It is said that there are three types of modality: deontic, epistemic and dynamic.
Here are sample sentences for each type of modality:
(1) You can stay as long as you want. [deontic]
(2) You may be right. [epistemic]
(3) He can drive better than you. [dynamic]
The first two names are said to be derived from the Greek for "biding" and "knowledge", respectively.
As far as 'epistemic' is concerned, the meaning derived from the Greek is such that I can easily understand why linguists who coined these terms did so.
As for 'dynamic' and 'deontic', however, it's not as clear to me why 'dynamic' and 'deontic' are chosen to coin the term 'dynamic modality' and 'deontic modality', respectively.
Specifically, what's so 'dynamic' about sentence (3)? (3) represents the subject's ability. I mean, what has being 'dynamic' got to do with anything about the subject's ability?
And what's so 'deontic' about sentence (1)? (1) represents the speaker's permission, and not any sort of 'duty' as expressed by the word 'deontic'.
Hence these questions:
What's the specific meaning of 'dynamic' in 'dynamic modality' that is descriptive of this type of modality?
Why use the difficult word 'deontic' when it doesn't actually represent the whole picture of the type of modality that it's supposed to represent?