Unless we have passports, we can't go abroad.
If I rewrite the above sentence in the following ways, is it acceptable or grammatical?
- If we have passports, we can go abroad.
- If we don't have passports, we can't go abroad.
Is there any change in sense? Thanks in advance.
From a perspective of formal logic, #0 and #2 are equivalent.
(0) Unless we have passports, we can't go abroad.
We have no passports => we cannot go abroad
(1) If we have passports, we can go abroad.
We have passports => we can go abroad
(2) If we don't have passports, we can't go abroad.
We have no passports => we cannot go abroad
(1) allows for the possibility that you can go abroad without a passport. (Maybe you're the Obama and the Pope, and no-one checks your identification)
Conversely, (0) and (2) allow for the possibility that we can't go abroad even if we do have a passport (maybe no one is allowed to travel)
Consider these three true statements:
(0b) Unless a bird has wings, it can't fly.
This is true, but a penguin has wings and can't fly.
Wings are a necessary, but not sufficient condition for flight.
(1b) If a bird is flying, it is alive.
This is true, but bird that is not flying might be alive too. Being alive is a necessary, but not sufficient condition for a bird to fly.
(2b) If a bird doesn't have wings, it cannot fly.
As with (0b), some birds with wings can't fly also.
From a less formal perspective: In some situations, it is natural to assume that statements like these work both ways. In a casual context, if I say "That restaurant doesn't give free refills on drinks before 6 o'clock," many people would assume that they do give free refills after six o-clock. This is fine when context is available to help out, but in technical or legal writing you need to pay attention to the formal logic structure. Or if you are talking with a nine year old.
