1. use these etymology quotations, to motivate ‘unless’ = ‘if not’?
2. teach why “less than” means 'if not'?
Traugott E.C. (1987) “UNLESS and BUT conditionals: a historical perspective.” In A. Athanasiadou and R. Dirven (eds.), On Conditionals Again. Amsterdam/Philadelphia: John Benjamins, page 145.We turn now to unless. The construction developed toward the end of the Middle English period, in the early fifteenth century. At this stage it is a comparative, lesse than, or in/on/of lesse than.
Idem, page 156.By late ME the construction must have become opaque, because we begin to find the forms unless/onless without than. This is presumably in part a folk etymology relating the connective to a negative derivative un- (it can be assumed that the negative inference from the protasis has been lexicalized into, that is, has taken on a morphological form of, the prefix of the conjunction).
Id. 157. See also Etymonline.As we have seen, unless derives from less than, and un- has no historical origin in a negative.
Lande N.P. Classical logic and its rabbit-holes: A first course (2013), pages 55-7.
Baronett S. Logic (5th edn 2022), 236In most statements, the word “unless” means if not.
Gensler H. Introduction to Logic (3 edn 2017), 132.“Unless” is also equivalent to “if not”; so we also could use “(∼B ⊃ D)” (“If you don’t breathe, then you’ll die”).
The word “unless” means “if not”.