This is an exercise in a book: We are asked to translate "one" using the passive voice for several sentences. Unfortunately, the exercise is not corrected.

One of those sentences is "One can not know." (actually the sentence is in French: "On ne peut connaitre")

But posse has no passive voice. Do you have any idea?

4 Answers 4


It is impossible to know what the book wants without more context (and even at that sometimes instructions are unclear. That said, you cannot make it passive. The only thing that can be made passive is scire, which would become sciri.

  • sciri non potest, "it is not able to be known" or "one is not able to be known"

Cf. Cicero's id de Marcello aut certe de Postumia sciri potest, "that can be known from Marcello or certainly from Postumia."

  • 1
    It seems that it is what the author of the book was expecting. I found an example in the lesson : "One can not live." translated as "Non potest vivi." Verbatim, it is like "To be known can not". Can we say that the infinitive passive verb "sciri" is the subject of the verbe potest ? Jun 17, 2021 at 5:32
  • 1
    @ArnaudMégret They're usually called the "complementary infinitive." So sciri is the complementary infinitive of the verb potest.
    – cmw
    Jun 17, 2021 at 12:05
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    The funny thing is that the passive form potestur does in fact occur along with a passive infinitive complement, but it is not typical.
    – Anonym
    Jun 17, 2021 at 21:45
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    @Anonym It's old Latin, found a handful of times and only once, I believe, by the archaizing Lucretius in Classical literature. It's certainly not something that a textbook would be wanting a beginner to know. But yes, it is possible (and weird!).
    – cmw
    Jun 17, 2021 at 23:23
  • @Anonym I wonder how exactly that came about, since it's not really passive - attraction from the passive infinitive and re-analysis of the verb's origins?
    – cmw
    Jun 17, 2021 at 23:27

My first thought would be that the intention is for you to render it as 'it cannot be known', so something like notum esse non potest or perhaps nosci non potest.

It sounds more natural with more added, so for example 'one cannot know X' as 'X cannot be known'; the construction using 'one' in English/French is really expressing a passive idea.

  • It sounds convincing. Can we say, in that case, that you use potest as a kind of impersonal verb (without subject) ? Jun 16, 2021 at 20:24
  • I'm not a good source for grammatical terminology, so I'll let someone else comment with how it could be analysed (and whether my suggestion has any mistakes I haven't noticed).
    – dbmag9
    Jun 16, 2021 at 21:49
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    I think the first option should be neuter (notum)
    – brianpck
    Jun 17, 2021 at 3:37
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    Perhaps non should go before potest and not before notum. It sounds to me like you are saying it can be unknown.
    – Figulus
    Jun 18, 2021 at 1:03

You could use the impersonal licet:

Scire non cupias, quod scire non licet.
Desire not to know what cannot (or must not) be known.

(Apparently Isidorus, Synonyma, according to this book.)


The English impersonal pronoun "one" is not easy to produce, in Latin. The nearest would be the neuter, impersonal gerundive-type construction e.g. "nunc est bibendum" = "now one must drink"; literally: "now it-ought-to-be-drunk". It was Horace's exhortation to the Romans to celebrate the downfall of Antony & Cleopatra (30BC).

Here, "non sciendum est" = "It ought-not-to-be-known";

rendered to: "one must not know".

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    This is indeed a good way to say "one must not know". But do you think this approach can be used for "one cannot know"?
    – Joonas Ilmavirta
    Jun 17, 2021 at 13:42
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    @Joonas llmavirta: Thank you. It's not easy to translate this verbatim. Answers--though good--have had to skirt around-the-houses, somewhwat; but, nothing wrong with that. If "one cannot know"; then, presumably, one is not allowed to know: hence, Seb's clever use of "licet". If one is not allowed to know; then, "one must not know"! I am with Horace's school-of-thought. If you require the English, impersonal pronoun, "one", Horace is your man.
    – tony
    Jun 17, 2021 at 16:40

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