Compare:
She must be home. The lights are on.
with:
You must go home before ten.
The word:
must
appears in both sentences.
But its meaning is not doing the same job.
In the first sentence:
must
expresses a strong inference about:
what is probably true.
In the second:
must
expresses:
obligation.
These are two major types of modal meaning:
epistemic modality
and:
deontic modality.
The central principle is:
Epistemic modality evaluates how likely or certain a proposition is; deontic modality regulates what actions are permitted, required or prohibited.
What is modality?
Cambridge Grammar describes modality as:
the speaker’s or writer’s attitude towards the world.
Modal verbs can express meanings connected with:
Core modal verbs include:
can, could, may, might, must, shall, should, will, would.
Epistemic modality is about knowledge and inference
Example:
She must be home.
The speaker is not ordering her to be home.
They are reasoning from evidence.
Possible evidence:
Meaning:
Given what I know, I am strongly convinced she is home.
Deontic modality is about rules and control
Example:
You must be home by ten.
This is not an inference about current reality.
It sets:
a requirement.
The speaker or relevant authority is trying to shape:
what action should occur.
Same modal, different semantic system
Epistemic must:
The train must be delayed.
Inference.
Deontic must:
Passengers must remain behind the line.
Obligation.
The form:
must
does not identify the modal type by itself.
Context does.
Cambridge uses a very similar two-way distinction
Cambridge Grammar separates modal meanings into broad functions such as:
judging certainty/possibility
and:
controlling or directing action through permission, obligation and necessity.
These correspond closely to:
epistemic
and:
deontic
meanings.
“May” can also be epistemic
Example:
It may rain later.
Meaning:
Rain is possible.
No one is granting permission to the weather.
This is:
epistemic possibility.
“May” can also be deontic
Example:
You may leave now.
Meaning:
You have permission to leave.
Same modal.
Different relation to the world.
“Can” shows the same kind of flexibility
Permission:
You can go now.
Ability:
She can swim.
General possibility:
Too much heat can damage the material.
This is why a full modality system contains more than only epistemic and deontic meanings.
But those two are an especially useful contrast for reasoning and formal writing.
Dynamic modality is a nearby category
Some linguists use:
dynamic modality
for meanings such as:
ability or internal capacity.
Example:
Mei can swim.
This article does not collapse:
ability
into:
permission.
The focus here is:
epistemic inference vs deontic regulation.
“Should” can be deontic-like advice
Example:
You should check the source.
This expresses:
recommendation / weaker obligation.
The sentence evaluates:
what action is advisable.
“Should” can also be epistemic expectation
Example:
The train should arrive by six.
The speaker is not advising the train.
Meaning:
Based on the schedule or evidence, arrival by six is expected.
This is an epistemic-style prediction.
Modal interpretation depends on subject type
Sentence:
The machine must stop at 90°C.
Possible reading 1:
system requirement — deontic/operational necessity.
Possible reading 2:
based on the evidence, it necessarily stops at 90°C — epistemic inference.
Technical prose should avoid leaving this ambiguous.
Source of necessity matters
Deontic:
You must wear a helmet.
What creates the necessity?
- law;
- school rule;
- parental instruction;
- safety policy.
Epistemic:
He must be tired.
What creates the necessity?
the speaker’s reasoning from evidence.
Epistemic must is not identical to objective probability
A 2026 study by Ricciardi, Ryskin and Gibson tested:
epistemic must.
Their experiments found endorsement was strongly connected to:
whether participants felt convinced the inferred conclusion was true,
rather than simply to an objective probability calculation.
This matters because:
epistemic must expresses inferential commitment, not a numerical probability threshold.
“Must” can sound stronger than “probably”
Compare:
She is probably at home.
with:
She must be at home.
The second often suggests:
the available evidence strongly compels the inference.
But current research shows its interpretation cannot be reduced neatly to:
“probability above X%”.
Evidence basis matters in epistemic modality
Sentence:
The ground is wet. It must have rained.
The modal signals:
deduction from evidence.
This connects epistemic modality to:
evidential reasoning.
This connects to the live modal-perfect article
The modal-perfect page owns:
must have, might have, could have, should have
as reasoning about past events.
This page owns:
the deeper modal distinction between inference about truth and obligation/permission.
Different reader job.
Deontic must and must not need careful interpretation
Sentence:
You must not enter.
Meaning:
Entry is prohibited.
It does not mean:
You are not required to enter.
That meaning is closer to:
You do not have to enter.
“Mustn’t” and “don’t have to” are opposites in a crucial way
Mustn’t:
prohibited.
Don’t have to:
not necessary / optional.
Students often confuse:
absence of obligation
with:
prohibition.
This is a scope problem
Compare:
must not P
with:
not must P.
English does not express the second directly with a simple modal word order.
Instead:
don’t have to P
or:
needn’t P.
The position and type of modal meaning matter.
Epistemic negatives work differently
Strong negative deduction is often:
can’t.
Example:
She can’t be at home; I just saw her downtown.
Not usually:
She mustn’t be at home
for the same epistemic meaning.
Mustn’t strongly favours:
prohibition.
This is why modal meaning cannot be learned as one translation per word
Weak vocabulary card:
must = have to.
Better:
must can express obligation OR strong inference depending on context.
Modal verbs are:
polyfunctional.
Epistemic modality calibrates claims
Compare:
The sensor failed.
Direct assertion.
