In the question on Sherlockian logic, Batavulus, in his answer gave an alternative translation of the clause "it must be believed"/ "one must believe it", which is:
"fieri non potest quin sit credendum." =
"it cannot happen that it may not be believed."
The English was a quick off-the-cuff translation (by me); an appalling double-negative. It implies doubt, from subjunctive "sit" = "may"/ "might" (character Sherlock Holmes was not in any doubt) and the key seems to be the meaning of "quin".
Joonas (CHAT: 3/2/2021) suggested:
"there is no option but to be believed."
The beauty of this is that it not only captures the required meaning, but by adroit sleight-of-hand (the translator's art) it negates any need to define "quin" & "sit"--brilliant!
The conjunction "quin" functions in a number of contexts; more than one may be applicable e.g. "(so) that..not" which would create another double-negative. Alternatively, "why not?". Although a question appears to be reinforcing the doubt, Allen & Greenough p.449 (2b): "...and quin with a Present Indicative may have the force of a command:-
"quin accipis" (Ter. Haut. 832) = "Here, take it" ("Why not take it?").
The translation would then be:
"It cannot happen it is to be believed."
This is the opposite of what is required and ignores "sit".
What are the roles of "quin" and "sit" in "fieri non potest quin sit credendum"?