Source: p 319, A Concise Introduction to Logic (12 Ed, 2014), by Patrick Hurley

in propositional logic it is usually simpler to equate “unless” with “or.”

In and using only ordinary English (and no Logic), how can 'unless' be interpreted to mean 'or'? Can semantic change explain how 'unless' = 'or'?

I wish to understand how 'unless' = 'or' directly and intuitively. So please do not refer to or use the alternative (Stipulative) definition that 'unless' = 'if not'.

unless = 1. prep.phr. On a less or lower condition, requirement, footing, etc., than (what is specified). With preceding negative, expressed or implied.

PS: I quoted the obsolete definition above, because I wish to understand 'unless' from first principles and because all the other definitions use some variant of 'if not'.


"Unless" does not equal "or" 'directly and intuitively'.

  • Can you please transform this into a comment, and delete it as an answer? If the answer really is 'no', then I wish to leave this question unanswered lest some linguist in the future discovers the intuition. – Greek - Area 51 Proposal Jan 2 '16 at 16:21

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