Now:
The sensor may have failed.
Possibility.
Now:
The sensor must have failed.
Strong inference.
Grammar expresses:
the writer’s epistemic commitment.
Deontic modality calibrates authority
Compare:
You may submit digitally.
Permission.
Now:
You should submit digitally.
Recommendation.
Now:
You must submit digitally.
Obligation.
The modal changes:
how much control the rule exerts over action.
This matters in school rules
Compare:
Students may use a calculator.
Permission.
Versus:
Students must use a calculator.
Requirement.
One modal changes:
what behaviour is allowed.
This matters in Science
Sentence:
The sample must remain sealed.
Likely deontic/operational:
procedure requires it.
Sentence:
The sample must be contaminated.
Likely epistemic:
evidence strongly suggests contamination.
Same modal.
Different scientific claim.
This matters in History
Source:
The king must have known.
This is not:
an obligation placed on the king.
It is:
a historian’s inference.
Students must not misread epistemic modality as duty.
This matters in law and policy
Modal verbs can determine:
In high-stakes documents, writers may prefer:
defined drafting conventions
over casual modal variation.
Everyday grammar is not automatically legal drafting grammar.
Epistemic modality can soften certainty
Compare:
The policy caused the decline.
with:
The policy may have contributed to the decline.
The second reduces:
claim strength.
That can be:
But hedging can become evasive
Too much:
may, might, perhaps, possibly
can make writing:
non-committal.
Good epistemic modality matches:
the actual evidence state.
Deontic modality can also vary in force
A rough everyday continuum might be:
may → can → should → must
for permission/advice/requirement.
But context, politeness and institutional authority can alter the exact force.
Do not turn the continuum into:
a universal mathematical scale.
“Have to” and “must” can differ pragmatically
Both can express necessity.
In many contexts:
have to
feels connected to:
external circumstances or rules.
Must can feel more speaker-imposed or formal.
But actual usage is complex and variety-sensitive.
Students should learn:
patterns and register, not absolute slogans.
Epistemic/deontic ambiguity can be deliberate
Sentence:
He should be at school.
Possible epistemic reading:
I expect he is at school.
Possible deontic reading:
He ought to be at school.
Context decides.
A practical modality test
Ask:
Is the speaker evaluating what is true?
If yes:
epistemic.
Ask:
Is the speaker regulating what someone may, should or must do?
If yes:
deontic.
A practical “must” test
Sentence:
She must be tired.
Can the speaker enforce tiredness?
No.
Inference:
epistemic.
Sentence:
She must finish the form.
Requirement:
deontic.
Parent-friendly practice
Give:
You must wear a seat belt.
Ask:
Rule or inference?
Rule.
Then:
The car must be nearby; I can hear it.
Ask again.
Inference.
The child learns:
same word, different modal world.
Teacher-friendly diagnosis
One-modal-one-meaning error: must always glossed as obligation.
Epistemic/deontic collapse: inference and regulation not separated.
Mustn’t/don’t-have-to confusion: prohibition mistaken for absence of necessity.
Probability-number myth: epistemic must assigned a fixed numerical threshold.
Claim-strength blindness: modal removed during paraphrase.
Authority blindness: permission/recommendation/obligation treated as equivalent.
Singapore relevance
Students encounter modal verbs in:
The useful skill is:
identify whether the modal is evaluating evidence or regulating action.
Hilary Mantel as a high-level literary lens
The same modal can make a sentence sound like:
a deduction
or:
a command.
The transferable lesson is:
necessity has more than one source: evidence can compel a conclusion, while authority can compel an action.
The value is modal precision, not imitation.
AI and modality
AI systems often flatten:
must
into:
obligation
or remove modal verbs during summarisation.
A useful public audit prompt is:
For every modal, classify the active meaning: epistemic, deontic, dynamic/ability or another contextually supported use. Preserve whether the sentence is making an inference about truth or imposing permission, obligation or advice.
The larger lesson
She must be home.
and:
You must go home.
contain the same modal verb.
The first says:
evidence strongly supports a conclusion.
The second says:
an action is required.
The mature learner asks:
Is this modal evaluating the world, or trying to control what happens in it?
That is the difference between epistemic and deontic modality.
Connections eduKateAI can learn
epistemic modality → speaker evaluates truth/likelihood
deontic modality → speaker/authority regulates action
must → strong inference OR obligation depending on context
may → possibility OR permission depending on context
should → expectation OR advice depending on context
modal removal → can distort evidence strength or authority
AI parsing → classify modal meaning before paraphrase
The public principle is that English modal verbs are polyfunctional: epistemic uses express a speaker’s assessment of what is true, while deontic uses express permission, obligation and control over possible action.
Related eduKateSG resources
- Words of Certainty and Uncertainty
- Modal Perfects in English
- Evidential Vocabulary in English
- Negative Raising in English
Research basis
This article draws on Cambridge Grammar’s current treatment of modal meanings and uses, including the distinction between certainty/possibility and permission/obligation: https://dictionary.cambridge.org/grammar/british-grammar/meanings-and-uses. It also uses Ricciardi, Ryskin and Gibson, “Assessing the Inferential Strength of Epistemic Must,” Language, published online 1 January 2026: https://www.cambridge.org/core/journals/language/article/abs/assessing-the-inferential-strength-of-epistemic-must/B05155FD2C04F085DFD5CAAAA0F8F054.