Friday, February 19, 2016

(Logic II) Translation of "unless"

Nice tip from Peter Suber:

Unless. Sometimes "unless" should be translated as inclusive disjunction, and sometimes as exclusive disjunction. For example, "I'll go to the party unless I get another offer" means that I'll go if nothing else comes along. In many contexts it also means that I might go anyway; the second offer might be worse. So I'll go or I'll get another offer or both (inclusive disjunction). Consider by contrast, "I'll go to the party unless Rufus is there". In many contexts this means that if I learn Rufus is going, then I'll change my mind and not go. So I'll go or Rufus will go but not both (exclusive disjunction). For symbolizing exclusive disjunction, see Tip 2, above. Because there is no hard and fast rule, paraphrase the English before translating. (I thank Susanna S. Epp for helpful correspondence on the subtlety of "unless".

Also, as mentioned in class (and in line with several of Suber's paraphrases above), we can usually symbolize unless claims in the form of conditional (material implication) claims.

So I think Hurley's answer was both wrong and incomplete.


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