patient subjects and intransitive verbs

Except for the case of passives, patient subjects always have intransitive verbs, because patient subjects are not acting on someone or something else but are themselves being acted upon.

Often a transitive verbs with an agentive subjects will have an identical intransitive counterpart that of necessity takes a patient subject.

Yoko emptied the bathtub.  >>  The bathtub emptied.

The alarm cleared the room. >>  The room cleared.

Hester dimmed the lights. >> The lights dimmed.

The cook thickened the soup. >>The soup thickened.

Orson opened the door.>>  The door opened.