The question "What is the difference between 'impossible' and 'implausible'?" has generated an interesting discussion on the differences, if any, between "totally implausible" and "impossible".
More precisely, the debate arose from the following statement:
Probably, in some contexts, "totally implausible" could mean
"impossible."
Since it seems that there is controversy on this matter, as the following comments show,
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"there's certainly scope for disagreement about what exactly totally implausible means. There's no doubt in my mind that the negating im- prefix is doing something subtly different when applied to “plausible” vs. “possible”. Even if not everyone (or perhaps no-one) agrees with me on that point."
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"implausible is often used thus, as if implausible and improbable were synonyms; and what is "totally improbable" is, by definition, impossible. But I think this is a misuse: "not seeming probable" is not the same thing as "not probable". Plausible speaks to perception, probable to fact."
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"Not sure I agree that totally implausible could mean impossible. Why does the addition of the adverb change the meaning of implausible to impossible?"
I wonder: could "totally implausible" mean "impossible?"
Best Answer
I take Barrie's point that plausibility is primarily concerned with appearance rather than actuality. But I think there's a limit to how far one can take such strictly literal interpretations.
In most contexts, probable/possible mean likely/unlikely, but whereas the corresponding negated form improbable means not likely, impossible doesn't mean not unlikely - it means definitely not.
By the same token, convincing/plausible usually mean very believable/just about believable. I'm aware some people use plausible to mean convincing, but I think most people need something very/totally plausible to actually be convinced.
As John Lawler often reminds us, negatives [are] probably the most complex portion of English semantics and the weirdest part of its syntax.
And as Cerberus points out here, [probability] is a complicated issue [in the context of linguistics].
In my version of English, if something is implausible, it's unlikely, but at least feasible. But if it's totally implausible, it has no element of plausibility/feasibility whatsoever, and I personally do not distinguish that from impossible.