It's not, and it's not supposed to be. Many Esperanto words have a few different meanings. Often that's because the respective word in national languages is ambiguous - like artikolo, which can be a grammatical article (English the, Esperanto la) or an article in a magazine.

Of course. You're right. I was ignoring a huge pile of obvious counterexamples like "artikolo."

Just to add, I did a little hunting about and found this little newsgroups message from Bertilo, in which he asserts that "malfermita" can be used for both "opened by someone" and just "open":

Thanks for the link. I can easily accept the "open/opened by someone" usage as a practical matter. I was just never able satisfactorily derive it, given the clearly transitive nature of "malfermi." It's just not helpful to try to think about language as though it were a branch of mathematics.

This discussion of rompi and rompiĝi got me thinking more about the use of the iĝi verbs. According to Ivy Kellerman's book:

b. Intransitive verbs may be similarly formed from the roots of transitive verbs, and indicate an action of the verb not immediately due to the subject's acting upon itself (as in the case of reflexive verbs, 41) and not caused by any direct agency (as in the case of the passive voice, 169): La pordo fermiĝas, the door closes (goes shut). La veturilo moviĝas, the vehicle moves. La branĉo rompiĝas, the branch breaks. Grupo da personoj kolektiĝis, a group of persons gathered.

How do you treat the -iĝi version of a transitive verb for which agency is strongly implied? For instance: "La kato laviĝis." Have I said simply that the cat is washed, without specifying the washer?

Incidentally, Kellerman's book (published in 1908) may be a bit old-fashioned, but it's quite good. The reading samples are much more interesting than the usual blather about furniture and grocery shopping found in language texts. And it's also free. You can download the Gutenberg project's ebook.

arkadio:"La kato laviĝis." Have I said simply that the cat is washed, without specifying the washer?

This would translate to English as "the cat washed", which in practice would be interpreted as the cat washed itself. You often see these reflexive forms with the iĝ suffix, such as laviĝi, baniĝi, moviĝi and others. It doesn't necessarily show that it was the subject doing the action to itself, but usually it will be understood that way depending on the meaning of the verb.

tommjames:

arkadio:"La kato laviĝis." Have I said simply that the cat is washed, without specifying the washer?

This would translate to English as "the cat washed", which in practice would be interpreted as the cat washed itself. You often see these reflexive forms with the iĝ suffix, such as laviĝi, baniĝi, moviĝi and others. It doesn't necessarily show that it was the subject doing the action to itself, but usually it will be understood that way depending on the meaning of the verb.

The version of "The cat washed" that I can get from "La kato laviĝis" belongs in the same category as these sentences:
(1) La libroj vendiĝis. (The books sold.)
(2) La gitaro ludiĝis bele. (The guitar played beautifully.)
The English translation would use "wash" ergatively. Upon hearing "La kato laviĝis," I would imagine a stuffed cat that had been successfully laundered. I guess it could be a real cat that had been bathed.

arkadio:Upon hearing "La kato laviĝis," I would imagine a stuffed cat that had been successfully laundered. I guess it could be a real cat that had been bathed.

Could be I guess, would depend on the context. But generally when someone laviĝas the meaning is they are washing, as in washing themself. I wouldn't compare a cat who laviĝas to a book that vendiĝas, because a book is inanimate and it doesn't make sense to think of a book selling itself, or doing anything to itself for that matter. Same with a guitar playing itself. If someone said "tiu gitaro ludiĝas bone" I think I would interpret that as "that guitar plays well", as in it is nice to play. Whether an iĝi verb should be taken as a reflexive action really depends on the meaning of the verb itself, the type of subject, as well as the context.

tommjames:

arkadio:Upon hearing "La kato laviĝis," I would imagine a stuffed cat that had been successfully laundered. I guess it could be a real cat that had been bathed.

Could be I guess, would depend on the context. But generally when someone laviĝas the meaning is they are washing, as in washing themself. I wouldn't compare a cat who laviĝas to a book that vendiĝas, because a book is inanimate and it doesn't make sense to think of a book selling itself, or doing anything to itself for that matter. Same with a guitar playing itself.

