s together that they determine the modal base — the set of worlds that modals like “must” or “may” go on to compare with the set of worlds described by the rest of the sentence. This way, we fully account for the fact that the truth of modal sentences depends not just on the context of the conversation, but on the way the world is.

As a final point, it’s worth mentioning that no matter which kind of structure we choose to represent modal sentences, none of them ever quite match up with how the sentence is actually pronounced. That is, the Logical Form of the sentence (how it’s interpreted), at least in these cases, is reliably different from its Phonetic Form (how it’s said). In fact, as it’s turned out, the idea that there are mismatches between how sentences are spoken and what they mean has played a big role in modern syntactic theory — one we’ll continue to talk about in the future!



So how about it? What do you all think? Let us know below, and we’ll be happy to talk with you about modality and possible worlds. There’s a lot of interesting stuff to say, and we want to hear what interests you!

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