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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 alēthia, 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.

You may find deontic modality referred to as event modality because it expresses the speaker / writer's view of the desirability of an event being the case.  We don't use that term in this guide but it is an alternative.
You may also encounter alethic modality described as circumstantial modality because it refers to circumstances outside the control of and without relation to the speakers involved in any kind of speech event.  Alethic modality would, philosophically speaking, exist even if there were no beings capable of expressing it.

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 four categories concerning how the ideas are realised in the structures of the language (there are two examples of each).
Click here when you have an answer.


Whose rules?

Because deontic modality is to do with rules, it is tempting to refer only to obligation when discussing the forms that can be used to realise the function of allowing or forbidding.
However, rules are of different sorts and are often realised in different ways through the language.  Here are some sorts of rules that apply to deontic modality.

The law
External obligations to do something or not to do something are often rendered in stern terms, allowing little flexibility.
There is a built in threat that a failure to obey will result in unpleasant consequences.  As a result, the modality is expressed through imperatives (often using no plus a noun phrase or gerund) and strong modal auxiliary verbs such as must, may, shall etc.  For example:
    No parking at any time
    Passengers must not cross the line here
    The tenant shall leave the premises in a fit state
    No alcohol may be served after 11 pm
    Give way to oncoming traffic

and so on.
Rules based on ethical systems and religions are not enforceable in the same way as the law of a country but are equally stern (often even sterner because the unpleasant consequences last longer; for eternity in some views).  Rules derived from ethical systems are also, therefore, often couched in similar terms to laws enshrined in constitutions.  For example:
    Thou shalt not kill
    Say your prayers three times a day
    Love your neighbour
    Alcohol is unlawful

Personal codes of conduct
These are often derived from ethical systems (and there is some overlap) but are also often not dependent on external views so are less obviously obligations.  Nevertheless, they lead to expressions of deontic modality such as:
    I cannot tell a lie
    I swear to tell the truth
    You should be kind to animals
    Eating meat is wrong

and so on.
Personal codes of conduct often lead to admonitions to others, imperatives, softened or not, in an effort to get others to conform to the same code so we get:
    Take your litter home
    Turn off unwanted lights
    Please consider our neighbours and leave quietly after 10 o'clock

Organisations rather than people also have codes of conduct which, while being non-obligatory and probably unenforceable except in unusual circumstances, carry a sense of obligation and, sometimes, the threat of sanctions.  For example:
    This space to be kept tidy
    Our customers pay your wages: treat them with respect

Personal commitments
These are allied to personal codes of conduct but instead of enforcing an obligation on others (or everyone), the terms used commit only the speaker or writer to the obligation.  It is a self-imposed obligation and sometimes called, therefore, commissive modality.  Examples are:
    I'll pay you back tomorrow
    I'll get that done for you
    I'll come and help
    I promise to be faithful

Organisations, too, indulge in commissive modality when they state, for example:
    We will reply to your request within six working days
See below for more on this form of modality.
These are often the result of a personal code of conduct influencing a request (as in the last example above) but they are also polite attempts to get others to do something and fall within the purview of deontic modality.  Examples include:
    Please queue here
    Please park tidily

and such requests are often couched in terms that make them seem like laws when they are in fact unenforceable such as:
    Private property
    No parking
    Please close the gate

While not placing an obligation on another, advice, too, is a form of deontic modality because it concerns persuasion and that is a form of obligation.  Examples include:
    You should see a dentist about that tooth
    Why don't you write her a letter?
    How about looking for another job?

and so on.


Overlapping categories

Obligation vs. requirement

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:

all express more or less the same idea using three of the 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 a form of alethic modality.  An alternative analysis is to call these forms of modality teleological because they refer to the end or aim of the person concerned.
Alethic modality is more frequently used to refer to the physical properties of the world around us.  For example:

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:

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 alethic and epistemic clothes.

Whose obligation?

