The verb quiet can be used as a causative, with or without the particle down. Some examples from the web:
- Shut Up and Play 9 summer apps that will quiet your kids. (source)
- It’ll help calm and quiet your kids, before they settle down to sleep. (source)
However, only with down can quiet be used as an intransitive verb with a reflexive interpretation.
- I decided to quiet down.
- * I decided to quiet.
(6) is ungrammatical on any reading, whereas (5) is grammatical only on the reading ‘I decided to quiet myself down’. Why is the particle down important here? In my dissertation (chs. 4 and 5), I proposed that certain argument positions introduced by prepositions and particles can be left unsaturated in a way that leads to a reflexive interpretation. Direct objects without a preposition, particle or small clause of any kind are special; following work by Alec Marantz, such direct objects are not arguments in the same sense as external arguments, applied arguments, or the subjects of small clauses, but are incorporated more directly into the verb’s event structure. That is, the direct object in (1-2) doesn’t have the same status as the direct object in (3-4); in (3-4), the object is a thematic argument of the particle down. This is the sort of argument that can be left semantically unsaturated, such that it forms a complex predicate where the external argument gets two theta-roles in (5).
Of course, there are intransitive verbs like wash, which seem to be intransitive with a reflexive interpretation. Why isn’t a particle important for these verbs?
- I decided to wash up.
- I decided to wash.
One possibility is that verbs of the sort in (8) are primarily interpreted as activities, with the causative-result being inferred but not entailed. However these verbs are built and interpreted, the particle is necessary for the event structure to entail a reflexive causative-result. Since a verb like quiet in (1-6) names a result and not an activity, it requires a particle in order to be interpreted as an intransitive reflexive, ruling out (6) in contrast to (8).