Is this use of 'not unless' correct?

If we travel in distant land, still uncharted on the map, and find a new mountain, can we say that we invented the mountain? Not unless we were insane. I thought Not means we cannot, so this is really "We cannot (=Not) (say it was invented) unless we were insane. or should i have said "not unless we... show more If we travel in distant land, still uncharted on the map, and find a new mountain, can we say that we invented the mountain? Not unless we were insane.

I thought Not means we cannot, so this is really "We cannot (=Not) (say it was invented) unless we were insane.
or should i have said "not unless we are sane"?

can someone explain this to me?
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