Could "It needn't have been Jill that wrote the note" be interpreted other than deontic?

Question

I know that the modality can be ambiguous and many clauses can be interpreted in many ways. I find that the clause "It needn't have been Jill that wrote the note" can be interpreted as deontic, but is dynamic also an option or not?

Thank you for your help!

Answer 1

In the terminology of Huddleston & Pullum, there are three forms of modality: epistemic, deontic, and dynamic. Following their rules around negation, we can roughly rephrase your sentence to use each of those three unambiguously:

1. Epistemic: It is not known whether Jill was the one who wrote the note.
2. Dynamic: Jill had the ability/capacity not to be the one who wrote the note.
3. Deontic: Jill was not obligated/required to be the one who wrote the note.

"It needn't have been Jill that wrote the note" could be interpreted as (1); you could use the sentence in the epistemic sense to correct someone who claimed that Jill was certainly the author.

It could also be interpreted as (3); you could use that sentence in the deontic sense to point out that someone else could have taken on the task.

But the interpretation (2) does not seem plausible. It seems like a quite unusual use of the word "need," and it's an odd idea to express semantically in context. You would use it to (say) refute the idea that Jill was coerced into writing the note.

Answer 2

Let's think about what the sentence is saying and then what it is doing.

It is equivalent (in truth value) to the following propositions:

1. The statement "Jill wrote the note" is not necessarily true.
2. The statement "Jill wrote the note" may/might not be true.

Proposition 1 is true if and only if something like 1a is true:-

1a. The statement "Jill did not write the note" does not contradict anything we know to be true about Jill or the note.

Proposition 2 looks (and is) more uncertain than this. "May"/"might" are modal forms. If I say "I might/may come and see you tomorrow", my appearance or non-appearance at your house tomorrow makes no difference to the truth or otherwise of my statement. It is not a prediction or a promise. It is modal. So is 'need' as in 'She need not have' done it. If it turns out that she did in fact do the deed, even this does not make the sentence wrong.

As to 'deontic', meaning 'to do with obligation or necessity'. The Routledge Encyclopaedia of Philosophy explains it thus:

Modality, as it is usually understood in contemporary philosophy, has to do with necessities and possibilities. Deontic modality is a kind of modality which has to do with what is necessary or possible according to various rules, such as the norms of morality, the principles of practical rationality or the laws of some country. Deontic modality can be contrasted with alethic modality and epistemic modality. The former has to do with what propositions are necessarily or possibly true given various metaphysical, logical or nomological laws. The latter has to do with what propositions are necessarily or possibly true given various bodies of evidence or information.

Words commonly thought to express deontic modalities include the auxiliary verbs ‘must’, ‘have to’, ‘may’, ‘can’, ‘should’ and ‘ought to ’... On these criteria, I think it would be fair to use the term deontic.