Hurm… I don't know whether "sine qua non" can go with "denying". In [url=http://72.14.203.104/search?q=cache:BIkd8X2aNk8J:www.jurisearch.com/newroot/caselink.asp%3Fseries%3DF.2d%26citationno%3D468%2BF.2d%2B769+%22sine+qua+non+for+denying%22&hl=ja] the google[/url] I found an
Indeed, the problem seems to be that sine qua non does not fit comfortably with deny. The phrase literally means 'without which not' and deny involves a negative, so essentially it's a problem of understanding a sentence with two negatives.
I can easily understand the example if we say 'approving a motion' instead of 'denying a motion', but I find I have
"A has maintained that being non-skeptical about some view of the world is not a sine qua non for rejecting moral realism."
"As already indicated, A opposes the view that ethical skepticism’s being local is a sine qua non for holding that there are no objective moral values." ?
Well, they both make my head hurt a bit, but the second version makes my head hurt a little less!
However, not to be facetious, I think that with this type of subject, the author can reasonably expect his reader to make an active effort to weigh the words and find the meaning.
You could always say, instead, something like an indispensable condition, but
So, I think I will change the first one, using "indispensable condition":
"A, on the contrary, has maintained that being non-skeptical about some view of the world is not an indispensable condition for rejecting moral realism."
Anyway, there's still "indispensable", and hence several negative expressions.
It's grammatically correct, but awkward because of the implied double negative. sine qua non is usually most comfortable in a purely affirmative context.