How to motivate ‘unless’ = ‘if not’, with etymology?

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My students still reckon Lande’s 2 motivations for ‘unless’ = ‘if not’ (below) too abstract, formalistic! Thus I need a 3rd simpler, motivation with merely etymology. But I never studied linguistics! How do I

1. use these etymology quotations, to motivate ‘unless’ = ‘if not’?

2. teach why “less than” means 'if not'?

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.

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.

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).

Idem, page 156.

As we have seen, unless derives from less than, and un- has no historical origin in a negative.

Id. 157. See also Etymonline.

mid-15c., earlier onlesse, from (not) on lesse (than) "(not) on a less compelling condition (than);" see less.

The first syllable originally was on, but the quality of negation in the word and the lack of stress changed it to un-.

Lande N.P. Classical logic and its rabbit-holes: A first course (2013), pages 55-7.

In most statements, the word “unless” means if not.

Baronett S. Logic (5th edn 2022), 236

“Unless” is also equivalent to “if not”; so we also could use “(∼B ⊃ D)” (“If you don’t breathe, then you’ll die”).

Gensler H. Introduction to Logic (3 edn 2017), 132.

The word “unless” means “if not”.