logo  ELT Concourse teacher training
Concourse 2

Deontic and alethic modality: expressing degrees of requirement

don't turn left, turn right

Deontic modality refers to degrees of obligation (from the Greek déon, meaning roughly that which is done).
Alethic modality refers to given truths (from the Greek alētheia, meaning truth).
They are treated together here because there is a considerable overlap between that which is a given truth (a force of circumstance) and that which one person chooses to oblige another to do.

In the example above, we have used a simple imperative verb form ((don't) turn) to express high degree of obligation but we can also use other ways to express lower levels of requirement or obligation, using for example, should or ought (to).
Here are some examples.  Your task, should you choose to accept it, is to divide them into 3 categories (there are two examples of each).
Click here when you have an answer.

  • You must not turn left.
  • Turning left is forbidden
  • They forbid you to turn left
  • Turning right is obligatory
  • The sign tells you to turn right
  • You have to turn right


Overlapping categories

We can take these categories one by one to do a bit of analysis but first we need to see where the categories of obligation (deontic modality) and requirement (alethic modality) overlap.  For example:

  • One must study hard to succeed in the examination
  • Success in the examination requires / needs hard study
  • Success in the examination is only achievable by hard study

all express more or less the same idea using three different structures as above, but do not carry the sense of a person obliging another to do something.  They express what the speaker / writer sees as universal truths rather than advice or admonition.  This is alethic modality.
Alethic modality is also frequently used to refer to the physical properties of the world around us.  For example:

  • You have / need to get the temperature up to 327.5 °C to make lead melt
  • You can only / must combine hydrogen and oxygen in a ratio of 2:1 to form water
  • Occasionally, a leap second must be applied to Coordinated Universal Time (UTC) to keep it in line with mean solar time.

The following examples lead to the same conclusion as the first set above regarding examination success but carry the sense of obligation by an external authority rather than expressing a universal truth:

  • You must study hard to succeed in the examination
  • I require you to study hard to succeed in the examination
  • Your success in the examination will be achieved by your hard study

The categories overlap because one strong way of expressing obligation is to dress up what you say as if it were a universal truth rather than strong advice or person-to-person obligation.  See the comments on need below for more on dressing deontic in epistemic clothes.


Levels of obligation with modal auxiliary verbs

The issue here is the strength of the modal auxiliary verbs.  Some coursebooks and many teachers are tempted to invent some kind of cline between 0% and 100% obligation with a total lack of obligation at one end and an absolute command at the other.  There are problems with this approach:

  1. It is almost impossible to get two native speakers to agree exactly where on the cline each verb should go.
  2. The verbs vary in deontic strength from mild advice to absolute requirement depending on:
    1. the roles taken or assumed by speakers
    2. power relationships between people
    3. the topic: laws, physical or otherwise are often cases of alethic modality but opinions are the realm of deontic modality

The same modal verb, then, can imply anything from mild advice to absolute command depending on who is using the language to whom to talk about what.  The examples in the table below have to be interpreted in that light.  The approach taken there is that the speaker / writer has some kind of authority to express obligation to another and it is not an exchange between equals.

This is not the place for a detailed look at modal auxiliary verbs.  That sort of guide appears elsewhere.  Go to the general map of modality to navigate to where you need to go.
Identifying three general levels of obligation – high, mid and low – can be a more helpful approach.  Then we get an analysis like this:

Level Type Example Issues and complications
High obligation or Alethic modality Positive You must finish now
A square must have four sides
You have to go there
You have to travel faster than 340 metres per second to break the sound barrier
You will put your address here
Between equals in terms of authority (such as friends) the modal verbs must and have to are often used simply to express advice as in, e.g.:
You must try this cheese
You have to hear this

will is quite rare in this sense and implies absolute authority
Negative You must not park here
You can't do that
Parallel lines cannot meet
You may not write yet
You won't speak to her again
can't and may not are used to express negative deontic obligation but not to express positive obligation in either case.
will not / won't is even rarer in negative obligation statements and implies absolute authority
must not is used to express strong deontic modality but is very rarely used to express negative alethic modality:
?Parallel lines must not meet
and may not is never used in this sense:
*A square may not have 5 sides
Mid-level obligation Positive You should take an umbrella
They ought to say sorry
There are some who aver that ought to expresses a greater sense of moral duty than should, which expresses simple advice.
Negative This shouldn't be left here
oughtn't to be so rude to her
As with the positive, both verbs carry the same sense of mid-level obligation or duty but ought to is somewhat rarer.
Low or absent obligation Positive You can go when you like
You may leave at any time
can and may carry the same approximate sense of a lack of firm obligation in the positive.  In the negative they express high-level prohibition (see above).
That is not intuitive and gives many learners problems.
Negative You don't have to stay
needn't go
These two are parallel in the present but have different senses in the past:
She didn't have to go (so didn't)
She needn't have gone (but did)


Commissive modality

A sub-category of deontic modality is one in which the speaker commits to a self-imposed obligation.  In BrE, this is often realised through the use of shall in expressions such as:
    You shall have the money by Thursday
although in spoken language the contracted form ('ll) disguises the use of shall and many believe it is a use of will.  The modal auxiliary verb will is often used in this way but expresses volition or willingness rather than a commitment as in
    I'll give you money on Thursday, if you like
which is dynamic modality rather than commissive or deontic modality (and not, except incidentally, the use of the modal verb to refer to a future action).

