A Student's Introduction to English Grammar

§8.1 Uses of the modal auxiliaries 55

declarative clauses with deontic meanings of modals are used to try and influence

what happens rather than simply to make assertions.

The link between the two families of meanings is that the concepts of necessity and
possibility - the key concepts in modal logic - apply to both. But with epistemic
modality, necessity and possibility relate to whether or not something is the case, is
true, whereas with deontic modality they relate to whether or not something hap­
pens, or is done. In [52i], for example, I'm saying in [a] that it is necessarily the case
that he overslept, and in [b] that it is necessary for him to apologise: in neither do I
countenance any other possibility.
Epistemic and deontic meanings are not in general associated with different
expressions. Many examples are ambiguous, allowing either kind of interpretation
for the modal:

[53] Yo u must be very tactful. [epistemic or deontic]

There is an epistemic interpretation of this under which it means I have evidence
that leads me to believe you're very tactful. And there is also a deontic one that I
might use to tell you there is an obligation or need for you to be very tactful (and
perhaps thus to tell you to behave with tact).

Dynamic interpretations

Some of the modals have uses concerned with properties or dispositions of per­
sons or other entities involved in the situation:

[54] She can speak five languages.
ii I've asked him to help us but he won 't.
iii I daren 't tell you any more.

These are called dynamic interpretations, and are somewhat peripheral to the con­
cept of modality. In [i], can is used to describe an ABILITY of hers; in [ii], the nega­
tive form of will talks about VOLITION (his unwillingness to help us); in [iii], dare
says something about whether my COURAGE is sufficient for me to tell you any
more. (Dare is unique among the modals in that it has ONLY a dynamic use.)
With can we find clear cases of ambiguity between a dynamic and either an epis­
temic or a deontic interpretation:

[55] Yo u can 't be serious.

II She can drive.

[epistemic or dynamic]

[deontic or dynamic]

The epistemic interpretation of [i] denies the possibility that you are being seri­

ous: it suggests a context where you have said something that I take to be absurd.

The dynamic interpretation says something about your personality: you are inca­

pable of being serious.

In [ii] the deontic reading is that she has permission to drive, while the dynamic

one attributes an ability to her - she knows how to drive.