Are these introductory logic textbooks wrong to teach ‘unless’ = ‘or’? [duplicate]
Closed as duplicate by Peter Taylor on Jan 26, 2024 at 10:16
This question has been addressed elsewhere. See: Why does “unless” mean “if not”?
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Colin Fine answered that
This contradicts the textbooks beneath. Who is correct?
Usually, "P unless Q" is "symbolized as P ∨ Q. Stephen Cole Kleene, Mathematical logic (1967 - Dover ed 2002), page 64.
Lande N.P. Classical logic and its rabbit-holes: A first course (2013), pages 55-7.
The wedge symbol is used to translate “or” and “unless.” A previous chapter explained that “unless” is equivalent in meaning to “if not.” This equivalence holds in propositional logic as well, but in propositional logic it is usually simpler to equate “unless” with “or.” For example, the statement “You won’t graduate unless you pass freshman English” is equivalent to “Either you pass freshman English or you won’t graduate” and also to “If you don’t pass freshman English, then you won’t graduate.” As the next section demonstrates, the wedge symbol has the meaning of “and/or”—that is, “or” in the inclusive sense. Although “or” and “unless” are sometimes used in an exclusive sense, the wedge is usually used to translate them as well.
Hurley P. A Concise Introduction to Logic (13 edn, 2018), page 319.
Translate “unless” as “or.”
Gensler H. Introduction to Logic (3 edn 2017), 132.
In addition, the word “unless” sometimes functions like the word “or.” For example, the statement “You can’t go to the party unless you clean your room,” can be rewritten as “Either you clean your room or you can’t go to the party.”
Baronett S. Logic (5 edn 2022), 318
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