I wasn't clear. I didn't mean guitars playing themselves or books doubling as salesmen.

If someone said "tiu gitaro ludiĝas bone" I think I would interpret that as "that guitar plays well", as in it is nice to play.

Yes, that is exactly what I meant. It isn't perfect English, but people say things like that all the time, e.g. "The car handles nicely." I came across the "vendiĝi" example in Kellerman or Richardson or some other Esperanto text. In some ad I saw recently, a company asserts that an item "ships easily." Bad English, true, but I think I can render this in Esperanto as "La ero ekspediĝas facile."

Whether an iĝi verb should be taken as a reflexive action really depends on the meaning of the verb itself, the type of subject, as well as the context.

arkadio:I didn't mean guitars playing themselves or books doubling as salesmen.

I didn't think you did I was just making the point, that vendiĝi for a book couldn't ever really be reflexive in the same way that laviĝi can be for a cat, so they're not exactly the same. But I think I just misunderstood your "belongs in the same category". I took it to mean "always works the same way", but I see what you meant now, you meant it is getting washed in the same way you might say "la vestoj laviĝas en la lavmaĉino". This would of course be the better interpretation if we're talking about a non living entity like a stuffed cat that clearly cannot wash itself. For something living though both meanings are possible, it would just be the context that makes it clear. Of course if you want the reflexive meaning you can always go for "lavas sin" to be more precise.

arkadio:How do you treat the -iĝi version of a transitive verb for which agency is strongly implied? For instance: "La kato laviĝis." Have I said simply that the cat is washed, without specifying the washer?

Or perhaps it was a native english speaking person who didn't want or was afraid of using the accusative form.
If you really want to use it reflexively, then you should say:
La kato lavis sin.
If you want to say that someone else washed the cat then you should use the passive voice:
La kato estis lavita (lavata).
I can't imagine a good reason to leave it open, if the cat was washed or if "ŝi lavis sin mem".
.
The same is true for:
La libro vendiĝis.
Normally you would say:
La libro estas (estis) vendita.
But if you want to emphasize that the book was sold without any efforts you could say:
La libro vendiĝis.
or
La libro vendis sin (mem).

horsto:If you really want to use it reflexively, then you should say: La kato lavis sin.
[...]
I can't imagine a good reason to leave it open, if the cat was washed or if "ŝi lavis sin mem".

The reflexive iĝ forms are a standard and frequently appearing part of the language so I wouldn't go avoiding them just as a matter of course. You often see troviĝi for trovi sin, leviĝi for levi sin, moviĝi for movi sin, turniĝi for turni sin etc. These forms can all have the non-reflexive meaning but in context they are usually clear enough.

tommjames:

horsto:If you really want to use it reflexively, then you should say: La kato lavis sin.
[...]
I can't imagine a good reason to leave it open, if the cat was washed or if "ŝi lavis sin mem".

The reflexive iĝ forms are a standard and frequently appearing part of the language so I wouldn't go avoiding them just as a matter of course. You often see troviĝi for trovi sin, leviĝi for levi sin, moviĝi for movi sin, turniĝi for turni sin etc. These forms can all have the non-reflexive meaning but in context they are usually clear enough.

From my point of view it's not me who avoids the reflexive iĝ forms, but people avoid to use the accusative or the passive voice, because it's more comfortable to use the reflexive iĝ forms. You avoid to think about the accusative (la kato lavis sin) or to think about the tense to use in the passive voice (la kato estis lavita/lavata).
Of course, for some transitive verbs it's normal to use the reflexive iĝ forms, as you said, for example komenci, trovi, naski, and so on. It would sound strange if you say: la konferenco komencis sin or la urbo trovas sin en Usono.
But it's completely different in the case of: la kato laviĝis.
For me that is a poor and lazy way to express the situation: You know what happens, but you express that in a way that doesn't allow the reader to understand exactly what happens.