When verbs that recommend an action to some degree (such as should, must, ought) are negated, there are two possible interpretations: (a) the action is simply not recommended, nor anything else; or (b) it is recommended to desist from the action. In English, it depends on the verb which interpretation is correct, but for each verb it is generally unambiguous.
- you should go → recommendation; you should not go → recommendation to stay
- you need to go → requirement; you need not go → simple absence of requirement, but go if you want
Some expressions are ambiguous and the meaning depends on context, possibly even on intonation.
All this raises the question how Latin's own oportere reacts to negation. In short, what does this sentence mean?
Te ire non oportet.
- You must not go?
- You need not go?
It is not self-evident, is it? To my surprise, the dictionaries I usually rely on are by and large not helpful. For example, Lewis & Short do not explicitly say how it works, and they give a few negated examples, but without translation.
I think of the core meaning of oportet as being along the lines of “it is proper,” with the negation meaning “it is improper” (there being, in my mind, no middle ground). So it expresses a more or less strong recommendation to desist from the action, and the sentence above means “It is not proper that you go; you must not, ought not go” or something to that effect.
But maybe I am wrong. I was made aware of this problem by a recent question asking for a Latin phrase for “it goes without saying.” A (since-deleted) answer suggested a phrase based on a quote by Thomas Aquinas, who said (Summa Theologiae 1, Q 52):
Et secundum hoc patet quod non oportet dicere quod Angelus commensuretur loco.
And sure enough, there is a translation that claims this means “there is no need for saying …” – I think this makes little sense in context, but then again this translation is actually officially sanctioned by the Catholic church.