There are other ways to realise the sense, e.g.:
    I commit to getting the money to you by Thursday
    I have made a commitment to get the money to you by Thursday
    The money will be with you by Thursday – promise


Sources of potential confusion with modal auxiliaries

  • These auxiliaries are also used to express degrees of truth of a proposition as the speaker / writer understands it (epistemic modality) and, if the use is not made clear by very explicit contexts which include the speaker / writer's intentions, there is a danger of confusing learners.  For example:
    • She must / should / ought to be in London by 6 o'clock (probably expressing epistemic modality: the speaker's understanding of the truth of the proposition)
    • She must / should / ought to be in London at 6 o'clock (probably expressing deontic modality: the speaker's obligation on her or comment on an external obligation)
  • In the past, there are three issues:
    • although the levels of obligation are unchanged, the form of e.g.,
          They shouldn't have said that
      looks like a present perfect and learners may be tempted consider it conceptually like that.  It is, however, in sense, akin to a past simple because the event referred to is finished in the past.  For more, see the guide to modality and aspect.
    • the past of should (not), ought (not) to and need not imply that an obligation in the past was not fulfilled:
          She shouldn't have gone (but did so the obligation was not fulfilled)
          We should have said sorry (but didn't so the obligation was not fulfilled)
          I ought to have been there (but wasn't so the obligation was not fulfilled)
          I oughtn't to have said that (but did so the obligation was not fulfilled)
          I needn't have gone alone (but did so the obligation was not fulfilled)
    • the past of have to, however, implies that the obligation was fulfilled
          I had to see the doctor yesterday (and did so the obligation was fulfilled)
          I didn't have to see the doctor (so didn't so the lack of obligation was fulfilled)


Personal and impersonal expressions of obligation: verbs and attributive clauses

I advise you against that  

These examples are of common ways in which personal expression of obligation can be achieved.  The strength of the obligation conveyed is a semantic issue to do with the meaning of the verbs:

  • I forbade them to do it
  • I permitted them to go
  • The teacher expected them to come on time
  • She advised him to wait

Using a passive construction removes the personal effect of the verbs:

  • They were forbidden to do it
  • They were permitted to go
  • They were expected to come on time
  • He was advised to wait

Using attributive clauses with the dummy it-structure further reduces the level of the personal although whether these should all be analysed as a form of passive with a verbal structure or a use of participle adjectives is disputable:

  • It was compulsory to stay
  • It was expected that they came on time

One minor way is through the use of a marginal modal (or semi auxiliary):

  • We were supposed to stay


Face saving and face threatening: dressing deontic modality in epistemic clothes

Deontic modality, expressed through verbs like must, have to, ought to and should is sometimes seen as threatening the face of the hearer because it implies that the speaker is in authority and the hearer subservient.
For this reason, speakers will often dress up the imposition by selecting a different formulation.  A common one is the use of the semi-modal need which implies less of a personal imposition and more of a fact without which something cannot occur.
In this sense, it can be seen as a form of alethic or epistemic modality, because it refers to the truth of a proposition.  The communicative effect, however, is often deontic.

There are times when need, must and have to may be used interchangeably but there are some important differences in nuance.
For example:
    You must buy a ticket
    You have to buy a ticket
    You need to buy a ticket

may be considered synonymous and, in many cases, they are.
All of these are examples of deontic modality.  So, for example,
    You have to fill in a form
    You must fill in a form
refers to the hearer’s duty and the obligation placed on him/her by the speaker.

However, in
    You need to fill in a form
The semi-modal need implies that it is a general rather than personal requirement, to do with the logical necessity of something being true (epistemically).
This form is often used, for example, in the passive as:
    To be processed, the form needs to be submitted before the end of the month
which states a conditional necessity rather than a personal obligation.

The verb often implies some undesired consequence in the way that must and have to do not.  For example, choosing the formulation
    You need to be at the station by 6 (or you'll miss the train)
expresses the preference for avoiding the unwelcome logical consequence while
    You must be at the station by 6
expresses more direct obligation.

Face-saving for the hearer is often the motivation for selecting need over the more direct model verbs of obligation.  In, for example:
    Everyone needs to be in the office by 8 o'clock
no personal obligation is implied because this is simply company policy and not my imposition on you or anyone else.  It is a statement of fact, sounds more polite and distances the speaker from any sense of assumed authority.
Compare, too, the use of need in, for example:
    The bank needs to have these figures tomorrow
    You have to / must give the bank the figures tomorrow
because the first saves the face of the hearer by not imposing authority on him/her.

Finally, we can ignore (if we wish) any statement using must or have to because we are independent operators.  So, for example:
    You have to come at 6 o’clock
is an obligation I can ignore if I choose and that might upset someone else, but
    You need to come at 6 o’clock
is probably not because it implies that I will miss something important if I don’t, not that someone else will be upset.

Related guides:
non-modal-verb modality covers some of the above and extends it to include epistemic and other forms of modality
epistemic modality which follows a similar format to this one
dynamic modality which follows a similar format to this one
the modality map for more choices