WikiDiscuss

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wrote:

>
> In Spanish, the equivalent construct "X a menos
> que Y",
> always requires Y to be put in the subjunctive
> mood.

This is virtually automatic with a subordinate
"que" clause; whether it is significant is harder
to tell (as is whether that significance carries
over to English, where the corresonding sentence
is not subjunctive — or not unequivocally so).
And, of course, that also does not speak to the
logical — thus Lojban — situation.

> I'm not convinced "unless" can be a purely
> truth
> conditional OR. I don't want to say that
> "I am mortal, unless you are mortal" is true,
> even though "I am mortal OR you are mortal" is.
>
Yes, there does seem to be something else than
simple OR going one. As noted, I suspect it is
pragmatic — but that doesn't relieve us of the
need to figure out what it is and display it.

> "Unless" would seem to require its complement
> to be either false or very unlikely /
exceptional.

This seems to be in the right direction,
corresponding to the emphatic negative
conditional "If he did it, I'll eat my hat." =
"He definitely didn't do it" and the like.

I'm not sure what you mean by "complement,"
appparently the sentence after the "unless"? But
that doesn't seem to fit the usual cases: "He
will come unless he gets caught in traffic" which
is neither obviously false nor even unlikely but
merely setting up an explanation in case the
other sentence turns out to be false.

What does seem to be the case is that there are
two uses of "unless," one purely factual, the
other with an added fillip, presumably the same
one that occurs also (though in a negative
presentation) in the conditional cited above,
emphasizing the truth (or falsity) of one factor
by the obvious falsity of the other. I think
this is much less common with "unless" than "if"
but the underlying mechanism is the same, truth
functionality (whether MTT or MTP) and trivial
falseness of one component. But none of this is
subjunctive or contrary-to-fact, just a pragmatic cuteness.