Deontic and Epistemic Modality

Modal verbs can be classified into two types, deontic and epistemic. Deontic modality expresses obligation and permission, and epistemic modality expresses possibility and prediction. Context is often needed to determine if deontic or epistemic modality is expressed.

In the sentence You must be good to get into Oxford University, two interpretations are possible. Deontic modality is connected to necessity. To get into Oxford University, it is necessary to be a good student. However, epistemic modality states that it is evident that the person is a good student because he/she has already entered Oxford University.

The same modal can be used to convery different types of modality. In the sentence He must be tired, the speaker is sure that the person is tired. It is an example of epistemic modality. In the sentence He must go to prison, the speaker believes it is necessary for the person to go to prison. It is an example of deontic modality. The modal verb must conveys both deontic and epistemic modality.

Modal verbs can express deontic and epistemic modality. Deontic modality can express different degress of obligation and permission. The modal must expresses a higher degree of obligation than should. Epistemic modality can express different degrees of possibility. The modal must expresses a higher degree of possibility than might.