OK, I’m a day late with this, but better late than never, eh?
I looked at this prefix briefly last time round, while writing about the fact that indifferent is not simply the opposite of different. It occurred to me that there are a few other similar cases (you might be thinking of inflammable and flammable, though as I mentioned before, in that case in- isn’t a negative prefix added to flammable).
There are of course many straightforward cases where affixing in- to an adjective or noun makes it a fairly simple antonym (opposite) of the word sans prefix: inaccurate, inaction, incomplete, indecisive etc. (you could also add impossible, imperfect, and impolite, as the im- prefix is basically in- with the N changed to an M to ease pronunciation).
And then there are words like insufferable, indescribable, or inconceivable. Still pretty straightforward antonyms. But when was the last time you used the word sufferable? Or conceivable? We tend to think of words with prefixes added as less common, somehow non-standard versions of the word with nothing added, but there are quite a few cases like these whereby the form with the negative prefix is much more common.
The explanation for this is fairly straightforward: how often do you ever need to use words like sufferable, conceivable or describable? Things being conceivable or describable is taken for granted. It’s the assumed default state of all things in existence. Most experiences are assumed to be sufferable, so you never find yourself coming home and saying, Well that was a quite sufferable evening. And do you ever really need to point out that an obstacle is entirely superable?
But the opposites of these conditions are all noteworthy because they’re unusual or unexpected in some way. Of course this phenomenon isn’t confined simply to in- (see previous sentence for example) but I’ve noticed it more with in- than with other prefixes. Perhaps it’s because in- can also be used with other meanings, e.g. movement inwards (inbound, incoming, ingrown etc.), so we don’t associate it simply with the concepts of negation or opposition.
A prefix like un- though, feels more exclusively negative in meaning, and is therefore more likely to be used in antonyms of common words. I’m going to try to pay attention to how often I use words with negative prefixes, and see how often I might use the word without the prefix: the examples might be innumerable, and the results inconclusive, but hopefully the experience won’t be uninteresting.