Another nonstandard hypothetical statement has the form, "p
"The plant will die unless you water it." This means the plant
will die if you do not water it; "unless" means "if not." Thus,
the following three propositions are equivalent:
p unless q.
p if not-q.
If not-q, then p.
Does "p unless q" also imply, "If q, then not-p"? Are we also
saying that if you do water the plant, it will not die? No--in
some contexts it might be assumed that we are also saying this,
but it is not asserted by the original statement. Once again,
think of the statement as a promise I have made to you. If you
water the plant and it still dies, I haven't broken my promise,
strictly speaking; I didn't promise that water alone would keep
the plant alive, only that the absence of water would kill it.
Consequently, "p unless q" is exactly equivalent to, "If not-q,
then p," and should be translated that way.