In the menagerie of terminology concerning modality, a distinction may also be drawn between different types of modality expressing requirement or obligation (or its lack):

  1. Deontic modality
    properly expresses that which is necessary, obligatory or possible given a set of legal or moral principles.  For example:
        You must not drive over 70 miles per hour in Britain
        You can't park here overnight
        People should respect their elders
  2. Boulemic modality
    expresses that which is necessary given a person's wants and desires.  For example:
        I must go now
        You must go to bed
        I have to make a note or I'll forget it
    In the first and third examples, the obligation rests on the speaker.  In the second, the obligation is transferred to the hearer but the desire for compliance still comes from the speaker.
  3. Teleological modality
    expresses that which is necessary given a person's aims or desires.  For example:
        She'll have to study hard if she wants to pass the examination
        If we want the meeting to start on time, we must get the opening speeches over quickly

External or internal?

In English language teaching, some make much of the concepts of intrinsic or extrinsic obligation and aver, for example, that:
    I have to eat more healthily
    I must eat more healthily
represent different kinds of obligation, the first externally imposed (by, e.g., a doctor or dietician) and the second internally imposed by the speaker's wishes.
This is not really sustainable for two reasons:

  1. There is little evidence for this and native speakers use the forms in free variation in most circumstances.  Evidence exists that the use of have to is increasing at the expense of must, at least in spoken BrE.  If this is true, the extrinsic-intrinsic distinction between the forms cannot be sustained.
    Most native speakers will, for example, see very little if any difference at all in meaning between:
        I must get an early night
        I have to get an early night
  2. The distinction is hard to sustain.  For example:
        I must see the dentist about this tooth
        I have to see the dentist about this tooth
    Aside from the fact that most people would not see much difference in meaning, is toothache an external or internal condition?
        You must learn the language if I want to live here
        You have to learn the language if I want to live here
    are arguably both forms of teleological modality and indistinguishable in meaning.
    nd so on.


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 auxiliary 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.

There is some validity in having a cline such as:

but it has to remain mobile so that you can move things depending on the variables concerned with stress and intonation as well as the role relationships between speaker and hearer.
From a police officer:
    You should get the brakes repaired
is an obligation but from a friend, it may simply be advice with no possibility of an imposed obligation.

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, linked below, to navigate to where you need to go.

Identifying three general levels of obligation – high, mid and low – can be a more helpful approach.

Now pause for a moment and consider which modal auxiliary verbs are used in each case.  There are six choices:

  1. absolute obligation to do something
  2. absolute obligation not to do something
  3. advice to do something
  4. advice not to do something
  5. no obligation to do something
  6. no obligation not to do something

Click here when you have a list.


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 will 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 auxiliary verb to refer to a future action.  In fact, it is the verb functioning as a primary auxiliary verb to make the aspect prospective).

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

follow me

Directive and Imperative modality

These are two parallel sub-categories of deontic modality which refer to the speaker / writer's view of how strong an obligation placed on someone is.
This category can itself be further analysed and, for teaching purposes, each type can be the topic of a lesson or mini-lesson.  Here is what is meant:


Sources of potential confusion with modal auxiliary verbs


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:

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

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:

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


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

Impersonal statements of deontic modality are often used as a way to avoid what is known as a face-threatening act (Goffman, 1967).
A direct order such as:
    Take these to the bank now
can be seen as threatening the face of the hearer because it places him or her in a subordinate, more humble position socially.  To avoid that, many speakers in authority roles may prefer to say:
    The bank expects these soon
and leave it to the hearer to make the connection between that and an imperative (which it remains).

Deontic modality, expressed through verbs like must, have to, ought to and should may be seen as a face threatening and making 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 or by an external authority.

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.
This may also be termed teleological modality in that it represents the need for a condition to be fulfilled before an aim is reached.

The verb need 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
modality and aspect for the guide to how aspect is realised with modal auxiliary verbs
epistemic modality which follows a similar format to this one
dynamic modality which follows a similar format to this one
multiple modalities for a guide which considers how different types of modality may be combined in single utterances
mood for a guide which considers the three main moods signalled by verb changes in English and some moods in other languages
ambiguity for a guide which considers the polysemous nature of many modal auxiliary verbs and the ambiguity which can arise
the modality map for more choices

Goffman, E, 1967, Interaction Ritual: Essays on Face-to-Face Behavior, New York: Doubleday
Palmer, FR, 2001, Mood and Modality, 2nd edition, Cambridge: Cambridge University Press