I was going through papers looking for a story I wrote a long time ago. I came across a photocopy of Chapter 8 of Wright's Arabic Grammar, VERBS OF VAGUE APPLICATION; PARTICIPLES. I wished I had come across this earlier so I could have put it in my new book before it was sent out.
§8: 1. The verb forms which have hitherto been discussed carry with them a specific mention of the agent, or 'doer' of the action: both أخْبَرَني (akhbarani) 'he (implying an already known individual) informed me' and أخْبَرَني الوزير (akhbarani-l-wazir) 'the minister informed me' are structures which state the identity of the informant, and are to that extent specific in their application. Parallel to these there exists a set of verb forms, distinguished by different vowel patterns, which do not state this identity but imply vaguely that 'someone or something unspecified' is the agent.
§8: 2. Verbs of vague application are characterized throughout all verb types by a vowel sequence u-i in the perfect and u-a in the imperfect. ... Forms of vague application of Type IX verbs do not exist. It will be noticed that the imperfect form of vague application of a Type IV verb is indistinguishable from that of a Type I verb.
§8: 3. When the specific verb has a direct object, the corresponding verb of vague application varies in its form as if the direct object were the agent; the direct object functions in the sentence structure as a surrogate for the unmentioned agent. ...
§8: 6. The participle is a recognizable word pattern which is primarily an entity-term associated with a verb, and connoting the agent of the verbal idea but without adding any other information about that agent. The participle form associated with the verb كتب يكتب (kataba yaktubu) means simply 'writer', that is, an entity definable solely by the statement 'he writes' or 'he has written' and carries no further information about the entity. Consequently, الكاتب is congruous in meaning with الذي كتب (aladhi kataba) or الذي يكتب (aladhi yaktubu) 'the person who wrote/writes', and كاتب (kaatib) with مَن كتب (man kataba) or من يكتب (man yaktubu) 'a person who has written/writes'.
I like this sinister grammatical form of unmentioned informers and vague application. I like the idea of a form that means an entity definable solely by the statement 'he writes' or 'he has written' and carries no further information about the entity.
W. Wright, A Grammar of the Arabic Language (3rd Edition)