# How does 'unless' mean 'or'?

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'.

• I will go to sleep unless she calls means either she will call or I will go to sleep. In formal logic, therefore, the first statement can be reduced to I will go to sleep, or she calls when you write out the formula. But you cannot simply swap out unless for or in ordinary English, because language is not about reducing statements to equations. Otherwise, we would never have had to invent propositional logic in the first place. – choster Dec 29 '15 at 22:28
• – Greek - Area 51 Proposal Jan 20 '16 at 21:25

"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